Planetary and Space 49 (2001) 859–906 www.elsevier.com/locate/planspasci

TOF-SIMS in Thomas Stephan ∗

Institut fur Planetologie=Interdisciplinary Center for Microscopy and Microanalysis (ICEM), Westfalische Wilhelms-Universitat, Wilhelm-Klemm-Str. 10, 48149 Munster, Germany

Received 21 June 2000; received in revised form 27 February 2001; accepted 27 February 2001

Abstract Time-of-.ight secondary spectrometry (TOF-SIMS) was introduced into cosmochemistry about a decade ago. Major advantages of TOF-SIMS compared to other ion microprobe techniques are (a) parallel detection of all secondary with one polarity in a single measurement—both polarities in subsequent analyses, (b) high lateral resolution, (c) su4cient mass resolution for separation of major mass interferences, and (d) little sample destruction. This combination makes TOF-SIMS highly suitable for the analysis especially of small samples, like interplanetary and presolar dust grains, as well as tiny inclusions within . Limitations of this technique are mainly referring to isotopic measurements and quantiÿcation. The possibility to measure molecular and atomic ion species simultaneously extends the applications of TOF-SIMS to the investigation of indigenous hydrocarbons in extraterrestrial material, which might have been essential for the formation of . The present work gives an overview of TOF-SIMS in cosmochemistry, technical aspects as well as applications, principles of data evaluation and various results. c 2001 Elsevier Science Ltd. All rights reserved.

Table of contents

1. Introduction...... 860

2. Principles of secondary ion ...... 861 2.1. Generation of secondary ions ...... 861 2.2. Separation of secondary ions ...... 862 2.3. Quantiÿcation ...... 862 2.4. Static and dynamic SIMS ...... 863 2.5. Charge compensation ...... 863 2.6. Mass interferences ...... 863 2.7. Mass resolution in time-of-.ight mass spectrometry ...... 865

3. Time-of-ight secondary ion mass spectrometry ...... 865 3.1. Secondary ion mass ...... 865 3.2. Description of the TOF-SIMS instrument ...... 866 3.3. TOF-SIMS mass spectra ...... 869 3.4. Quantiÿcation ...... 870 3.5. Element analysis ...... 872 3.6. SIMS sensitivities ...... 876 3.7. analysis ...... 877 3.8. Organic ...... 878 3.9. Data processing ...... 878

∗ Tel.: +49-251-8339050; fax: +49-251-8336301. E-mail address: [email protected] (T. Stephan).

0032-0633/01/$ - see front c 2001 Elsevier Science Ltd. All rights reserved. PII: S 0032-0633(01)00037-X 860 T. Stephan / Planetary and Space Science 49 (2001) 859–906

4. Meteorites ...... 879 4.1. Ca,Al-rich inclusions in carbonaceous ...... 879 4.2. Organic molecules in , Murchison, and ...... 880 4.3. Further studies with TOF-SIMS ...... 884

5. Interplanetary dust ...... 885 5.1. Volatile elements ...... 887 5.2. identiÿcation ...... 887 5.3. The collector project ...... 888 5.4. impact residues from space probes ...... 890 5.5. Further TOF-SIMS studies ...... 890

6. ...... 891 6.1. Isotope measurements ...... 891 6.2. Element mapping with TOF-SIMS ...... 892

7. TOF-SIMS in comparison ...... 893

8. TOF-SIMS in space ...... 893 9. Summary ...... 895 Acknowledgements ...... 895 Appendix ...... 895 References ...... 900

1. Introduction range from the analysis of rare elements in var- ious samples (e.g., Zinner and Crozaz, 1986; Bottazzi The analysis of micrometer-sized samples like, e.g., in- et al., 1994; Floss and JolliK, 1998; Fahey, 1998) to iso- terplanetary dust, presolar grains, and small inclusions in tope measurements (Zinner, 1989) in meteorites (e.g., meteorites has become more and more important in cosmo- McKeegan et al., 1998; Leshin et al., 1998; Chaussidon . Recent technical developments lead towards the et al., 1998; Eiler et al., 1998), interplanetary dust (e.g., ultimate analytical goal, the localization and identiÿcation of McKeegan et al., 1985; Zinner, 1991), presolar grains every single and its chemical bonding state in a sample. (e.g., Zinner et al., 1989; Zinner, 1991, 1998), and also The transmission (TEM) with an ex- terrestrial (e.g., Froude et al., 1983; Chaussidon and Li- tremely high lateral resolution allows even seeing individual bourel, 1993; Didenko and Koval, 1994; Schuhmacher crystallographic layers. Therefore, this technique has been et al., 1994) and lunar (e.g., Compston et al., 1984) . established widely into cosmochemistry (e.g., Tomeoka and Technical developments of conventional SIMS instruments Buseck, 1984, 1985, 1988; Bradley, 1994a). New develop- during the last years led to increased sensitivities at high ments in mass spectrometry like resonant mass mass resolution and high spatial resolution. This culminated spectrometry (RIMS) enable to analyze the isotopic com- in the development of the Cameca IMS 1270, optimized position of an element that is present with just a few for high mass resolution at high transmission (de Chambost (e.g., Hurst et al., 1979; Pellin et al., 1987; Ma et al., 1995; et al., 1992, 1997), and the Cameca Nanosims 50 with Nicolussi et al., 1996, 1998a). a lateral resolution of 50 nm (Hillion et al., 1994, 1997; Secondary ion mass spectrometry (SIMS) has also be- Stadermann et al., 1999a, b). come an increasingly important tool in geo- and cosmo- Finally, since the early 1990s, also time-of-.ight (TOF-) chemistry during the last two decades. After ÿrst develop- SIMS has been introduced to the analysis of extraterrestrial ments in the 1960s, a generation of instruments with high samples (Stephan et al., 1991; Radicati di Brozolo et al., mass resolutions like the Cameca IMS 3f and the SHRIMP 1991). (sensitive high mass resolution ion microprobe) allowed to Common to all diKerent SIMS techniques is that sec- introduce SIMS into this scientiÿc ÿeld mainly during the ondary ions are released from a sample by sputtering 1980s. Since then, this technique has been used for trace with a primary ion beam. In conventional ion microprobes, element analysis and isotope measurements in terrestrial as using mainly double-focusing magnetic mass spectro- well as extraterrestrial geological samples. The applications meters (DF-SIMS), diKerent secondary ions are usually T. Stephan / Planetary and Space Science 49 (2001) 859–906 861 measured sequentially during continuous sputtering. Since this process is destructive, the number of analyzed sec- ondary ion species is limited depending on the available amount of sample material or, because the analyzed ma- terial may change during sputtering, on the heterogeneity of the sample. In TOF-SIMS, the secondary ions are used more e4ciently. All secondary ion species with one po- larity, either positive or negative, are here detected quasi- simultaneously. Destruction of the sample is minimized by pulsing of the primary ion beam. The high transmission and in most cases su4cient mass resolution in combination with a high lateral resolution (∼0:2 m) makes TOF-SIMS highly suitable for the analysis of small samples. The aim of the present work is to summarize 10 years Fig. 1. The principle of secondary ion mass spectrometry: On the sample of analytical experience with TOF-SIMS in cosmochem- surface, an -rich primary ion beam generates secondary ions, which istry. After a more general introduction into the principles are separated and detected with a mass . of secondary ion mass spectrometry, TOF-SIMS will be explained in detail with special emphasis on applications in cosmochemistry. The principles of TOF-SIMS data ac- quisition are illustrated as well as evaluation procedures to achieve useful information on the elemental, isotopic, and molecular composition of the respective samples. The most intriguing results of TOF-SIMS studies in cos- mochemistry are reviewed in subsequent sections of the manuscript.

2. Principles of secondary ion mass spectrometry Fig. 2. Primary ions transfer energy in a collision cascade to the target atoms. Ion implantation (A) or backscattering (B) may occur. A third al- Secondary ion mass spectrometry in general is a tech- ternative, for thin samples only, is forward scattering (C). Atoms from the nique used for surface analysis. Samples are bombarded target material can leave the sample after several collisions as secondary with primary ions at typical of 10–30 keV. (backward sputtering: a and b; transmission sputtering: c). Secondary particles are released in a so-called sputtering process from surface-near layers. Besides , these low energies (several electron volts) and originate from the secondary particles are atoms and molecules, which are in uppermost atomic layers of the target. part (approximately 1%) positively or negatively charged. The collision cascade has a characteristic dimension of The secondary atomic and molecular ions are extracted 10 nm in SIMS. Crystalline targets allow typically much with electric ÿelds and then separated and detected in a higher penetration depths along open directions in a mass analyzer (Fig. 1). Depending on the type of mass ana- so-called channeling process. The collision cascade model is lyzer, one discerns between either conventional SIMS, with not applicable here, because binary collisions are relatively magnetic or quadrupole mass spectrometers, or TOF-SIMS, rare. The charging state of the sputtered particles also cannot with time-of-.ight mass spectrometers. be derived from this simpliÿed model, and multi-element targets are not considered. 2.1. Generation of secondary ions 2.1.2. Emission of secondary ions 2.1.1. The sputtering process The of an element is decisive for the The sputtering process can be described qualitatively, at generation of positively charged secondary ions. Conse- least for amorphous and polycrystalline samples, by cas- quently, ions from alkali and alkaline–earth are cades of atomic collisions (Sigmund, 1969). An impinging formed most e4ciently during the sputtering process. On primary ion experiences a series of collisions in the target the other hand, for negative secondary ions, the electron material (Fig. 2). Recoiling atoms with su4cient energy go a4nity plays the major role, and have the highest through secondary collisions and create further generations ion yields. Besides this, the ionization probability depends of recoiling atoms. Both, primary ions and recoil atoms, have strongly on the charging state of the respective particle a chance to leave the target material as backscattered ions within the sample, i.e., its chemical environment. This char- or secondary atoms. The majority of sputtered particles re- acteristic, known as “ eKect”, complicates the proper sult from clouds of high-order recoil atoms. They have very quantiÿcation of SIMS results (see below). 862 T. Stephan / Planetary and Space Science 49 (2001) 859–906

2.1.3. Post-ionization of sputtered neutrals (Kovalenko et al., 1991, 1992; Clemett et al., 1992, 1993, About 99% of all secondary particles escape the sample 1996, 1998; Thomas et al., 1995). electrically neutral and are not detectable in classical SIMS. However, by subsequent ionization with electron or laser 2.2. Separation of secondary ions bombardment (Oechsner, 1994; Wittmaack, 1994; KQotter and Benninghoven, 1997; Arlinghaus et al., 1997; Arling- 2.2.1. Conventional SIMS haus and Guo, 1998), the sputtered neutrals can be measured In conventional SIMS instruments, secondary ions are in a mass spectrometer. The mass spectrometry of secondary generated continuously by use of a static primary ion beam, neutrals (SNMS) has two major advantages: (1) The ion- and the diKerent ion species are detected sequentially, ization e4ciency is increased compared to the SIMS pro- e.g., in a double-focusing magnetic mass spectrometer or cess. (2) Matrix eKects are reduced because the secondary a quadrupole spectrometer. At a given instant, only sec- particles are ionized after they have left the solid body, ondary ions of a previously selected mass-to-charge ratio and when the chemical environment has lost most of its reach the detector. Thus, most secondary ions are not mea- in.uence. sured. Even in multi-detector instruments, only a few ion In general, multiphoton processes are necessary to exceed species with similar mass-to-charge ratios can be detected the ÿrst ionization potential for atoms (4–17 eV for elements simultaneously. Most secondary ions are therefore lost aside from noble ) and molecules. Individual during the analysis. energies of most laser systems are typically too low to ionize the atom or directly. Two major techniques for 2.2.2. TOF-SIMS post-ionization with laser are established, resonant and Contrary to this, time-of-.ight mass spectrometry al- non-resonant ionization. lows to measure all secondary ions with one polar- In the non-resonant multiphoton ionization (Kampwerth ity quasi-simultaneously. The sample is bombarded in et al., 1990; Terhorst et al., 1992, 1994; Niehuis, 1994; TOF-SIMS with a short-time primary ion pulse with typical Schnieders et al., 1996, 1998; Schnieders, 1999) an atom lengths between several hundred picoseconds and a few or molecule has to transit through a series of intermediate nanoseconds. This is repeated every few hundred microsec- states to reach its ionization energy. These “virtual” states onds, resulting in a duty cycle of typically 10−5. are not quantum-mechanical excitation states and have there- All secondary ions with one polarity are accelerated in −15 fore very short lifetimes, of the order of 10 s. This pro- an electric ÿeld with potential U and mass separated in a cess can be described as simultaneous of several ÿeld-free drift tube. Particles with diKerent mass-to-charge . Crucial for the ionization probability is the total ratios m=q reach the detector after the time-of-.ight t: photon .ux density. Practically all atomic and molecular 2eU species are aKected in this non-resonant laser ionization. m=q = t2: (1) 2 The resonant case uses quantum-mechanical transition s states for ionization that are speciÿc for an atom or a Here, s is the eKective .ight path, corresponding to the length molecule (Hurst et al., 1979; Pellin et al., 1987; Ma et al., of the drift tube. 1995). Therefore, the photon energies of one or a few lasers have to be tuned carefully to match exactly the required 2.3. Quantiÿcation excitation energies. The results are extremely high ioniza- tion yields for speciÿc atoms or molecules. The ionization 2.3.1. Element ratios of other neutral species at the same time is eKectively sup- Quantiÿcation, i.e., to determine useful element ratios, is pressed, if single photons cannot ionize these species and a problem in SIMS due to the above-described matrix eKect. if the photon .ux density does not reach values necessary Since the emission of secondary ions from multi-element for non-resonant multiphoton ionization. Because of that, samples is theoretically not well understood, only an em- isobaric interferences at same nominal masses are avoided piric approach is possible (Meyer, 1979). Standards with (e.g., interferences of 92Mo, 94Mo, and 96Mo with 92Zr, a as similar as possible to the re- 94Zr, and 96Zr) even with a relatively low mass resolution spective sample have to be used. These standards have to of the spectrometer. have well-deÿned element abundances. Since usually only In cosmochemistry, resonant laser post-ionization is up a small volume of the standard is analyzed, this volume to now mainly used together with laser ablation instead of has to be representative for the entire standard. This leads primary ion bombardment. Applications have been the anal- to high demands on purity and homogeneity of the stan- ysis of titanium, zirconium, and molybdenum isotopic com- dard. Amorphous or polycrystalline standards and samples positions of individual presolar or are desired, because crystalline targets lead to unwanted ef- grains (Ma et al., 1995; Nicolussi et al., 1997a, b, 1998a, b). fects like channeling. Only relative abundances can be deter- Microprobe two-step laser desorption=laser ionization mass mined with SIMS, even if all these conditions are satisÿed. spectrometry (L2MS) has been used for the analysis of or- The secondary ion sensitivity for each element is calculated ganic molecules in various kinds of extraterrestrial material from the standard relative to a reference element E0, often T. Stephan / Planetary and Space Science 49 (2001) 859–906 863 silicon. Using these relative sensitivities, the respective 2.5. Charge compensation element ratios E=E0 in the analyzed sample can be calcu- lated from the determined secondary ion intensities SI: Since most samples of geo- or cosmochemical interest are electrical insulators, sample charging during primary ion [SI(E)=SI(E0)]Sample bombardment has to be considered (Werner and Morgan, (E=E0)Sample = (E=E0)Standard: (2) [SI(E)=SI(E0)]Standard 1976; Werner and Warmoltz, 1984; Blaise, 1994). Charging aKects the positioning of the primary ion beam, which is The relative sensitivity de.ected by the electric surface potential, and, on the other hand, it aKects the energy of the secondary ions, because SI(E)=SI(E0) they are not extracted from a well-deÿned electric potential. RS(E; E0)= (3) E=E0 For conventional SIMS analyses, samples are therefore often coated with 20–100 nm of or (Zinner, depends on the respective ion species, the polarity, the 1989). The primary ion beam then sputters a small (up to sample material (matrix), as well as the analytical condi- 20 m) hole into this coating, and the underlying material tions, like primary ion type, energy, and angle of incidence, can be analyzed. Small samples, single grains up to 30 m together with the detection e4ciency for the secondary in size, can be pressed into gold foil (McKeegan et al., ion species. 1985; Zinner, 1989). Although, strictly speaking, the actu- ally investigated sample area is still insolating, charging is no longer critical in both cases. Electrons for charge com- 2.3.2. Isotopic ratios pensation are picked up from the surrounding conductive The matrix eKect plays a much less pronounced role for neighborhood, which also takes up surplus electrons when isotopic ratios. Nevertheless, eKects of mass fractionation the sample is bombarded with negative primary ions. occurring in the SIMS process or during subsequent mass Surface coating has to be avoided in TOF-SIMS, because separation and detection of the secondary ions have to be only the uppermost atomic monolayers are analyzed. Even if taken into account if high accuracy of isotope measurements coating of a single spot can be removed by sputtering, larger is required (Slodzian et al., 1980, 1998). Again, standards areas would not be accessible. However, a charge com- with known isotopic ratios can be used to monitor these pensation system has been developed especially for pulsed eKects. primary ion sources used in TOF-SIMS (HagenhoK et al., 1989). Electrons from a pulsed low-energy electron source 2.4. Static and dynamic SIMS reach the target between two primary ion shots. This process is self-regulative due to the low energy (10–20 eV) of the Secondary ion mass spectrometry in general is a destruc- electrons. The secondary ion extraction ÿeld is switched oK tive technique. However, one often discerns between static during the electron pulse. For the analysis of negative sec- and dynamic SIMS due to the diKerence in sputtering in- ondary ions, a slight negative retarding ÿeld at the extractor tensity. Static SIMS describes the case where the sample prevents the electrons from reaching the detector. surface is not altered during the SIMS analysis. Dynamic SIMS, on the other hand, is much more destructive but facilitates quantiÿcation. Interaction of the primary ion 2.6. Mass interferences beam—often O− for positive and Cs+ for negative secon- dary ions—with the sample generates more reproducible A major problem in SIMS in general and TOF-SIMS in conditions in dynamic SIMS. The high abundance of oxy- particular is the separation of mass interferences. There- gen facilitates the formation of positive secondary ions and fore, this subject is discussed in detail here, especially for the actual (pre-sputtering) oxidation state within the sam- TOF-SIMS. The ability to separate two neighboring mass ples loses its in.uence. Cesium, the element with the lowest peaks in mass spectrometry is described as the mass resolu- ionization potential, supports the generation of negative tion or the mass resolving power (m=Vm). This mass reso- secondary ions. With this strategy, dynamic SIMS becomes lution can be adjusted in DF-SIMS over a wide range at the much less susceptible to matrix eKects than static SIMS. expense of the ion transmission of the instrument. The mass Static SIMS conditions cannot be achieved strictly speak- resolution in TOF-SIMS mainly depends on the accuracy of ing, because primary ion bombardment even at lowest in- the .ight time measurement as will be discussed later. tensity alters the sample surface. However, TOF-SIMS is a The term “required mass resolution for separation of two good approach to static SIMS conditions especially with a peaks” is used in the following for theoretical values result- less-focused primary ion beam. The probability for a pri- ing solely from the mass diKerence of the respective peaks. mary ion to hit an area on the sample already altered by sput- It does not include considerations on peak shape, required tering can be neglected, and sputter times of several hours accuracy, or varying peak intensities, which often lead to are necessary to consume one monolayer (Benninghoven, more strict limitations for a proper peak separation. For 1973, 1975, 1985, 1994). the experimentally determined mass resolutions of an actual 864 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 3. Mass resolutions required for the separation of isobaric interfer- Fig. 4. Mass resolution necessary for separating (m−1A1H) from ences resulting from of other elements (resolving mA from mB). element peaks (mB). mass spectrometer, the width (Vm) of a mass peak has to with an isotope of the same or a neighboring element. The be determined at a certain height of the peak. The full-width required mass resolutions are typically lower here and can half-maximum (FWHM) deÿnition is used throughout this be achieved in TOF-SIMS, at least for low masses, by using work if not speciÿed otherwise. The actual mass resolution very short primary ion pulses. At masses above ∼90 amu for a su4cient separation of two peaks might be required (amu = unit = 1=12 of the mass of 12C) the re- at a lower peak height, depending on the relative intensi- quired mass resolution exceeds m=Vm=104 (Fig. 4), which ties of the respective ion signals and the required accuracy. is approximately the maximum mass resolution reachable This has especially to be considered in time-of-.ight mass with present TOF-SIMS instruments. spectrometry, where peak integrals and not peak heights are determined. There are four major groups of mass interferences with 2.6.3. Oxides and hydroxides typical characteristic features: (1) Isobaric interferences Oxides and hydroxides account for similar problems as 16 from of diKerent elements, (2) hydrides, (3) oxides hydrides. Since O (15:995 amu) almost exactly lies on the and hydroxides, (4) simple hydrocarbons. These groups are nominal mass, any and its oxide have the same de- now discussed in greater detail. viation in mass from the nominal mass. This mass diKerence typically increases for low-mass nuclides with increasing mass number. Between 88 and 140 amu it is practically con- 2.6.1. Isobaric interferences from other nuclides stant, before it decreases again for higher masses. Therefore, Isotopes from diKerent elements at the same nominal mass in the middle mass range, around 110 amu for oxides and are one group of isobaric interferences. Two stable nuclides 130 amu for hydroxides, their separation from atomic ions exist for 52 isobars and even three naturally occurring nu- requires non-achievable high mass resolutions, well above clides can be found for ÿve isobars. Mass resolutions be- m=Vm =104 (Fig. 5). tween m=Vm =104 (48Ca and 48Ti) and 6 × 107 (187Re and 187Os) are required to separate their corresponding mass peaks from each other (Fig. 3). This is not achievable with 2.6.4. Hydrocarbons the available TOF-SIMS instruments, as will be shown later. Hydrocarbons occur almost throughout the entire mass However, for the analysis of element abundances, these in- spectrum. Nevertheless, their interferences with atomic ions terferences have to be considered but are not crucial. They are rather easy to separate, especially when several hydro- can be corrected in most cases by using other isotopes from gen atoms are present. Each additional H-atom shifts the −3 the respective element (see below). peak 7:98 × 10 amu to the right of the nominal mass (to- wards higher masses). Except for the light elements up to and the very heavy elements thorium and uranium, 2.6.2. Hydrides elemental mass peaks lie left (towards lower masses) of Due to adsorbed atoms or hydrocarbons, hy- the respective nominal mass. Upto 56Fe the mass-to- drides are important in surface analysis and are therefore ratio decreases (56Fe is the nuclide with the highest bind- often observed, especially in TOF-SIMS mass spectra. ing energy). Consequently, the required mass resolution for Sometimes they can reach the same intensity as the ele- separation of hydrocarbons has a minimum at masses around ment peak itself. Hydrides from one isotope may interfere 56 amu (Fig. 6). T. Stephan / Planetary and Space Science 49 (2001) 859–906 865

From the linear correlation between t2 and m (Eq. (1)) follows  −1=2 m 1 Vt2 +Vt2 1 = + reg p ; (6) 2 Vm Ran c m where the constant c depends on instrument parameters, length of the .ight path and extraction voltage. Actual values of mass resolutions achievable in TOF-SIMS will be discussed later, when the TOF-SIMS instrument used in this study is described.

3. Time-of-ight secondary ion mass spectrometry

The following section will concentrate on practical as- m−16 16 Fig. 5. Mass resolution needed for the separation of oxides ( A O, pects of TOF-SIMS. First, for a better understanding, the solid symbols) and hydroxides (m−17B16O1H, open symbols) interfering with nuclides at mass m. principles of conventional DF-SIMS and TOF-SIMS instru- ments are compared.

3.1. Secondary ion mass spectrometers

3.1.1. DF-SIMS Double-focusing magnetic mass spectrometers in SIMS are separating secondary ions by electrostatic and magnetic sector ÿelds (Fig. 7). In the electrostatic ÿeld, the secondary ions are dispersed according to their energy. Centrifugal and electrostatic forces are in equilibrium here: mv2 = qE: (7) r The radius r depends in a given electrostatic ÿeld E on the kinetic energy (mv2=2) per charge q. With the aid of the energy slit (cf. Fig. 7), selection of secondary ion energies is possible, and the system can be used as an energy ÿlter. In the following magnetic ÿeld B, the secondary ions are Fig. 6. Mass resolution required for the separation of hydrocarbons dispersed according to their mass-to-charge ratio and their 12C1H n m−12n from element peaks at mass m. energy. It can be regarded as a ÿlter for momentum (mv). Here, centrifugal and magnetic forces are in equilibrium mv2 = qvB: (8) 2.7. Mass resolution in time-of-;ight mass spectrometry r

In general, mass resolution in TOF mass spectrometry depends on time resolution of the .ight time measurement. According to Eq. (1) follows m t = : (4) Vm 2Vt This resolution depends on the resolution of the mass ana- lyzer Ran (Mamyrin et al., 1973), the time resolution Vtreg of detector and registration electronics, and the duration Vtp of the ionization event that in SIMS is the pulse width of the primary ion shot (JurgensQ et al., 1992; Jurgens,Q 1993)

 −1=2 m 1 Vt2 +Vt2 = +2 reg p : (5) Vm R2 t2 Fig. 7. Schematic view of a DF-SIMS instrument. Not shown are an ion-optical elements like de.ectors or electrostatic lenses. 866 T. Stephan / Planetary and Space Science 49 (2001) 859–906

The combination of electrostatic and magnetic ÿelds al- lows compensation of energy dispersions from the electro- static and magnetic analyzers. The result is separation of secondary ions according to their mass-to-charge ratio re- gardless of their energy, the principle of double-focusing mass spectrometry. Nevertheless, the energy slit can be used for narrowing the energy bandwidth of the secondary ions. The width of the secondary ion energy distribution decreases with increasing complexity of the molecular ion and depends on the relative mass diKerence between the constituents of the molecule (Crozaz and Zinner, 1985). Consequently, energy ÿltering is not e4cient for the removal of hydrides and most monox- ides, but complex interferences can be eKec- tively suppressed (Crozaz and Zinner, 1985). This is done by setting the sample potential and the width of the energy slit to values that only secondary ions within a certain energy window above a speciÿc energy reach the detector. This en- Fig. 8. A schematic TOF-SIMS instrument. ergy ÿltering technique has been successfully applied, e.g., to the measurement of rare earth elements (Crozaz and Zin- their velocity before they reach the detector. Since slightly ner, 1985; Zinner and Crozaz, 1986). diKerent starting energies result in slightly diKerent .ight A special feature of the experimental set-up shown in times, even for ions of the same species, a re.ector is used Fig. 7 is that it can transport ion images, showing the lateral for energy focusing (Mamyrin et al., 1973). Ions with higher distribution of the respective ion species. Secondary ion energies penetrate its electric ÿeld deeper and are therefore images can be generated by using a defocused primary ion delayed compared to lower-energy ions. By adjusting the beam. Direct imaging through ion optical projection is often electric ÿelds carefully, all secondary ions of one species called ion microscope technique in contrast to SIMS instru- can reach the detector practically simultaneously. ments where secondary ion images are generated by scan- ning the primary ion beam over the sample (ion microprobe 3.2. Description of the TOF-SIMS instrument technique). Energy ÿltering in DF-SIMS also reduces chromatic aber- The Cameca=ION-TOF TOF-SIMS IV instrument used rations of secondary ion images. Analogous to light , in this study is a re.ector-based instrument as described the focal length of an ion-optical lens varies with the energy in the previous section and shown in Fig. 8. The present of the ions (equivalent to wavelength or color of light). TOF-SIMS IV instrument at the Institut fur Planetologie in Munster is equipped with two primary ion sources for anal- ysis, an electron impact ion gun mainly for primary 3.1.2. TOF-SIMS ions (Niehuis et al., 1987) and a gallium ion All types of TOF-SIMS instruments are based on the same gun (Ga-LMIS; Niehuis, 1997). In addition, the instrument principle: The velocity of an ion with a given kinetic en- has a sputter source, a combination of a cesium thermal ion- ergy depends on its mass. One discerns between two major ization source and an electron impact source for argon or groups of TOF-SIMS instruments. Besides analyzers with sputtering ions (Terhorst et al., 1997). It can be used three hemispherical electrostatic analyzers (Schueler et al., for cleaning of the sample surface as well as for depth pro- 1990; Schueler, 1992), most TOF-SIMS instruments use a ÿling. All ion beams reach the sample with an angle of 45◦ more simple design with an ion re.ector (Mamyrin et al., and the secondary ions are extracted perpendicular to the 1973; Niehuis et al., 1987; Niehuis, 1990). Only the latter sample surface with an extraction potential of 2 kV (Fig. 8). type is explained in detail here, because most analyses dis- The sample is at ground potential. cussed in this study were performed with this type of in- After the re.ection, the secondary ions are post-accelerated strument. However, the major diKerence between the two to energies of 8–10 keV before they reach the detector. The set-ups is that the triple sector spectrometer allows direct detector consists of a microchannel plate for ion-to-electron imaging through ion optical projection (ion microscope). conversion, a for electron-to-photon conversion, Spatial information gets lost in the re.ector instrument and and a photomultipier, which is placed outside the vacuum the ion microprobe mode is used for imaging. chamber. All three steps lead to an ampliÿcation of the sig- A schematic view of a re.ector TOF-SIMS instrument nal, but the major advantage of this set-up is the separa- is given in Fig. 8. Secondary ions are extracted from the tion of high voltage at the detector and low voltage of the sample surface by acceleration in an electric ÿeld. In the electronics. A constant fraction discriminator converts the subsequent drift tube, the ions are separated according to incoming signal to a normalized signal, which is further T. Stephan / Planetary and Space Science 49 (2001) 859–906 867

Fig. 9. The pulsing and bunching system of the electron impact primary ion gun. Focusing elements like lenses and de.ection units for beam alignments were omitted. Primary ions, mainly Ar+, are generated in a conventional electron impact . A 90◦-de.ection unit is used to cut ion packages with pulse lengths of 10–15 ns out of a continuous ion beam. These pulses are compressed to pulse lengths of ∼1:2nsina bunching system. Fig. 10. The gallium liquid metal ion source. Focusing elements like lenses and de.ection units for beam alignments were omitted. Primary ion packages with pulse lengths of ∼5 ns are produced from a continuous processed in a time-to-digital converter (TDC). The time beam with a blanking system. These packages can be chopped a second resolution of the TDC is adjustable between 50 and 1600 ps, time or compressed along their .ight direction in a bunching system to further reduce their pulse lengths. but it is mostly operated at 200 ps. A second detector system is used for detection of sec- ondary electrons. With this, a secondary electron image sim- (∼10 m), due to the relatively large volume where ion ilar to those generated in a scanning electron microscope formation takes place. Projection of this ionization volume can be obtained by scanning the primary ion beam over the onto the sample surface leads to inevitable dispersion of the sample. A pulsed low-energy (∼18 eV) electron source is ion beam. Acceleration of the ions to diKerent energies in used for charge compensation. the bunching unit also reduces the lateral resolution. These energy variations increase chromatic aberrations of the target 3.2.1. The argon primary ion source lens. A schematic view of the pulsing system of the electron impact primary ion gun is given in Fig. 9. This primary ion 3.2.2. The gallium primary ion source source is mainly used with 40Ar+ as primary ion species. Ion The gallium primary ion source is shown in Fig. 10. A pulses with typically lengths of 10–15 ns are generated from tungsten needle with a thin liquid gallium ÿlm is exposed a continuous ion beam with a 90◦ de.ection unit. Each ion to a high electric ÿeld. At typical extraction potentials of package is compressed along its .ight direction to typically 5–10 keV the liquid gallium forms a so-called Taylor-cone 1:2 ns pulse length in an electrodynamic bunching system. at the tungsten tip and Ga+ ions are emitted. These ions stem Ions from the rear of the package are accelerated to catch from a very small region with a typical size of 10 nm. This up with those from the front when they reach the target. small spot size is responsible for the achievable high lateral (Niehuis et al., 1987). resolution of the Ga-LMIS (beam diameter ∼200 nm). The argon electron impact ion source has its main ad- The Ga-LMIS can be operated in diKerent modes, which vantage in producing short primary ion pulses with rela- mainly diKer in length of the primary ion pulse, primary tively high intensities (cf. Table 1). The major drawback ion intensity, and achievable spatial resolution (cf. Table 1). of this type of ion source is the limited lateral resolution Typical pulsing systems produce single ion packages with

Table 1 Comparison of diKerent analytical modes for argon and gallium ion sourcesa

Primary Pulsing mode Pulse Spatial Primary Secondary ion species width resolution ions=cycle ions=cycle

40Ar+ Blanked+bunched 1:2ns 10m 500 2.5 69Ga+ Blanked 5 ns 200 nm 50 0.25 69Ga+ Blanked+chopped 1:5 ns 200 nm 10 0.05 69Ga+ Blanked+bunched 600 ps 5 m 50 0.25 69Ga+ Burst mode n × 1:5 ns 200 nm n × 10 n × 0:05

aA value of one secondary ion per 200 primary ions is assumed as a rough estimate that is based on analyses of standards. For these conditions, one sputtered particle per primary ion can be assumed. The actual values strongly depend on sample material and surface conditions. The typical repetition rates of 5–10 kHz result in several thousand secondary ion counts per second spread over the entire mass spectrum. 868 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 11. Timing schemes for blanker and chopper. The blanker is used to select one or a series (burst mode, dotted line) of 1:5 ns ion pulses continuously produced by the high- (40 MHz) chopper. Blanker Fig. 12. Mass resolution (FWHM) for typical measurements with gallium and chopper are both pulsing units which are applied in diKerent ways. and argon primary ion sources. Data points show selected peaks with no Sometimes, these names are used in the literature the other way round. apparent mass interference. The gallium ion source was operated in three diKerent modes. The mass resolution of the analyzer is m=Vm = 10500 in all cases. lengths of ∼5 ns by blanking a continuous ion beam. There are two possible techniques to further reduce the pulse length. Each ion package can be chopped a second time argon primary ion source as well as for diKerent analyti- (Fig. 11) or compressed along its .ight direction in a cal modes of the gallium source. From best ÿts according bunching system analogous to the argon source (Niehuis et to Eq. (6) for the data points of undisturbed mass peaks, al., 1987; Schwieters et al., 1991, 1992). a resolution Ran for the analyzer of about 10,500 has been In the ÿrst case, the primary and therefore the secondary inferred. This resolution describes the asymptotic limit at ion intensity decrease with the pulse length. Extremely short high masses, whereas the accuracy of the time measurement, pulses result in extremely small signals, often not su4cient mainly from Vtp, shapes the ÿt curve at low masses. The for trace element studies. On the other hand, the spatial res- time resolution of the registration system Vtreg was 200 ps. olution (beam diameter) remains unchanged in this case. Primary pulse lengths Vtp of 1:3 ns for the argon source Typical pulse lengths achieved with high-frequency chop- and 5:2; 1:6, and 0:6 ns for blanked, blanked+chopped and pers are 1:5 ns. blanked + bunched modes of the gallium source, respec- The electrodynamic bunching system allows an axial tively, have been determined. compression of the ∼5 ns ion package to ∼600 ps pulse A simultaneous increase of all three parameters, mass res- length. Since no primary ion gets lost, secondary ion intensi- olution, spatial resolution, and intensity of the secondary ion ties are usually by a factor of ÿve higher than in the chopped signals, is restricted, because the number of charged particles mode. This is mostly su4cient for trace element studies. within a small volume is also limited due to space-charge On the other hand, chromatic aberrations of the target lens eKects. Extension of the ion package in .ight direction re- here limit the achievable spatial resolution to ∼5 m. sults in low mass resolution. Extension perpendicular to the .ight direction results in low spatial resolution. The diKer- 3.2.3. Mass resolution — spatial resolution — signal ent pulsing modes of the gallium source in Table 1 show intensity the mutual in.uence of these parameters. The three parameters mass resolution, spatial resolution, Nevertheless, if high lateral resolution and high mass res- and intensity of the secondary ion signals are strongly re- olution are required, the secondary ion signal can be in- lated to each other in TOF-SIMS. The mass resolution can creased by applying a series of 1:5 ns primary ion pulses, be adjusted in DF-SIMS over a wide range by changing the spaced 25 ns apart from each other, in every shot cycle. widths of the diKerent slits (Fig. 7) at the expense of the This so-called “burst” mode (Fig. 11) results in a series of ion transmission of the instrument. This is not the case for overlapping mass spectra, which can be summed up by ap- TOF-SIMS. Here, the mass resolution depends on the dura- propriate computer programs. For primary ion bursts of ÿve tion of the primary ion pulse, the length of the .ight path, single pulses, approximately the same intensity is reached the energy focusing in the re.ector, the time resolution of as in the “blanked” mode (cf. Table 1). This leads to use- the detection system, and, ÿnally, on the mass itself. The de- ful results for masses up to ∼200 amu, where the last peak pendence on time resolution of the .ight time measurement from one nominal mass begins to overlap with the ÿrst peak has been discussed above. of the next nominal mass. By varying the number of pri- The mass dependence of the mass resolution at full-width mary ion shots within one burst, the usable mass range can half-maximum (FWHM) is demonstrated in Fig. 12 for the be adjusted. An increasing number of ion shots within one T. Stephan / Planetary and Space Science 49 (2001) 859–906 869

Fig. 14. Timing scheme for simultaneous use of two ion sources for insulating samples. A gallium primary ion source pulsed with 10 kHz performs the actual TOF-SIMS analysis. Sputtering with a pulsed argon Fig. 13. The dual beam sputter source. The special ion source combines a source occurs between two gallium shots. Secondary ions are only ex- cesium thermal ionization source with an electron impact source for argon tracted directly after the gallium pulse. During the sputter pulse, a slight sputtering ions. As in previous ÿgures, focusing elements like lenses and retarding ÿeld at the extractor prevents secondary ions from reaching the de.ection units for beam alignments are not shown. detector. burst leads to a decreasing upper limit for the usable mass the residual . The primary ion source is not able to range (e.g., ∼50 amu for bursts of 10 shots). Therefore, the prevent this su4ciently with its low duty cy- burst mode is not applicable to the investigation of large cle, especially when a large area is scanned with the ion molecules. beam. Therefore, an analytical mode has been developed, where the sample is analyzed with a gallium primary ion 3.2.4. Sputtering source and sputtered simultaneously with an argon sputter- Since TOF-SIMS analyzes the uppermost surface of a ing source (Fig. 14). The sputter pulse is applied between sample, can be a severe problem. Therefore, two gallium pulses during charge compensation. The inten- the surface often has to be cleaned. To avoid adsorption sity of the sputter pulse is limited to ∼100 pA at 10 kHz 4 of molecules present in the ambient air after the cleaning repetition rate (∼6 × 10 ions=shot) due to charging of procedures, this is preferentially performed directly under the sample. Although the electron current for charge com- ultra-high vacuum conditions in the TOF-SIMS instrument pensation easily exceeds 1 A, the e4ciency of charge prior to the analysis. It can be done by sputtering with the compensation mainly depends on the number of electrons primary ion gun. The TOF-SIMS IV instrument is equipped reaching the area of actual ion bombardment, ∼50 min with an additional ion source exclusively for sputtering. Be- diameter for the sputter source. However, the sputter current sides for surface cleaning, the sputter gun can also be used of 100 pA is su4cient to suppress adsorption eKectively for depth proÿling (Williams, 1998). The TOF-SIMS IV during the analysis. The eKects of sputtering on various ion sputter source (Fig. 13) is a combination of a cesium ther- species are demonstrated in Fig. 15. Since the abundance of mal ionization source and an electron impact source for ar- ions directly depends on the presence of hydrogen gon or oxygen sputtering ions (Terhorst et al., 1997). The on the surface, hydride secondary ion intensities behave beam diameter is ∼50 m. analogously to the hydrogen signal. Secondary ion yields can be in.uenced and drastically increased by selecting speciÿc ion species for sputtering. 3.3. TOF-SIMS mass spectra Bombardment with oxygen ions, e.g., enhances the positive secondary ion signal (Wittmaack, 1998); cesium ions in- TOF-SIMS mass spectra contain a wealth of information crease negative ion abundances. Although such an increase due to parallel detection of an in principle unlimited mass is desirable, the occurrence of molecular ions like oxides, range. Before the rules of data evaluation are explained, ex- stimulated by oxygen sputter ions, may complicate the inter- amples for positive and negative secondary ion mass spectra pretation of the mass spectrum. Therefore, sputtering with (mass range 1–100 amu) obtained from a glass standard af- inert ion species like argon is more advantageous and has ter extensive argon sputtering are given in Figs. 16 and 17. been performed in this study. Negative ion spectra are disturbed by electrons, which After removing adsorbed hydrocarbons through sput- partially result from charge compensation. They are also tering, even under ultra-high vacuum conditions (∼2 × generated as secondary or tertiary electrons from interaction 10−10 mbar), the sample surface can regain atoms and of primary as well as secondary ions with various molecules like hydrogen or hydrocarbons present in within the instrument. Some of these electrons appear as 870 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 15. EKects of argon sputtering on the secondary ion intensities. A 50 × 50 m2 area of glass StHs6=80-G (cf. appendix) was bombarded with 5 ns gallium shots. After 580 scans (256 × 256 pixels, one shot per Fig. 16. Highly compressed mass spectrum of positive secondary ions pixel) the sample was simultaneously sputtered with an argon beam. This obtained from glass standard KL2-G (cf. appendix) after extensive argon leads to an increase of most secondary ion signals by factors of 50–200. sputtering. Only the mass range from 1 to 100 amu is shown. Surface adsorbed hydrocarbons are e4ciently suppressed: The H+ and C+ signals decrease by factors of 2 and 10, respectively. Stable signals are reached after 700–800 scans (∼80 min) with simultaneous sputtering. 3.4. Quantiÿcation After switching oK the sputter source (after scan 2210) hydrocarbon signals increase again to values similar to those before sputtering. Signals Quantitative evaluation of the mass spectra is one of for other elements decrease slightly, but still remain clearly above the the major objectives in TOF-SIMS analysis. From each values before sputtering. signal in the spectrum, the abundance of the secondary ion species and ultimately the concentration of the respec- distinct lines or broad peaks in the respective spectrum, but tive element, isotope, or molecule in the target have to a general baseline increase is also observed throughout the be deduced. However, only elemental or isotopic ratios entire mass range. can be determined in SIMS. Absolute concentrations can- The spectra in Figs. 16 and 17 are highly compressed, not be obtained due to insu4cient knowledge of sputter and peaks at identical nominal masses cannot be sepa- volumes, absolute ion yields, transmission of the instru- rated in these diagrams. The actual ÿne structures at 29 ment, and detection e4ciencies. Prerequisites to determine and 43 amu, respectively, are shown in Fig. 18 for low- these ratios are, besides knowledge of the respective SIMS and high-resolution mass spectra of positive secondary sensitivities, linearity of the detection and registration sys- ions. 29Si+ and 28Si1H+ can only be distinguished in the tem, as well as control of instrumental mass fractionation high-resolution spectrum at 29 amu. At low mass resolu- eKects. tion, only hydrocarbon peaks can be separated. This is also obvious at nominal mass 43 amu. The unambiguous iden- 3.4.1. Linearity of the detector tiÿcation of every single peak is even complicated for the Linearity of the detection system over a wide dynamic high-resolution spectrum. Especially for some peaks (peaks range is indispensable for a quantitative evaluation of mass 3, 4, and 7–11 in Fig. 18) that lie between the element and spectra in TOF-SIMS, where the entire spectrum is measured the pure hydrocarbon peak, assignments made in the ÿgure simultaneously. Isotopic and element ratios up to several or- caption are equivocal. ders of magnitude occur, and secondary ion sensitivities span T. Stephan / Planetary and Space Science 49 (2001) 859–906 871

Fig. 18. Comparison of diKerent mass resolution settings near masses 29 and 43 amu (top: 5 ns pulse length; bottom: 0:6 ns pulse length), respec- tively. At 29 amu the ÿve peaks in the high-resolution spectrum can be 29 + 28 1 + 12 1 16 + 12 1 14 + identiÿed as (1) Si , (2) Si H , (3) C H O , (4) C H3 N , 12 1 + 43 + and (5) C2 H5 , respectively. At 43 amu (6) Ca with a contribu- 42 1 + 27 16 + 26 16 1 + 28 12 1 + tion from Ca H , (7) Al O , (8) Mg O H , (9) Si C H3 12 1 16 + 12 1 14 + 12 1 + (10) C2 H3 O , (11) C2 H5 N , and (12) C3 H7 emerge in the high-resolution spectrum.

peak integral I becomes cor  I I = −N ln 1 − exp ; (9) cor N

where Iexp is the measured integrated peak intensity and N the number of primary ion shots. The statistical one-sigma Fig. 17. Mass spectrum (1–100 amu) of negative secondary ions from error of the corrected signal is   glass standard KL2-G after argon sputtering. Broad peaks are caused by NI 1=2 electrons from various origins. They are also responsible for the higher exp VIcor = : (10) overall background in negative mass spectra compared to positive spectra N − Iexp (cf. Fig. 16). The correction of multiple peaks at one nominal mass be- comes more complicated, but even entire mass spectra can be corrected for dead time eKects by applying similar formu- a vast range. Therefore, high count rates for major peaks are lae. This has been demonstrated in greater detail by Stephan often compulsory to achieve su4cient ion intensities for mi- et al. (Stephan, 1992; Stephan et al., 1994a). Limitations of nor isotopes or trace elements. For single-ion counting, the these corrections result from two deÿciencies. First, the ac- typical detection technique in TOF-SIMS, dead time eKects tual dead time cannot be described by a single numerical for high-intensity mass peaks have to be considered. Since value. A probability has to be used instead, which signal intensities in TOF-SIMS .uctuate on an extremely is only roughly known. Second, constant peak intensities short time scale (nanoseconds), these eKects become more during the measurement are indispensable for an exact dead complex than for constant or exponentially decaying signal time correction. However, simpliÿed correction techniques intensities (SchiK, 1936; Esposito et al., 1991). However, deliver results with su4cient precision (∼10%) for element dead time eKects have to be considered also in conventional analysis even for correction factors up to three. Restrictions SIMS. Especially in isotope studies, they contribute signif- are more severe for isotope studies, where higher accuracy icantly to measurement uncertainties (Storms and Peterson, (few per mill) is required. 1994). The actual dead time of the TOF-SIMS IV instrument is 3.4.2. Mass fractionation ∼40 ns, which mainly results from the exponentially decay- Instrumental mass fractionation eKects play a less pro- ing scintillator signal. Therefore, only one secondary ion per nounced role in TOF-SIMS. Here, general mass fractiona- nominal mass can be detected for each primary ion shot. tion eKects have not been observed so far. If there are any However, dead time correction of TOF-SIMS spectra may eKects, they only have to be considered in isotope analy- be performed using Poisson statistics. If only one mass peak sis, where high accuracy is required. But even there, statis- is present at a given nominal mass, the dead time corrected tical errors dominate at least for less abundant isotopes the 872 T. Stephan / Planetary and Space Science 49 (2001) 859–906 analytical uncertainties. For high-intensity ion signals, dead ity dominates. For positive secondary ions, a mass interfer- 2 + 1 + time eKects often exceed mass fractionation eKects. How- ence is observed for ( H ) from H2 molecular 1 + ever, instrumental mass fractionation has always to be con- ions. In addition, H3 -ions are often present in secondary sidered, before deviations from “normal” isotopic ratios can ion spectra. The analysis of hydrogen with TOF-SIMS is af- be claimed. fected by the residual gas in the vacuum apparatus, because it is typically dominated by hydrogen. Even at a pressure 3.5. Element analysis of several 10−10 mbar in the analysis chamber, hydrogen is still adsorbed onto the sample surface during primary ion Peak assignments for quantitative analyses follow the bombardment and can be observed in every mass spectrum. same principles as described by Eugster et al. (1985). For Therefore, the analysis of indigenous hydrogen isotopic ra- each isotope of a given element, at most the whole inten- tios was not possible so far, although H2 forms practically sity at the respective mass window can be assigned. This no negative secondary ions and consequently 2H− is not in- mass window is deÿned by the expected center of mass and .uenced by any mass interference. the prevailing mass resolution. A value for the expected to- tal elemental ion intensity can be calculated from each as- 3.5.2. Lithium, , and boron sumed isotope peak. Since mass interferences may aKect one Lithium, beryllium, and boron all form positive secondary or more of these peak intensities, the minimum value de- ions. Molecular interferences are almost negligible during duced from the diKerent isotopes is the upper limit for the their analysis. Like all alkali metals, lithium is not observed intensity of the respective element. A proper analysis of the as hydride in the positive spectrum. It would be unequiv- respective element can only be inferred, if the calculated to- ocally identiÿable as 7Li1H+ at nominal mass 8 amu. Any tal elemental ion intensities for major isotopes agree within contribution of 6Li1H+ at mass 7 amu to the 7Li+ mass peak statistical error limits. If this is not the case, at least one iso- would be easy to subtract, particularly since 6Li has an iso- tope is aKected by interference and the others might also be topic abundance of only 7.5% in “normal” lithium. in.uenced by similar interferences. Consequently, one has Beryllium has only one stable isotope at nominal mass to interpret the calculated elemental ion abundance as an 9 amu. Here no mass interference occurs except 27Al3+, upper limit. Strictly speaking, all calculated ion abundances which can easily be separated (required mass resolution are upper limits, if interferences cannot be unequivocally m=Vm = 490). For boron, the convenient isotopic ratio of excluded. about four for 11B=10B allows a deÿnite determination of the Figs. 19 and 20 comprise peak assignments and the prin- B+-intensity. BeH+ in most cases plays a minor role and, if ciples of quantitative evaluation for positive and negative at all, only for the less-abundant isotope 10B+. secondary ions, respectively. Moderate mass resolution, su4cient for a complete separation of most interfering molecules like hydrocarbons, was assumed (e.g., 5 ns pri- 3.5.3. Carbon mary ion pulse length with the above-described gallium Carbon forms positive as well as negative secondary source). In the following, the evaluation is discussed for ions during primary ion bombardment. Like observed for diKerent elements. Prerequisite for the here-described prin- hydrogen, negative ions form the majority. Especially the ciples are “normal” (=cosmic) isotopic ratios of most polarity of the carbon secondary ions depends on their elements. —Throughout this study, isotopic abundances are chemical environment. Hydrocarbons generate C+ as well used according to Anders and Grevesse (1989). —Cosmic as C− secondary ions, whereas carbonates and in particular − isotope ratios can be assumed within statistical error limits carbides mainly form C -ions. CH, CH2, and CH3 are also in most cases. Nevertheless, one has to be aware of the observed in both polarities. Consequently, separation of the possibility of non-solar isotopic ratios during the analysis 12C1H-signal from the 13C peak is essential for the mea- of extraterrestrial material. Especially for the analysis of surement of 12C=13C isotopic ratios in both polarities. A presolar grains, this has to be taken into account and will be minimum in mass resolution of m=Vm = 2910 is necessary discussed in more detail below. Some of the interferences for this separation. Since the intensity of the 12C1H-ions like hydrides, considered as non-separable in the present is often comparable with the 12C-signal, this mass resolu- section, can be resolved by using very short primary ion tion is required at ∼1% of the peak height in case of solar pulses or mathematical peak deconvolution techniques. The carbon (12C=13C=89:9). latter will be explained in an extra section. Isotopic compo- For a proper measurement of isotopic ratios, preferentially sitions for some elements, especially the light ones, can be the amount of the hydride CH has to be minimized. This determined even at low mass resolution. This is discussed can be done by sputtering of the sample, since formation of now, together with the principles of element analysis. hydrides is often due to the residual gas in the vacuum system (see above). To separate the two peaks at 13 amu, of- 3.5.1. Hydrogen ten mathematical peak separation techniques have to be ap- Although hydrogen forms positive as well as negative plied (cf. section on presolar grains). A comparison between secondary ions during the SIMS process, the negative polar- TOF-SIMS and DF-SIMS spectra for masses 12 and 13 amu, T. Stephan / Planetary and Space Science 49 (2001) 859–906 873

Fig. 19. Flow chart showing the principles of peak assignments and determination of positive secondary ion abundances for elements from hydrogen to nickel. respectively, is shown in Fig. 21. Although the TOF-SIMS former can be identiÿed as 12C14N−, whereas the latter is spectrum (Fig. 21 top) was obtained under nearly optimal formed by 13C14N− and 12C15N−. These two ion species conditions—.at Te.on sample, only little surface adsorp- cannot be separated in TOF-SIMS (required mass resolution tion of hydrocarbons (low CH-signal), and short primary ion m=Vm = 4270). The 14N=15N-ratio can be calculated now pulses (∼1:2 ns)—the two peaks at 13 amu were not sep- with the known 12C=13C-ratio according to arated su4ciently. In contrary to this, these two peaks are 12C14N− I separated completely with DF-SIMS (Fig. 21 bottom). 14N=15N= = 26 (11) 12 15 − 13 12 C N I27I26 − C= C 3.5.4. Nitrogen or Nitrogen has only a small ionization yield in the SIMS process. Nevertheless, nitrogen can be measured as CN− if 15 14 I27 13 12 carbon is present. For the measurement of 14N=15N isotopic N= N= − C= C; (12) I26 ratios, ÿrst, the 12C=13C isotopic ratio has to be calculated. Then, the mass peaks at 26 and 27 amu are determined. The where Ix is the peak integral at x amu. 874 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 20. Flow chart showing the principles of peak assignments and determination of negative secondary ion abundances for elements from hydrogen to .

17O (CI: 0.038%) and the high 16O1H− intensity, only 16O and 18O can be measured satisfactorily in TOF-SIMS.

3.5.6. , chlorine, bromine, and iodine All halogens form preferentially negative secondary ions. Chlorine, the element with the highest electron a4nity, has the maximum negative secondary ion yield. Nevertheless, especially samples with high .uorine concentrations often show signals at 19 amu also in the positive secondary ion spectrum. Fluorine and iodine are both monoisotopic ele- ments, whereas chlorine and bromine have two stable iso- topes each. Convenient isotopic ratios of about three for 35Cl=37Cl and about one for 79Br=81Br, respectively, allow measuring all four nuclides. Since 79Br is often disturbed by molecular interferences, 81Br is the isotope of choice for a Fig. 21. Comparison of TOF-SIMS (top) with DF-SIMS (bottom, courtesy of F.J. Stadermann) spectra at masses 12 (12C) and 13 amu (13C and proper bromine analysis (Stephan et al., 1994b). 12C1H). With TOF-SIMS, positive secondary ions were retrieved from a Especially chlorine and .uorine are omnipresent in our Te.on sample by Ar+ primary ions. The DF-SIMS analysis was performed environment. Therefore, many samples show con- on a terrestrial silicon carbide grain. Mass resolutions (m=Vm) are given tamination on the surface, often together with the alkalis at 12 amu for full width at 50, 10, and 1% peak height. and potassium. Consequently, cleanness of the sample surface is especially important for a proper 3.5.5. Oxygen halogen analysis. Oxygen is the classical element providing negative secondary ions in most SIMS applications. In oxidized 3.5.7. Sodium, potassium, rubidium, and cesium compounds, it takes up the electrons released during the The same high demands on surface purity apply to the formation of positive metallic ions. Nevertheless, because alkalis, especially sodium and potassium, which are often of its high abundance in rocks, oxygen is also observed observed as surface contamination. The alkalis have the as positive secondary ions, though with lower intensity. highest positive secondary ion yield, analogous to the high Therefore, oxygen is qualiÿed as reference element besides yield for negative ions for halogens. Hydrides and doubly silicon for the comparison of negative and positive sec- charged secondary ions are not observed. The monoisotopic ondary ion spectra. Because of the very low abundance of element sodium can unequivocally be analyzed at 23 amu, T. Stephan / Planetary and Space Science 49 (2001) 859–906 875 where no molecular interference appears. Potassium as 39K+ and 137Ba+ (11:23%) but can also be seen as 136Ba+ + 135 + 134 + 130 + can easily be separated from C3H3 at a low mass resolu- (7:854%), Ba (6:592%), and Ba (2:417%). Ba tion (m=Vm = 652). 40K (0.01167% of K) is not distin- (0:106%) and 132Ba+ (0:101%) are too rare in most spectra guishable from 40Ca (96.941% of Ca) due to a required to be detected. mass resolution of m=Vm = 28400 and its low abundance. 41 Although the abundance of K is higher (6.7302%), mass 3.5.9. Aluminum 40 1 41 amu is typically dominated by C H (required mass The monoisotopic element aluminum can be measured resolution m=Vm = 4770). Rubidium has two primordial at nominal mass 27 amu considering corrections from 85 87 isotopes Rb (72.165%) and Rb (27.835%). The latter 26Mg1H+ according to Eq. (13). AlO+ at 43 amu; Al2+ at 87 is interfering with Sr (7.00%, required mass resolution 13:5 amu and even Al3+ at 9 amu are also observed. The m=Vm = 296000). Cesium is monoisotopic and can be seen hydride might be present at 28 amu but is often not resolved at 133 amu. from 28Si+ (m=Vm = 2250). 3.5.8. Magnesium, calcium, strontium, and barium The alkaline-earth metals are the group with the second 3.5.10. Silicon highest yield for positive secondary ions. They often form Silicon has three stable isotopes, 28Si (92.23%), 29Si hydride-ions like, e.g., MgH+, complicating isotope analy- (4.67%), and 30Si (3.10%). Although it forms predomi- ses. 24Mg1H+ overlaps with 25Mg+; 25Mg1H+ with 26Mg+, nately positive secondary ions, also negative ions occur. and ÿnally 26Mg1H+ coincides with 27Al+ (required mass For silicates it is suited as reference element besides oxygen resolutions m=Vm = 3550; 2350, and 3040). The latter in- for the comparison of negative and positive secondary ion terference complicates the analysis of aluminum. However, spectra (see above). Furthermore, because of its ubiquity assuming a 25Mg=24Mg-ratio of 0.127, which mostly is jus- in rocks, it is used as normalization element for element tiÿed for the achievable accuracy, one can calculate a possi- abundances in meteorites or the (Anders and ble 26Mg-enrichment and also the contribution of 26Mg1H+ Grevesse, 1989). Similar to magnesium, hydrides occur and to mass 27 amu: interfere with 29Si; 30Si, and 31P. 26Mg MgH+ 27Al+ = I − I (13) 27 24 24Mg Mg+ 3.5.11. Phosphorous The monoisotopic element phosphorous forms normally with 31P−-ions, but also positive secondary ions are observed in I MgH+=Mg+ = 25 − 0:127 (14) samples with high phosphorous concentrations like phos- I24 phates. Similar to 27Al+ and its interference with 26Mg1H+, and 31P+ is in.uenced by 30Si1H+, which can be corrected anal- + ogous to Eq. (13). 26 24 I26 MgH Mg= Mg = − 0:127 + : (15) I24 Mg Similar to this, 40Ca1H+ interferes with 41K like dis- 3.5.12. Sulfur cussed above. Besides hydrides, also oxides are observed. Sulfur, which forms negative secondary ions, has two 32 34 To separate 24Mg16O+ from 40Ca and 40Ca16O+ from isotopes, S (95.02%) and S (4.21%), that are measurable 32 56Fe, mass resolutions of m=Vm = 2300 and 2480, respec- in TOF-SIMS. Especially for S, the O2 molecule plays an tively, are required. Hydroxides like 40Ca16O1H+ at 57 amu annoying role in the mass spectrum, but it can be separated also emerge in the mass spectra. 44Ca, the other isotope su4ciently at moderate mass resolution (m=Vm = 1800). available for quantitative calcium analysis interferes with 28Si16O(m=Vm = 2690). Therefore, 40Ca is more reli- 3.5.13. Scandium, titanium, vanadium, and chromium able for a quantitative analysis especially of silicates. 44Ca The monoisotopic element scandium has typically very can be useful only in Mg-rich, Si-poor samples like some low abundances in most rocks (Sc=Si = 3:42 × 10−5, - carbonates. dritic atomic abundance according to Anders and Grevesse, Doubly charged ions like 24Mg2+ or 40Ca2+ are also 1989). In addition, 45Sc+ interferes with 28Si16O1H+ and observed for alkaline-earth metals. Since the mass scale can therefore only be seen in TOF-SIMS spectra with high in TOF-SIMS spectra is actually a mass-to-charge scale, mass resolution. they appear at 12 and 20 amu, respectively. Consequently, In Ti-rich samples, all ÿve stable isotopes (46Ti: 8:0%, 24Mg2+ interferes with 12C+ (m=Vm = 1600), but this con- 47Ti: 7:3%, 48Ti: 73:8%, 49Ti: 5:5%; and 50Ti: 5:4%) can tribution can be calculated from the 25Mg2+ intensity at be found as positive secondary ions. Since besides 48Ca, 12:5 amu. no major interference is observed, and the share from 48Ca Strontium is measured mainly as 88Sr+ (82:58%) and can easily be calculated from the total calcium, 48Ti is used 86Sr+ (9:86%); 87Sr+ (7:00%) interferes with 87Rb+ (see to determine the element abundance. On the other hand, above). Barium is detected primarily as 138Ba+ (71:70%) 48Ti16O+ interferes with 64Zn+ as described below. 876 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Because of the low abundance of 50V (0.250%) and its 3.5.18. Lanthanides interference with 50Ti and 50Cr, vanadium can only be mea- Rare earth elements could be seen in the positive sec- sured as 51V. Chromium, ÿnally, is measured as 52Cr, but ondary ion spectrum. Nevertheless, an unambiguous deter- it also contributes signiÿcantly to the peaks at 50, 53, and mination of these elements is not possible so far, due to a 54 amu. However, 54Cr+ can be neglected in most cases due wealth of isobaric interferences and the presence of hydrides to much higher contributions from 54Fe+. and oxides—oxides of the low-mass lanthanides overlap with those of the high-mass ones.

3.5.14. Manganese, iron, cobalt, and nickel 3.5.19. Lead Iron usually dominates the positive mass spectrum at As a surface analytical technique, TOF-SIMS often “sees” 54 and 56 amu due to its high cosmic abundance. Never- lead present in the positive secondary ion spectrum result- theless, in Ca-rich, Fe-poor samples, 40Ca16O+ contributes ing from surface contamination, because this highly volatile signiÿcantly to the mass peak at 56 amu. Another possi- element is omnipresent in our environment. 28 + ble interference is the Si2 . Whenever these peaks are not separable (required mass resolution m=Vm = 2480 3.5.20. Other elements 40 16 + 28 + for Ca O and 2960 for Si2 ), the less-abundant Noble metals besides silver have typically low secondary 54Fe-isotope (5.8% instead of 91.72% for 56Fe) has to be ion yields and are therefore not suited for TOF-SIMS anal- taken for calculation of the iron abundance after correc- ysis. Noble gases normally are not ionized in the SIMS tion for contribution from 54Cr: 57Fe+ (2.2%) is in most process. Other elements not mentioned speciÿcally here are cases not discernable from 56Fe1H+ (m=Vm = 7730) and typically too rare in cosmic materials. Nevertheless, some often in.uenced by 40Ca16O1H+ (m=Vm = 1900). Espe- of them can certainly be analyzed with TOF-SIMS. cially the FeH=Fe-ratio is important to determine, since 54 1 + Fe H is the major interference at mass 55 amu for 3.6. SIMS sensitivities 55Mn+ (m=Vm = 5850). Similarly, the analysis of cobalt, 58 1 + which is also monoisotopic, is complicated by Ni H . For a quantitative analysis with TOF-SIMS, to transform 58 + 60 + Nickel can be measured as Ni and Ni , the former secondary ion intensities into element ratios, relative sen- 58 + 57 1 + with interferences from Fe and Fe H . sitivities must be determined. Therefore, carefully selected standards have to be analyzed. Since the actually investi- gated volume is extremely small, only a few atomic mono- 3.5.15. Copper and zinc layers thin, sample homogeneity and purity are crucial. As Copper and zinc have both low cosmic abundances described before, amorphous or polycrystalline standards are (Cu=Si = 5:22 × 10−4; Zn=Si = 1:260 × 10−3, chondritic desired. However, certain materials, glass as well as some atomic abundances according to Anders and Grevesse, minerals, used as standards in other techniques, do not meet 1989) and are therefore di4cult to determine. From the the requirements of TOF-SIMS concerning homogeneity on easy to conÿrm isotopic ratio of approximately 7 : 3 for a micrometer scale. Therefore, the quality of a standard must 63Cu=65Cu, an unequivocal identiÿcation of copper is some- always be monitored in the TOF-SIMS analysis. times possible. Zinc, although having more stable isotopes, In Fig. 22, typical TOF-SIMS sensitivities are shown for is harder to identify because of interferences from TiO+. positive and negative secondary ions. These results were derived from TOF-SIMS analyses of eight glass standards 3.5.16. Gallium (Jochum et al., 2000) with known composition. They meet Gallium was mainly used as primary ion species for the the above-mentioned strict requirements for standards con- TOF-SIMS analyses in this study. Although the ion source cerning homogeneity as was shown during the TOF-SIMS is equipped with “monoisotopic” 69Ga, some remaining analysis. The results are comparable to DF-SIMS sensitivi- 71Ga (∼0:2%) was still present, since the industrial isotope ties in the literature (e.g., Meyer, 1979; Lange et al., 1986). is not perfect. Therefore, both isotopes Unfortunately, .uorine and sulfur concentrations are not cannot be used for the analysis of indigenous gallium. known so far for these glass standards. Therefore, the data set on negative secondary ion sensitivities is quite incomplete. Previous investigations of halogens and sulfur showed that 3.5.17. Silver the selection of appropriate standards for these elements is Silver has a comparatively high ionization yield although rather di4cult. Sensitivities here vary over more than one it has a relatively high ionization energy (7:58 eV). Silver order of magnitude. or silver oxide is often used as for the analysis of All glass standards were measured twice, with simulta- organic substances since it also has a positive eKect on the neous argon sputtering and directly after sputtering. Sen- ionization probability of other elements and molecules (ma- sitivities before sputtering could not be determined since trix eKect). Both isotopes can be measured (107Ag: 51.839% no stable ion signals were reached (cf. Fig. 15). Except for and 109Ag: 49.161%) and show typically no interferences. O−-ions, diKerences between sensitivities during and after T. Stephan / Planetary and Space Science 49 (2001) 859–906 877

Fig. 22. Relative TOF-SIMS sensitivities for positive (solid symbols) and negative (open symbols) secondary ions relative to silicon derived from the analysis of eight glass standards with the gallium primary ion source. The sensitivities were determined from measurements with simultaneous argon sputtering and directly after sputtering. Only for negative oxygen ions signiÿcant diKerence between both measurements appears (the open square gives the O−-sensitivity after sputtering, the open circle during sputtering). Carbon sensitivities are determined from silicon carbide samples. sputtering seem not to exist for most elements, although mass interferences like oxides often prevent proper secondary ion signals typically decrease after sputtering isotopic analyses. (Fig. 15). For negative oxygen ions, the sensitivity relative If isotopic analysis with high accuracy is required, short to silicon increases by a factor of ∼1:7 after sputtering primary ion pulses are indispensable to achieve high mass compared to the sensitivity during argon sputtering. resolution. Even with these, neighboring peaks are often not Detailed results from the analyses of the individual glass su4ciently separated. Therefore, only mathematical peak standards are listed in the appendix. For chlorine, only the separation techniques provide further help. Since peaks in sensitivity obtained during sputtering from the most Cl-rich TOF-SIMS are typically not Gaussian and even not sym- glass (ATHO-G, 400 ppm Cl) is shown in Fig. 22. All other metric, the expected peak shapes have to be known for a samples showed traces of surface adsorbed chlorine. For proper deconvolution. Peak asymmetry is in part caused by other elements, geometric means for the sensitivity values dead time eKects that must be corrected by using Poisson given in the appendix are shown in Fig. 22. Error bars mark statistics as described above. Other eKects cannot so easily the range of values for diKerent measurements. Since no car- be corrected: Besides instrumental parameters like, e.g., ex- bon is present in the glass standards, C-sensitivities relative traction and re.ection ÿelds the peak shape also depends on to silicon were determined from the analysis of several sili- the initial energy distribution of the respective ion species. con carbide grains, although it is expected that matrix eKects However, if an undisturbed isotope of the same element is in silicone carbide hardly resemble those in glass. present, its peak may be used as an internal standard peak. This is the best available approximation under realistic con- 3.7. Isotope analysis ditions, since all isotopes of one element should have the same energy distribution. Peaks of other elements are not Only a few elements in typical rocks allow isotopic anal- suitable, because their energy distribution might be diKerent. ysis at moderate mass resolution. For hydrogen (negative To separate, e.g., the hydride peak 24Mg1H+ from 25Mg+, secondary ions), lithium, and boron, practically no interfer- the 24Mg+-peak can be used to ÿt the low-mass edge of ences are observed in the spectrum. Starting with carbon, the spectrum at 25 amu (Fig. 23). Here, the scaling fac- hydrides become a major problem, because they are not tor for 24Mg+ gives directly the 25Mg=24Mg isotopic ra- separable with moderate mass resolution. Hydrides are less tio. The diKerence between the two spectra represents the important for elements with isotopes that are separated 24Mg1H+-peak. by two mass units, e.g., copper or silver. Since these are This peak deconvolution technique has been success- typically present as trace elements in natural samples, other fully applied to various isotope systems. For the eight glass 878 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Table 2 Reproducibility of isotopic ratio TOF-SIMS measurements from the anal- ysis of eight diKerent glass standardsa

Isotopic ratio -values Standard Mean statistical (per mill) deviation error (per mill) (per mill)

(Positive secondary ions) (25Mg=24Mg) +5 9 2 (26Mg=24Mg) −13 7 2 (29Si=28Si) −23 14 4 (30Si=28Si) −67 14 5 (41K=39K) −42 56 7 (42Ca=40Ca) −9139 (43Ca=40Ca) −45 41 21 (44Ca=40Ca) −76 49 7 (46Ti=48Ti) +97 121 27 Fig. 23. A demonstration of the peak separation of 25Mg+ and 24Mg1H+. (47Ti=48Ti) +75 57 25 The shape of the 24Mg+-peak (dotted line, top x- and right y-axis) was (49Ti=48Ti) −29126 used as a model for the 25Mg+-peak. The hydride peak (solid line) (53Cr=52Cr) −10 60 39 emerges as the diKerence between the actual signal at mass 25 amu (57Fe=56Fe) +1 30 14 (shaded area) and the 24Mg+-peak, scaled to the same height. The spec- trum was measured with bunched gallium primary ions (∼600 ps pulse (Negative secondary ions) width) on glass BM90=21-G (cf. appendix). (13C=12C) ±0 94 168 (17O=16O) −72 81 44 (18O=16O) −88 36 13 standards, already used to determine elemental sensitiv- (29Si=28Si) −46 41 28 ities (see above), expected terrestrial values have been (30Si=28Si) −71 21 33 reproduced within deviations of a few percent for several (34S=32S) −83 117 108 isotopic ratios (Table 2). The accuracy achievable with this (37Cl=35Cl) −21 37 30 peak separation technique strongly relies on the abundance aData measured by T. Henkel (pers. comm.). The second column gives of the respective element in the sample and therefore varies the average deviation from standard isotopic ratios in per mill as -values for diKerent samples.   x y (xA=yA) ( A= A) = 1000 x y − 1 : ( A= A)Std: 3.8. Organic molecules The standard deviation of this average is shown in column three, whereas column four provides the mean statistical error of individual measure- Since the number of possible interferences for molecu- ments. Since carbon was only present as surface contamination on the lar ions can be very high, appropriate strategies had to be glass standards, the deviations from cosmic ratios are not representative for samples containing carbon in the bulk. developed for their proper identiÿcation. Nevertheless, an accurate mass determination is prerequisite to discriminate most possible interferences. Fragmentation during primary ion bombardment of most organic compounds occurs, ÿlling the respective mass spectrum with numerous peaks. Series obtained during subsequent data processing. Time proÿles of fragments with well-deÿned mass diKerences are striking as those in Fig. 15 show the time dependence of the re- features of organic mass spectra. They often allow the iden- spective secondary ion signals. Secondary ion images for tiÿcation of organic molecules or entire molecule classes. an unlimited number of mass intervals can be obtained Quantiÿcation of these results is even more complicated by scanning the primary ion beam over the sample. These than for element ratios, often limited to substances sitting on images show lateral element or molecule distributions simple, well-deÿned substrates, and requires internal stan- (cf. Fig. 24). dards (HagenhoK et al., 1992; HagenhoK, 1997). The amount of data, typically several hundred megabytes in one measurement, can be handled and processed by the 3.9. Data processing present generation of personal computers. A ÿrst evaluation can be performed during the measurement, providing total The amount of information obtained from a TOF-SIMS mass spectra, selected ion images, or depth proÿles. Never- analysis can be tremendous. In principle, each primary ion theless, for a detailed evaluation, all available information shot delivers a complete mass spectrum, although only a on every secondary ion has to be monitored. few secondary ions might be distributed over the entire mass Automated techniques are here desired for qualiÿed data range. Nevertheless, each secondary ion has to be registered reduction and evaluation in the future to get the immense individually to avoid loss of information. Mass spectra data stream under control, without losing any important in- for a carefully selected series of primary ion shots can be formation about the target. T. Stephan / Planetary and Space Science 49 (2001) 859–906 879

Fig. 24. Secondary ion distribution images from a CAI in Cold Bokkeveld. Field of view is 500 × 500 m2. Each image consists of 256 × 256 pixels. The number of primary ion shots per pixel is 1600 for positive and 1120 for negative secondary ions. Secondary ion species are given below each image (e.g., 16O−). Individual secondary ion images are normalized to the number given below the image, usually the intensity in counts of the most intense pixel (489 for 16O−), shown in red. A color bar represents the linear scale from (equals zero) to red. The second number below each image (9:57 × 106 for 16O−) is the intensity integrated over the entire image. (D. Rost and S. Schirmeyer, pers. comm.)

4. Meteorites tribution of lithium, which was found enriched in Fe-rich phyllosilicates within CAIs from several diKerent carbona- Meteorites are the principal objects of research in cosmo- ceous chondrites but not in phyllosilicates located in the chemistry. They are the major source of information about surrounding matrix. Examples of ion images for several our solar system, its origin and early history about 4:6Ga secondary ion species including 7Li+ are shown in Fig. 24. ago. The investigation of small, micrometer-sized entities They were obtained from a mineralogical thin section from within meteorites has become increasingly important during a CAI within the CM Cold Bokkeveld (Fig. 25). the last decades. Here, TOF-SIMS was expected to be an ap- The CAI itself can clearly be seen in the aluminum image. propriate technique due to its imaging capabilities. Elemen- Although in the 7Li+-image only 2790 ions were detected, tal mapping, isotope analysis, as well as the investigation a concentration of the signal within several regions of the of organic matter can be performed on phases with linear CAI is obvious. A closer examination of the results show dimensions even smaller than one micrometer. Several dif- that lithium is especially enriched in Fe-rich phyllosilicates ferent applications of TOF-SIMS in meteorite research are located between the Ca-rich rim and the interior presented exemplarily in the following. of the CAI (Schirmeyer et al., 1997). Although this hydrous phase is similar to the surrounding -bearing 4.1. Ca,Al-rich inclusions in carbonaceous chondrites minerals in the host , these minerals diKer by more than two orders of magnitude in their lithium contents. Ca,Al-rich inclusions (CAIs) are among the most primi- From TOF-SIMS studies on various carbonaceous chon- tive objects found in meteorites (MacPherson et al., 1988). drites, it can be ruled out that lithium was already present As high- condensation products, they stem in the proposed precursor material of these hydrous phases, from the earliest stages of solar system formation and are most likely melilite (Schirmeyer et al., 1996b, 1997). It must therefore of special interest in cosmochemistry. After their have been incorporated into the phyllosilicates during aque- formation, many CAIs experienced complex histories of al- ous alteration processes. teration. These objects where among the ÿrst extraterrestrial Phyllosilicates in carbonaceous chondrites were formed samples chosen for TOF-SIMS analysis (Stephan et al., by aqueous alteration in the early solar and=or 1991) to test the applicability of this technique in meteorite within meteorite parent bodies (BischoK, 1998). Still some research. CAIs from CM, CO, and CV meteorites were controversy exists about the time and environment of their investigated in a later, more extended study (Schirmeyer formation, and, more generally, about the possibility of et al., 1996a, b, 1997). Here a striking feature is the dis- aqueous alteration prior to the of the ÿnal parent 880 T. Stephan / Planetary and Space Science 49 (2001) 859–906

measurements with TOF-SIMS. Anyhow, within statistical error limits no evidence for deviations from the chondritic lithium isotope ratio other than caused by instrumental mass fractionation was observed with TOF-SIMS for the Li-rich phases in carbonaceous chondrites (Schirmeyer et al., 1997). These results were also conÿrmed by DF-SIMS analysis (Schirmeyer et al., 1997).

4.2. Organic molecules in Allan Hills 84001, Murchison, and Orgueil

Since the early 19th century, it is known that carbona- ceous chondrites contain organic matter (Berzelius, 1834; “Gibt dies moglicherweise einen Wink uber die Gegenwart organischer Gebilde auf anderen Weltkorpern? ”). In the second half of the 20th century, due to new analytical tech- Fig. 25. SEM-BSE (scanning electron microscope, backscattered elec- trons) image of the CAI shown in Fig. 24. (S. Schirmeyer, pers. comm.) niques and a rising interest in space science, a lot of ef- forts were made to characterize these organic components (Hayes, 1967; Anders et al., 1973; Cronin et al., 1988). In body. While Metzler et al. (1992) found clear evidence the 1960s, amino were found in several carbonaceous for such preaccretionary processes for CM chondrites chondrites, but contamination during handling and storage of (BischoK, 1998), solely asteroidal alteration was also pro- the samples could not be unambiguously ruled out (Hayes, posed (Browning et al., 1996; Browning and Keil, 1997; 1967). Reports of non-terrestrial bacteria (Claus and Buseck et al., 1997; Zolensky, 1997). Nagy, 1961) in those years were also soon compromised The TOF-SIMS observation of greatly diKerent lithium as terrestrial (Hayes, 1967). In the early 1970s, abundances in Fe-rich phyllosilicates within CAIs and those analyses of the fallen in 1969 showed in the adjacent matrix strongly suggest that both phases were clear evidence for extraterrestrial amino acids, aliphatic and formed in diKerent environments. At least the phyllosilicates aromatic hydrocarbons in this CM chondrite (Kvenvolden of the CAI must have been formed prior to the incorporation et al., 1970; OrÃo et al., 1971; Pering and Ponnamperuma, of the CAI into the meteorite probably under 1971). nebular conditions (Schirmeyer et al., 1997). This supports Although no connection of these organic compounds to the conclusions of MacPherson and Davis (1994) who sug- can be drawn (Chyba and Sagan, 1987)— gested alteration of refractory inclusions from the CM chon- in contrary they have been linked to the drite Mighei in the solar nebula, and not in the present or a (Allamandola et al., 1987)—prebiotic organic matter from previous generation of parent bodies. and delivered to Earth by meteorites and Lithium isotopic ratios are of great interest due to their interplanetary dust might have been important for the origin cosmological implications. In general, both lithium isotopes of life on our planet (Anders, 1989; Chyba et al., 1990; can be formed through three diKerent processes: (1) galactic Chyba and McDonald, 1995). cosmic-ray spallation of heavier nuclides, mainly 12C; 14N For our neighboring planet, organic compounds known and, 16O, (2)  + -fusion reactions, and (3) during pri- as polycyclic aromatic hydrocarbons (PAHs) in the Mar- mordial just after the (Meneguzzi tian meteorite ALH 84001 (Thomas et al., 1995) have been et al., 1971; Reeves et al., 1973; Olive and Schramm, suggested as being indicative for the existence of past life 1992; Steigman and Walker, 1992; Thomas et al., 1994a). on (McKay et al., 1996; Clemett et al., 1998). Since While big bang nucleosynthesis produces almost pure then, the signiÿcance of PAHs as in this case 7Li (7Li=6Li-ratio ∼104), interactions of galactic cosmic has been questioned by many authors (Anders, 1996; Bell, ray spallation and fusion reactions lead to 7Li=6Li-ratios of 1996; Becker et al., 1997, 1999; Stephan et al., 1998a–c, the order of one (Steigman and Walker, 1992). Therefore, 1999a; Zolotov and Shock, 1999). the 7Li=6Li-ratio in the solar system (∼12) can only be Chemically, PAHs represent the between explained by a mixture of lithium from diKerent nucleosyn- aliphatic hydrocarbons and graphite. While the former are thetic processes. Deviations from the CI-value in primitive characterized by a H=C-ratio of ∼2, this ratio becomes zero solar system material would be of great interest, but have for the latter. For PAHs, H=C is below 0.8 and generally de- not been unambiguously found so far. creases with increasing mass. PAHs are omnipresent in our Since lithium has two stable isotopes, 6Li and 7Li with solar system and even beyond (Allamandola, 1996). In prin- CI-abundances of 7.5 and 92.5% (Anders and Grevesse, ciple, they can derive from biogenic as well as non-biogenic 1989), respectively, and no mass interferences occurs in the sources. On Earth, PAHs derive mainly from anthropogenic secondary ion spectra, it seemed to be predestined for isotope emission caused by the combustion of fossil and other T. Stephan / Planetary and Space Science 49 (2001) 859–906 881

Fig. 26. Secondary ion (100 × 100 m2, 200 shots=pixel, 256 × 256 pixels) and SEM-BSE images for an area of ALH 84001 dominated by a carbonate surrounded by orthopyroxene. The PAH image is obtained by summing up signals from characteristic PAH masses between 115 and 339 amu (cf. Figs. 28 and 29). natural biogenic sources (La.amme and Hites, 1978; Wake- PAHs were identiÿed by L2MS analysis with a lateral res- ham et al., 1980a, b). PAHs attributed to a non-biogenic olution of about 40 m (Thomas et al., 1995). A meaningful origin can be found in ordinary and carbonaceous chondrites mass spectrum was obtained by mapping a surface region (OrÃo et al., 1971; Pering and Ponnamperuma, 1971; Hahn of 750 × 750 m2 at a spatial resolution of 50 × 50 m2 et al., 1988; Zenobi et al., 1989; Wing and Bada, 1991; (McKay et al., 1996). Therefore, a correlation of PAHs even Clemett et al., 1992; de Vries et al., 1993). They have with carbonates, between 1 and ∼250 m in size (McKay been detected in interplanetary dust particles (Clemett et al., 1996), could not be scrutinized by this technique. et al., 1993), Halley (Moreels et al., 1994), and also Here, TOF-SIMS has its advantages. The high lat- in circumstellar graphite grains extracted from primitive eral resolution in combination with parallel detection of meteorites (Clemett et al., 1996; Messenger et al., 1998). atomic as well as molecular secondary ions should be Therefore, the presence of PAHs in ALH 84001 is not predestined for an examination of the proposed spatial indicative for past life on Mars. Nevertheless, is has been association. First TOF-SIMS studies of ALH 84001 were argued that their spatial association with other proposed performed on polished thin sections of this meteorite biomarkers suggests a biogenic origin of PAHs in this Mar- (Stephan et al., 1998a, b). Although a homogenization tian meteorite (McKay et al., 1996). These biomarkers, most of the lateral PAH distribution by the polishing process prominently the “nanofossils”, are all related to chemically was expected, PAHs were found to be distributed inho- zoned Ca–Mg–Fe-carbonates (Mittlefehldt, 1994; Scott mogeneously, especially after removal of the uppermost et al., 1998; Treiman and Romanek, 1998; Warren, 1998), monolayers through sputtering. However, a spatial associa- which might have been deposited from hydrothermal .uids tion with carbonates could not be conÿrmed in this study. on Mars (McKay et al., 1996). These distinct observa- Although omnipresent in this , in car- tions were made by use of entirely diKerent techniques, bonates, compared to orthopyroxene and feldspathic glass, which, simply from their spatial resolution, hampers a PAHs even seem to be slightly depleted. Spatial distribu- correlation of their results. Due to the small size of the tions of PAHs and major rock-forming elements are shown supposed microfossils, 100 nm in longest dimension and in Fig. 26 for one section of ALH 84001. The area is domi- 20–80 nm across, a scanning electron microscope with ÿeld nated by a carbonate, prevailing in the Ca+-image, which is emission gun (FEG-SEM) with a lateral resolution of about surrounded by orthopyroxene (dominant in the Si+-image). 2 nm was used (McKay et al., 1996). On the other hand, In the meantime, the search for PAHs was extended to 882 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 27. Secondary ion images from a fracture surface of ALH 84001 (150 × 150 m2, 3200 shots=pixel, 256 × 256 pixels). Prevailing mineral phases are orthopyroxene (dominant in Mg+-, Si+-, and Fe+-images), feldspathic glass (Na+-, Al+-, and K+-images), and carbonate (Ca+-image). One carbonate-rich area is enriched in barium. Small chromite grains are apparent in the Cr+-image. They are rich in titanium. Lithium is also enriched in distinct spots, but is not obviously correlated to any other element.

fractured surfaces. Preliminary results (Fig. 27) here also Pure PAH substances, (C22H14) and coronene show no indication of PAH enrichments associated with (C24H12), as well as two carbonaceous chondrites containing carbonates. PAHs seem to be omnipresent on this surface. PAHs, Murchison and Orgueil, were analyzed to investigate Nevertheless, no homogeneous distribution of PAHs was this fragmentation more thoroughly (Stephan et al., 1998c, observed in the corresponding image. The variability can at 1999a). Pentacene and coronene showed fractionation dur- least in part be attributed to topographic eKects, since here ing primary ion bombardment, but to diKerent extents no .at surface was investigated. (Fig. 29). The linear line-up of rings in pentacene A more detailed analysis of the TOF-SIMS spectra is help- is more susceptible to fragmentation than their circular ar- ful to elucidate the origin of the PAHs in ALH 84001. These rangement in coronene. However, for both substances the mass spectra (Fig. 28) diKer signiÿcantly from those ob- very same PAH fragments were observed as in ALH 84001 tained by L2MS (McKay et al., 1996; Clemett et al., 1998; and the other two meteorites. From these results, one can Becker et al., 1999). This can be explained by diKerences infer that PAHs similar to those observed by L2MS had in ionization processes. Fragmentation of the PAHs occurs produced the observed TOF-SIMS spectra upon sputtering. during primary ion bombardment, increasing the low-mass Since unambiguous proof for a connection between PAHs PAH signals. and indigenous features in ALH 84001 is missing, it has T. Stephan / Planetary and Space Science 49 (2001) 859–906 883

Fig. 28. Typical TOF-SIMS spectrum of mass range 110–345 amu for positive secondary ions obtained from a section of ALH 84001. Relative maxima are separated by mass diKerences of 11 or 13 amu. Often several maxima appear separated from each other by a mass diKerence corresponding to H2. Since several structural exist for all mass peaks, only examples for major peak identiÿcations are given. been suggested that PAHs as well as amino acids in ALH 1980a, b). Heteroatoms would drastically increase the ion- 84001 originate from the icy environment like observed ization yields of the PAHs. Because of this and since they in other Antarctic meteorites (Becker et al., 1997; Bada would appear at diKerent characteristic masses, they cannot et al., 1998). Based on stable carbon isotope measurements, be overlooked in the mass spectra. it was proposed that besides terrestrial contamination, a sec- So far, the comparison between the TOF-SIMS spec- ond organic carbon component exists, which might be ex- tra of the three meteorites is not quantitative. To calculate traterrestrial, probably Martian (Jull et al., 1998; Becker the mean H=C-ratio for all observed PAH fragments, is a et al., 1999). ÿrst attempt to overcome this deÿciency. Here, the three To investigate the origin of PAHs in ALH 84001— meteorites yield similar values, 0.69 for ALH 84001 and terrestrial or Martian—extraterrestrial PAHs from the 0.65 for both carbonaceous chondrites. These values diKer carbonaceous chondrites Murchison and Orgueil were an- signiÿcantly from those obtained for pentacene (0.58) and alyzed with TOF-SIMS (Stephan et al., 1999a). The mass coronene (0.52). This diKerence and the similarities between spectra of PAHs from these two meteorites resemble re- all ÿve PAH spectra are also emphasized in Fig. 30, where markably well those obtained from ALH 84001 (Fig. 29). the data points from Fig. 29 for ALH 84001, Orgueil, and All three meteorites show essentially the same PAH frag- the two pure PAH substances are directly compared with mentation and, therefore, a similar, non-biogenic origin can those of Murchison. It is expected that future TOF-SIMS be deduced. For biogenic PAHs we would expect higher analyses of terrestrial biogenic PAHs will help to assess degrees of and the presence of aromatic hetero- these results and to discover the true of PAHs in ALH cycles like observed in terrestrial samples (Wakeham et al., 84001. 884 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 29. Relative abundances of PAH ions in TOF-SIMS spectra obtained from ALH 84001, Murchison, and Orgueil compared with those obtained from pure PAHs, pentacene and coronene. Coronene and pentacene structures are shown as well as the structural of (C14H10) a major fragment of pentacene during sputtering. Besides the number of carbon atoms above the upper x-axis, the numbers of hydrogen atoms are given for each major peak next to the ALH 84001 data points.

However, the present results, great similarities between association between PAHs and carbonates exists in PAHs in ALH 84001 and carbonaceous chondrites, supports this meteorite. the suggestion that these organic molecules derived from the exogenous delivery of meteoritic or cometary debris to the 4.3. Further meteorite studies with TOF-SIMS surface of Mars (Becker et al., 1999), similar to those com- pounds delivered to the early Earth (Anders, 1989; Chyba 4.3.1. Orgueil et al., 1990). Alternatively, an abiotic synthesis of PAHs on CI chondrites represent chemically the most pristine solar Mars is also conceivable, provided that this chemical process system material available for laboratory analysis. Except for is similar to the formation process of PAHs in Murchison the so-called CHON elements (carbon, hydrogen, oxygen, and Orgueil. This has also been proposed recently (Zolotov and nitrogen), lithium, and the noble gases, their composi- and Shock, 1999). Independent of the discussion about the tion is representative for the entire solar system (Anders and signiÿcance of other suggested biomarkers in ALH 84001 Grevesse, 1989). (e.g., Bradley et al., 1996, 1997, 1998; Leshin et al., 1998; Only little is known about the actual distribution of Scott, 1999), TOF-SIMS studies revealed that no spatial minor and trace elements in CI meteorites, despite their T. Stephan / Planetary and Space Science 49 (2001) 859–906 885

phases diKer often drastically, up to orders of magnitude, in their secondary ion yield from elements in the bulk matrix.

4.3.2. and Nakhla A general interdisciplinary study on Martian meteorites has just been started recently. A central role in this investi- gation plays TOF-SIMS (Stephan et al., 1999b). Besides the analysis of ALH 84001, two further Martian meteorites were investigated so far, namely Chassigny and Nakhla. In com- bination with SEM, electron microprobe, and TEM studies, so-called symplectic exsolutions in were analyzed that set constraints on the formation of these Martian rocks, the oxidizing conditions and the cooling history (Greshake et al., 1998). Fig. 31 shows secondary ion images as well as an SEM-BSE image from one of these objects found in Nakhla. A calcium-rich pyroxene (augite) can be seen as dark areas in the SEM image. Here calcium showed the high- est secondary ion intensities. The bright phase in the SEM image is dominated by chromium-rich magnetite, prevailing in the Cr+- and Fe+-images with a slight enrichment in alu- minum. Magnesium, silicon, and iron secondary ion images Fig. 30. Comparison of relative intensities of PAHs in ALH 84001 and show the highest intensities outside the exsolution lamella in Orgueil meteorites, as well as of pentacene and coronene standards, with the PAH intensities in Murchison. the . Therefore, these images have been normalized to the most intense pixels inside the interesting region and not to the most intense pixels of the entire images. The fact that iron yields higher secondary ion intensities in olivine importance for the understanding of early solar system evo- than in magnetite, although the iron concentration is higher lution. The knowledge of the actual distribution of trace in the latter, can be attributed to matrix eKects. elements in chondritic matter is of great interest also for the understanding of apparent enrichments of volatile trace elements in interplanetary dust (see below). 5. Interplanetary dust Imaging TOF-SIMS at high lateral resolution was there- fore used to analyze the element distribution within the In 1683, when the Italian–French astronomer G.D. Cassini Orgueil CI meteorite (Stephan et al., 1997a), which is ÿrst described the zodiacal light, he also gave the right generally used as the cosmic element abundance standard explanation for this phenomenon (in Cassini, 1730): It is (Anders and Grevesse, 1989). The TOF-SIMS investigation caused by sunlight scattered from dust particles concentrated revealed a rather homogeneous distribution for the major in the ecliptic plane. Since 1981, interplanetary dust parti- elements magnesium, aluminum, silicon, iron, and nickel. cles (IDPs) are collected routinely in the stratosphere using For sodium and potassium, enrichments throughout the en- high-.ying aircraft (Warren and Zolensky, 1994). They rep- tire section were observed in correlation with .uorine and resent, besides meteorites and lunar rocks, the third class of chlorine. This might be explained by surface contamination, extraterrestrial material available for laboratory investiga- a general problem in TOF-SIMS studies (see above). Other tion so far (Brownlee et al., 1977). Due to their small size— elements are often concentrated in small, up to 100 m typical IDPs have a size of ∼10 m—these samples require sized regions. Calcium occurs in carbonates, as observed analytical techniques with high lateral resolution and high before in CI chondrites (EndreX and BischoK, 1996). sensitivity. Chromium, typically together with iron, can be found in IDPs are of high interest in planetary research since chromites. Iron and nickel enrichments are often correlated they potentially allow to investigate the most pristine solar with sulfur, indicative for the presence of sulÿdes. Lithium, system bodies, asteroids and comets (Brownlee, 1994; Der- titanium, manganese, copper, bromine, and iodine showed mott et al., 1996). In particular for cometary material, they enrichments in small regions, only a few micrometers in represent the only available source so far for an earthbound size. Further mineralogical studies are needed to elucidate investigation. But also asteroidal IDPs should signiÿcantly the host phases of the enriched elements. diKer from meteoritic material. While meteorites are gen- It should be mentioned that although Orgueil is the ele- erally accepted to derive primarily from the inner ment standard in cosmochemistry, this meteorite is not suited belt (Wetherill and Chapman, 1988), dust from all over the as an element standard for TOF-SIMS, due to lack of homo- solar system, including the outer belt, should spiral towards geneity. Elements that are only present in distinctive mineral the due to Poynting-Robertson-drag (Robertson, 1937; 886 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 31. Secondary ion images from a symplectic exsolution from the Nakhla Martian meteorite (20 × 20 m2, 1600 shots=pixel, 256 × 256 pixels). Lower intensities towards the very right and bottom of the images are artifacts from ion beam shifts during analysis.

Klacka, 1994) and therefore contribute to the near-Earth ably similar to those observed in dust around and from dust population. Some of this dust eventually enters the comets Halley and Hale-Bopp (Jessberger and Kissel, 1991; Earth’s and can be collected in the stratosphere. Bradley et al., 1999a). Comets are probably the most prim- IDPs still preserve information on their precursors, although itive bodies in the solar system and therefore predestined several selection and alteration processes occur during their as deep-freeze storehouses for presolar material. The search lifetime. For a general overview of IDPs refer to, e.g., for interstellar matter in cometary IDPs has been one inspir- Zolensky et al. (1994), Brownlee (1996), Rietmeijer (1998), ing motivation for the analysis of these tiny grains (Bradley or Jessberger et al. (2001). and Ireland, 1996). Recent observations link them—or at Major goals in IDP research are still to unambiguously least parts of them known as GEMS (glass with embed- diKerentiate cometary from asteroidal IDPs and to distin- ded metal and sulÿdes)—to interstellar amorphous silicate guish between features caused by and grains (Bradley, 1994b; Bradley et al., 1999b). Together collection in the stratosphere, and indigenous properties. with presolar grains extracted from primitive meteorites, Nevertheless, the “chondritic porous” subset of IDPs, an- constituents of cometary IDPs are the most primitive mate- hydrous particles with highly porous microstructures, high rials available for laboratory investigations and maybe the carbon abundances, and often high atmospheric entry ve- best analogue to present interstellar dust. locities are most promising candidates for cometary dust. Due to its capabilities of analyzing small particles They consist mainly of Mg-rich, Fe-poor silicates, remark- (Zehnpfenning et al., 1994; Rost et al., 1998a), TOF-SIMS T. Stephan / Planetary and Space Science 49 (2001) 859–906 887 was expected to be an appropriate technique for the inves- were observed on several stratospheric particle surfaces tigation of IDPs. although some analyses were obstructed by non-removable silicone oil residues. —Stratospheric dust collectors are 5.1. Volatile elements coated with silicone oil, which is also used during particle handling. —A surface correlation of halogens on some par- Many stratospheric IDPs are more rich in volatile ele- ticles was unequivocally proven by extensive sputtering, ments than CI chondrites (Arndt et al., 1996a), while ma- reducing and ÿnally removing the halogens at least on cer- jor elements usually show chondritic abundances (Schramm tain particles (Rost et al., 1999a). In addition, several dust et al., 1989). Attempts to explain the enrichments range particles in sections showed again outer rings of halogen from postulating a new type of volatile-rich chondrite-like contaminants (Rost et al., 1998b, 1999b). matter (Flynn and Sutton, 1992; Flynn et al., 1996a) to Another source of contamination, not in the stratosphere invoking atmospheric contamination processes (Jessberger but from laboratory handling, was observed in TOF-SIMS et al., 1992). Before far reaching conclusions on nebular studies in form of surface correlated beryllium. This re- processes can be drawn, stratospheric processes, contamina- sults from the SEM-EDS analyses, which in case of these tion during capture and handling, artifacts from various se- particles were performed on beryllium substrates (Rost lection eKects or from analytical techniques and even from et al., 1999a). After such analyses, some particles tend to data treatment have to be excluded (Stephan et al., 1997b). stick to the substrate and carry beryllium when they were Due to their complex history, IDPs in the laboratory have removed. already experienced several potential sources of contamina- For other elements, ÿnally, no unambiguous evidence tion. For small particles, contamination in general is a more for or against contamination is available so far. For most severe problem, because of the high surface-to-volume ra- an unequivocal enrichment cannot be claimed since tio, than for the comparably large “normal” meteorites. High the data set available in the literature typically is too incom- porosity of IDPs also enhances the surface receptive to con- plete for a clear statement (Arndt et al., 1996a) and limita- tamination (Stephan et al., 1992a, 1994b). Furthermore, el- tions of the applied analytical and mathematical techniques ements with very low abundances in chondritic matter are complicate the interpretation (Stephan et al., 1997b). Trace most susceptible to contamination because then even a small element distributions are largely variable and re.ect chemi- absolute amount added can yield high enrichments. cal inhomogeneity on a micron scale (Bohsung et al., 1993; Bromine shows the highest enrichments in IDPs, with en- Stephan et al., 1993a, 1994b; Arndt et al., 1996a). Never- richment factors of up to 104 × CI (Arndt et al., 1996a). theless, since most IDPs chemically are remarkably similar Since bromine, on the other hand, has only a CI-abundance to CI chondrites, they represent an adequate sample of prim- of 3.56 ppm (Anders and Grevesse, 1989) and halogens are itive solar system material. widespread in the stratosphere (Cicerone, 1981), contamina- tion processes have to be considered as being responsible for 5.2. Mineral identiÿcation high bromine concentrations in stratospheric particles. First direct experimental evidence for such a contamination were TOF-SIMS has enabled the discovery of several unique Br- nano- attached to IDP W7029E5 (Rietmei- mineral phases by secondary ion imaging, although the jer, 1993) and a halogen-rich exterior rim of IDP L2006G1 capabilities of quantiÿcation are limited for this technique. found with TOF-SIMS (Stephan et al., 1994c). The distri- Calcium phosphate, probably apatite, was found for the butions of secondary halogen ions from a section of this ÿrst time to occur in an IDP (L2006E10, Fig. 33 top) IDP reveal an outer ring structure for .uorine, chlorine, and by TOF-SIMS investigation (Stephan et al., 1994c) and also bromine, though the latter image is disturbed by rather has meanwhile been reported in another IDP (Rost et al., high background (Fig. 32). Assuming a spherical particle, 1999b). For U2071B6 (Fig. 33 bottom), a manganese sul- originally 20 m in diameter, and a continuous 1 m thick ÿde (most likely alabandite) was observed (Rost et al., surface layer surrounding it, this layer represents ∼25 vol% 1998b). While apatite is widespread in many diKerent types of the entire particle. Although only a rough estimate, these of meteorites, alabandite usually is only found in mete- numbers illustrate the in.uence that surface contamination orites that formed under highly reducing conditions, e.g., might have on bulk chemistry of small particles. enstatite chondrites or (Brearley and Jones, 1998; Other clear evidence for contamination as a major clue to Mittlefehldt et al., 1998). However, no further connection halogen enrichments of stratospheric IDPs was found with between these meteorite classes and this speciÿc IDP can be test particles exposed to the stratosphere on a dust collector drawn from TEM studies so far (W. KlQock, pers. comm.). by Arndt et al. (1996b). Besides this, weakly bound bromine Although an unambiguous identiÿcation of the actual min- was observed in large IDPs (Flynn et al., 1996b). eral phase is not possible through TOF-SIMS analysis alone, To test more directly for surface contamination with indicative elemental correlations can help to recognize in- TOF-SIMS, the original surfaces of stratospheric particles teresting regions within a sample, which can be analyzed were analyzed and compared with sections from the very later by appropriate techniques like TEM for an unequivocal same particles (Rost et al., 1998b, 1999a, b). Here halogens mineral assignment. 888 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Fig. 32. Secondary ion images of positive as well as negative secondary ions from IDP L2006G1. The investigated area (30 × 30 m2; 128 × 128 pixels) was bombarded with Ga+ primary ions (1000 shots=pixel and 800 shots=pixel for positive and negative secondary ions, respectively). Although the background for Br−-ions (here the added signals from 79Br− and 81Br−) is relatively high, a correlation with the halogens .uorine and chlorine is obvious.

5.3. The collector project one avoids human selection eKects prevailing in most stud- ies. Only “chondritic looking” (black and .uKy) particles As long as no interplanetary dust particle has been cap- are picked from the collector .ags in most cases (J.L. War- tured gently in (cf. Brownlee et al., 1996, 1997), ren, pers. comm.) and only particles pre-classiÿed as “cos- retrieved to Earth, and recovered successfully from the cap- mic” due to their chondritic major element composition are ture medium (probably aerogel; Tsou, 1996), IDPs collected further investigated. Therefore, one major objective of this in the stratosphere are the ÿrst-choice samples of interplan- study is to track down non-chondritic interplanetary dust etary dust. Therefore, a comprehensive consortium study particles. was initiated to investigate all particles from one collection Table 3 gives an overview of the present status of this con- surface (U2071) with as many diKerent techniques as pos- sortium study. All 326 particles with diameters ¿ 10 mon sible (Stephan et al., 1994d). This suite of analytical meth- collection surface U2071 were investigated under an optical ods (cf. Table 3) includes TOF-SIMS for surface studies microscope, and 90 of them were selected for the ÿrst round (Stephan et al., 1995a; Rost et al., 1996, 1999a, b) as well as of investigations. After SEM imaging and EDS analysis of for the investigation of interior element distributions (Rost major element concentrations including carbon (Thomas et al., 1998b, 1999b) in sections within the same particles. et al., 1993, 1994b), these particles were pre-classiÿed Not only the investigation of contamination processes as as cosmic particles (C), terrestrial contamination, natu- mentioned above is the aim of this ongoing study, but to ral (TCN) and artiÿcial (TCA), aluminum oxide spheres investigate the mineralogical and chemical properties of an (AOS; solid rocket exhaust), and non-classiÿed parti- unbiased sample of stratospheric dust particles as thoroughly cles. The classiÿcation scheme was used according to the as possible. By studying all particles from one collector, Catalogs (Warren and Zolensky, 1994). T. Stephan / Planetary and Space Science 49 (2001) 859–906 889

Fig. 33. Secondary ion images from IDPs L2006E10 (ÿrst row; Stephan et al., 1994c) and U2071B6 (second row; Rost et al., 1998b) show the ÿrst reported observation of calcium phosphate (presumably apatite) and manganese sulÿde (most likely alabandite) in IDPs. Fields of view are 25 × 25 m2 (L2006E10) and 30×30 m2 (U2071B6). Both samples were scanned with 128×128 pixels, and 1000 shots=pixel (L2006E10) and 2560=2400 shots=pixel (U2071B6, positive=negative secondary ions).

Table 3 Analytical sequence and present status of the consortium study on particles from stratospheric collector U2071a

Analytical technique Particles from collector U2071

Optical microscope 326 particles ¿10 m SEM (images) 90 particles SEM-EDS 90 particles TOF-SIMS (surface) B3, B6, B7a,C3, D1, D4, D8, E6, E8,F3, G8, H1a,H9, I9, J9a,L1,L2 PIXE (bulk) B3, B6,C3, D1, D8, E6, F3, G8, H1a,H9, I9, L2 STIM (bulk) B3, B6,C3, D1, D8, E6, F3, G8, H1a,H9, I9, L2 TEM (section) B6,C3, D1, D4, E6, E8,F3, G8, H9, I9, L1 TFO-SIMS (section) B6,C3, D1, D4, E6, E8,H9, I9, L1 PIXE (section) in preparation

aUnderlined names represent particles pre-classiÿed as cosmic (type C) IDPs. Other types are terrestrial contamination, natural (TCN: E6) and artiÿcial (TCA: I9), aluminum oxide spheres (AOS: B3, D8, G8, L2), as well as non-classiÿed particles (type ?: D1). The TEM studies are presently in progress.

Surface analysis with TOF-SIMS followed to study con- only investigates the surface. Consequently, even for ele- tamination eKects (see above). Subsequently, the particles ments that are detectable with all three analytical techniques, were investigated with PIXE ( induced X-ray emis- the results are not straightforward to compare (Stephan sion) analysis including STIM (scanning transmission ion et al., 1995b). microprobe). PIXE allows trace element studies with high After these analyses, the particles are imbedded in epoxy sensitivity down to ppm levels for elements with atomic and cut with an ultramicrotome. The microtome sections numbers ¿ 11 (Traxel et al., 1995; Bohsung et al., 1995). were prepared for TEM studies and the particle residues STIM yields particle densities and masses (Maetz et al., in the epoxy were analyzed again with TOF-SIMS. This 1994; Arndt et al., 1996c, 1997). This ÿrst PIXE investiga- allows a direct comparison of mineralogical results (TEM) tion was done at limited proton current and dose to avoid and element distribution images (TOF-SIMS) of adjacent destruction of hydrous minerals (Maetz et al., 1996). The sections. information depth in PIXE is of the order of 30 m, the It is further planned to produce sections several microm- typical dimension of the dust particles. The information in eters thick from the remaining epoxy stubs for further PIXE SEM-EDS comes from the upper 1–2 m, and TOF-SIMS studies. As the last analytical step for these particles, PIXE 890 T. Stephan / Planetary and Space Science 49 (2001) 859–906 can now be performed at high proton current, dose, and 1992b). Although a proper quantiÿcation is not possible, therefore increased sensitivity. Since the ÿnal stage in the se- copper was identiÿed as the dominating element. These quence of analytical steps is now reached, precautions con- ÿndings seem to exclude an extraterrestrial origin of the re- cerning mineral destruction have not to be considered any spective particles, since high abundances of elements with longer. extreme low cosmic abundances like copper are indicative The general principle for this sequence of analytical tech- for space debris. Copper has a mean chondritic abundance niques is to go from non-destructive via less destructive to of only 126 ppm (Anders and Grevesse, 1989) and should the most destructive techniques. The analysis of contamina- not be present as major element in interplanetary dust. These tion eKects in the stratosphere has to be made as early as results clearly demonstrate the advantages of TOF-SIMS es- possible to minimize the risk of additional contamination in pecially when sample material is limited. DF-SIMS only al- the laboratory. It is expected that the great variety of ana- lowed to analyze a limited number of elements within the lytical techniques applied to individual IDPs in this ongoing thin layers. This is often not su4cient for a proper classiÿca- study will ÿnally be instrumental in a comprehensive under- tion of impact residues. The parallel detection in TOF-SIMS standing of the origin and history of interplanetary dust. led to the discovery of the copper enrichment. Beforehand, such enrichments were not expected and therefore copper 5.4. Particle impact residues from space probes was not selected for DF-SIMS analysis.

Each surface exposed in space is subject to dust par- ticle bombardment. In low Earth orbit, this dust consists 5.5. Further TOF-SIMS studies of interplanetary dust as well as man-made space debris. To discern between these populations and to reveal their As mentioned before, to decipher the origin of an inter- respective were major objectives of the capture planetary dust particle, its connection to asteroids or comets experiment A0187-2 on the LDEF (long duration expo- is a major aim of IDP research. The TOF-SIMS study of an sure facility) satellite (Amari et al., 1991, 1993; Bernhard anhydrous particle supported the idea of a relation between et al., 1993a, b; HQorz et al., 1995). chondritic porous IDPs and comets (Stephan et al., 1993b; At typical relative velocities of several kilometers per sec- Jessberger et al., 2001). The element abundances of this ond, an impinging dust particle evaporates almost entirely. particle, L2006B21, are similar to the element abundances Nevertheless, some material may be deposited in or around observed in comet Halley’s dust (Jessberger et al., 1988). the impact crater. These impact residues provide informa- Besides interplanetary dust collected in the stratosphere tion on the chemical composition of the precursor particles, and impact residues from surfaces exposed in low Earth or- although tremendous elemental fractionation processes oc- bit, also extracted from Antarctic have curring during particle impact have to be considered (Lange been investigated by TOF-SIMS (Stephan et al., 1995b). et al., 1986). The typical amount of sample material de- So-called COPS phases, iron oxide rich in carbon, phos- posited by high-velocity impact is conÿned to 1–5 atomic phorous, and sulfur (Engrand et al., 1993), were found on layers. TOF-SIMS with its high surface sensitivity here the surface and in voids of the micrometeorites. The lateral seems to be ideally suited for the analysis of these impact distribution supported the idea that COPS phases grew on residues. Due to parallel detection of all secondary ions with the surface of micrometeorites, possibly by modiÿcation of one polarity, TOF-SIMS allows to investigate numerous ele- indigenous material, and by incorporation of material dur- ments even within these extreme thin layers. This might help ing atmospheric passage and Antarctic residence (Maurette to elucidate the origin of the respective impacting particle. et al., 1994; Kurat et al., 1994; Stephan et al., 1995b; Jess- TOF-SIMS studies were performed on two impact berger et al., 2001). residues from the above-mentioned LDEF capture cell ex- First attempts to study organic molecules or in periment A0187-2 (Stephan et al., 1992b) as well as on IDPs using TOF-SIMS have been made in the past (Radicati several impact craters from the Hubble space telescope so- di Brozolo and Fleming, 1992; Stephan et al., 1996), but lar cell array (unpublished data) to reveal the origin of the no systematic investigation has been performed so far. Here impacting particles. the general contamination problem complicates the analy- Prior to the TOF-SIMS analysis of the LDEF impacts, ses. Residual silicone oil as well as carbon-containing em- both samples were investigated with DF-SIMS (Amari bedding materials obstruct a proper examination. et al., 1991). From these analyses, the two projectiles that Oxygen isotopic imaging with TOF-SIMS has been per- had produced the impact features were classiÿed as proba- formed for four IDPs in order to search for sub-micrometer- bly chondritic, based on relative abundances of magnesium, sized (Messenger, 1999). The comparison with aluminum, calcium, titanium, and iron. However, an unam- DF-SIMS results of the same particles (Messenger, 1998) biguous identiÿcation of the true nature of the impacting showed no signiÿcant diKerences in 18O=16O isotopic ra- particle is hampered by the huge elemental fractionation. tios. Nevertheless, within the large statistical error limits With TOF-SIMS, high secondary ion intensities for cop- (30–90 per mill in 18O), the values obtained by TOF-SIMS per were found in both LDEF residues (Stephan et al., are also not discernable from solar ratios. T. Stephan / Planetary and Space Science 49 (2001) 859–906 891

6. Presolar grains

The year 1987 became a major turning point in cosmo- chemistry through the ÿrst discovery and isolation of indi- vidual pristine presolar dust grains (for reviews on presolar dust cf. Anders and Zinner, 1993; Ott, 1993; Zinner, 1998). This discovery was the reward for more than 20 years of ef- forts to locate the carriers of several highly anomalous components in meteorites (Tang et al., 1988; Tang and Anders, 1988a, b). Major types of presolar dust found so far are (Lewis et al., 1987), silicon carbide (Berna- towicz et al., 1987; Zinner et al., 1987), graphite (Amari et al., 1990), (Huss et al., 1992; Hutcheon et al., 1994; Nittler et al., 1994), and (Nittler et al., 1995). Titanium carbide was found inside graphite and sil- icon carbide grains (Bernatowicz et al., 1991, 1992). Alu- minum within silicon carbide and graphite grains showed a correlation with nitrogen, an indication for the presence of aluminum nitride (Zinner et al., 1991a; Virag et al., 1992; Stephan et al., 1997c).

6.1. Isotope measurements

Conventional DF-SIMS has played the major role in the study of presolar grains, because this technique allows to measure isotopic compositions of individual grains down to ∼1 m in size (Zinner, 1998). These isotopic composi- Fig. 34. Negative secondary ion mass spectrum at 13 amu obtained from a presolar SiC grain (KJG2-1412). The peaks can be deconvoluted with tions, often diKerent from mean solar system values by sev- two diKerent methods. In the upper spectrum, the 12C-peak (dotted line, eral orders of magnitude, clearly prove the extrasolar origin top x- and right y-axis) was ÿt to the left edge of the spectrum at 13 amu of these grains. Due to the tremendous magnitude of the (shaded area, bottom x- and left y-axis). Here, the scaling factor 9:9±0:9 anomalies, the required precision for this kind of analysis is directly gives the 12C=13C isotopic ratio. The diKerence between the two 12 1 rather low. spectra (solid line) represents the C H-peak. In the lower spectrum, the 12C-peak is compared with the right edge of the shaded area resulting TOF-SIMS seemed to be a promising technique for the from the 12C1H-peak. Here the 13C-peak is the diKerence of both spectra analysis of individual presolar grains. Especially the high (solid line). The 12C=13C-ratio is 10:9 ± 2:1. Both results within their lateral resolution together with the simultaneous detection uncertainties agree with the DF-SIMS result, 12C=13C=9:795. of the entire mass range were expected to allow new insights into the properties of individual presolar grains. Isotopic studies should be possible but were not the major purpose ter allows using more actual channels of the spectrum for of these investigations. the ÿtting routine and is therefore less susceptible to statis- However, in the ÿrst TOF-SIMS study of presolar grains tical variations. (Stephan and Jessberger, 1996; Stephan et al., 1997c) it was TOF-SIMS allowed a proper analysis of the carbon iso- tried to reproduce already known carbon isotopic ratios for topic ratio for 13 diKerent SiC grains so far (Fig. 35). In silicon carbide (SiC) grains from the Murchison meteorite, most cases, these results agree well with those obtained by previously investigated by DF-SIMS (Hoppe et al., 1994). DF-SIMS (Hoppe et al., 1994). Nevertheless, the limited As discussed before, the major problem for carbon iso- mass resolution only allows the analysis of grains signiÿ- tope analysis at high lateral resolution (necessary for small cantly enriched in 13C compared to chondritic isotope ratios. presolar grains) is the separation of 13C from 12C1H. Since A signiÿcant increase in mass resolution is impeded by the this is typically not possible satisfactorily with TOF-SIMS, required high lateral resolution. mathematical peak deconvolution techniques have to be ap- The analysis of nitrogen isotopic ratios was unsatisfactory plied. In principle, similar to the technique described above due to the limitation in mass resolution. Since 12C15N− can- (Fig. 23), two peak ÿtting routines are possible (Fig. 34): not be resolved from 13C14N−, the relatively high statistical The 12C-peak can be used (1) as a prototype for the 13C-peak error in the 13C analysis prevents an appropriate determina- or (2) to ÿt the 12C1H-peak. Although the ÿrst approach tion of 15N. (Fig. 34 top) seems to be more promising—both carbon iso- Silicon and magnesium isotopic measurements were re- topes should indeed have the same peak shapes—the second cently performed successfully with TOF-SIMS on presolar method (Fig. 34 bottom) is often more appropriate. The lat- SiC grains from Murchison and Semarkona meteorites by 892 T. Stephan / Planetary and Space Science 49 (2001) 859–906

TOF-SIMS has been used to search for possible subunits within SiC grains (Stephan et al., 1997c). TOF-SIMS studies of presolar SiC grain KJG2-0411 re- vealed an inhomogeneous distribution for several elements (Stephan et al., 1997c). Secondary ion images of aluminum, silicon, calcium, and titanium are shown in Fig. 36. Sili- con is distributed homogeneously, whereas aluminum is en- riched in a distinct area, which appeared after sputtering. Consecutive analyses of the grain showed that Al-rich ma- terial is concentrated in the center of this presolar grain. A correlation between Al=Si- and CN=Si-ratios (Fig. 37) indicates again the presence of aluminum nitride (Stephan et al., 1997c). Whether AlN is present as distinct phase or in solid solution with the SiC as suggested before (Bernatow- icz et al., 1992), cannot be clariÿed with TOF-SIMS. The Fig. 35. Comparison of 12C=13C isotopic ratios from TOF-SIMS with those from DF-SIMS for 13 diKerent SiC grains from the Murchison KJG increase of the Al=Si-ratio towards the center of the grain fraction (Amari et al., 1994). All analyses were performed on grains with might be due to destruction of AlN on or near the surface 12C=13C-ratios below the chondritic (CI) value. during chemical processing when the grains were extracted from the meteorite (Stephan et al., 1997c). Although sig- nal intensities are rather low, calcium showed also a corre- Fahey and Messenger (1999). The burst mode (see above) lation with aluminum and nitrogen (Fig. 36). Titanium, on was used in the investigation to increase the secondary ion the other hand, is enriched in a diKerent region of the same 26 data rates. From Mg secondary ion imaging, it was pos- grain, maybe indicative for the presence of TiC. sible to identify several so-called X-grains (Zinner et al., In a type X presolar SiC grain, isotopic heterogeneities 1991b; Amari et al., 1992; Hoppe et al., 1996) among silicon were found recently with TOF-SIMS (Henkel et al., 2000). carbides from Murchison (Fahey and Messenger, 1999). These analyses clearly demonstrate the potential of TOF-SIMS for the investigation of presolar grains. High 6.2. Element mapping with TOF-SIMS lateral resolution in combination with parallel detection of all secondary ion species is ideally suited to localize and However, the imaging capabilities of TOF-SIMS are identify interesting grains or subgrains, even if the ability to much more impressive than its abilities in isotope analysis. measure isotopes is limited. Although no presolar grain has Interstellar grains within interstellar grains (Bernatowicz been found in situ with TOF-SIMS so far, this technique et al., 1991) have been identiÿed with the TEM as titanium should be in principle suited for this kind of search. In situ carbide (TiC) grains inside graphite and SiC grains (Berna- localization would also allow investigating the real link towicz et al., 1991, 1992). Although a general correlation between SiC and AlN, because chemical processing during of aluminum and nitrogen was found for SiC grains (Zinner isolation of the grains may selectively destroy AlN. The et al., 1991a; Virag et al., 1992), no separate Al-bearing major advantage of in situ analysis is, however, that infor- phase like aluminum nitride (AlN) has been detected with mation about the petrographic environment of the presolar the TEM (Bernatowicz et al., 1992). Therefore, imaging grains could be obtained. This would help to clarify possible

Fig. 36. TOF-SIMS images of SiC grain KJG2-0411 show a rather heterogeneous distribution of aluminum, calcium, and titanium. Field of view is 12 × 12 m2. The sample was scanned with 128 × 128 pixels and 800 shots=pixel. T. Stephan / Planetary and Space Science 49 (2001) 859–906 893

ergy, and data transmission constraints. Compared to other types of mass spectrometers, the time-of-.ight principle should be advantageous. Since no magnet or other heavy components are needed, the mass of the instrument can be kept comparatively low. Power consumption can also be low for this kind of mass spectrometer. Time-of-.ight mass spectrometers were therefore used and are still used aboard several space probes. Three missions to comet 1p=Halley applied TOF mass spectrometry after impact ionization of dust particles with high relative velocities (Kissel and Krueger, 1987a; Hornung and Kissel, 1994). The PIA (par- ticle impact analyzer) instrument on Giotto and the PUMA (pyle-udarnyi mass analizator, pyle-udarnyi mass analizator, Russian for dust impact mass analyzer) instru- Fig. 37. Al=Si and CN−=Si− show a linear correlation that indicates AlN ments on both Vega missions to comet Halley (Kissel et al., as the dominating aluminum-bearing phase in KJG2-0411. Data point (A) 1986a, b) delivered a wealth of information on the elemen- corresponds to the Si-rich, Al-poor area in the lower left of the particle tal and isotopic composition of Halley’s dust (Jessberger et in Fig. 36. (B) is the Al-rich area to the right. (C) is the titanium hot al., 1988, 1989; Jessberger and Kissel, 1991). A modern spot. (D) represents an earlier measurement of the entire particle before sputtering. variant of these instruments, CIDA (cometary and interstel- lar dust analyzer), was launched in February 1999 aboard the Stardust space vehicle to comet 81p=Wild 2 (Brownlee et al., 1996, 1997). connections of presolar grains with other constituents of the The ÿrst SIMS instrument envisioned for space .ight was respective meteorites, e.g., CAIs, , or matrix. an astronaut operated DF-SIMS instrument for on-site anal- ysis of the lunar surface in the framework of the Apollo missions (Rudenauer,Q 1994), which however never .ew. 7. TOF-SIMS in comparison The DION system on both Soviet space probes to the Mar- tian satellite Phobos was the “largest” SIMS instrument ever To put TOF-SIMS into an analytical context, a com- built. Secondary ions sputtered from the surface were to parison with other micro-analytical techniques used in be analyzed in a quadrupole mass spectrometer at a cruis- cosmochemistry—DF-SIMS, TOF-SNMS, laser desorp- ing altitude of 50 m above Phobos (Balebanov et al., 1986; tion TOF-SNMS, EMPA (electron microprobe analysis), Hamelin et al., 1990; Rudenauer,Q 1994). A time-of-.ight PIXE, SXRF (synchrotron X-ray .uorescence analysis), instrument was also on board both Phobos missions, the and RBS (Rutherford backscattering)—is given in Table 4. LIMA-D experiment (Sagdeev et al., 1986; Pellinen et al., A more comprehensive description of conventional SIMS 1990). The plan was to use the cruising altitude of 50 m as techniques, various instruments, and their applications in .ight path for ions generated by high-energy laser pulses. cosmochemistry can be found in Ireland (1995). Micro- Unfortunately, both spacecraft were lost prior to arrival at probe techniques in Earth are explained in detail Phobos. by Potts et al. (1995). The numbers for spatial resolution, A new era in TOF-SIMS will hopefully commence in information depth, and detection limits provided in Table 4 2011, when the ÿrst instrument of this type is planned to are typical values for instruments used in this ÿeld, which reach comet 46p=Wirtanen on ESA’s spacecraft. Af- may not always re.ect the most recent developments in ter reaching its target, it is planned to collect cometary dust the respective techniques. However, these numbers clearly intact at low relative speed. The dust particles then will demonstrate their major diKerences, which often complicate be analyzed in a TOF-SIMS instrument named CoSIMA a proper comparison of the various results. The diKerent (cometary secondary ion mass analyzer; Kissel et al., 1988). methods also vary in the type of information they pro- (cometary matter analyzer), an earlier version of vide. For the analysis of large organic compounds, only this instrument, was also designed by J. Kissel to .y on “soft” ionization methods are suitable and the simultane- NASA’s cometary rendezvous and asteroid .yby mission ous detection of all ion species like in time-of-.ight mass (CRAF) that was cancelled in 1992 (Zscheeg et al., 1992; spectrometry is required. Beck, 1994; Beck and Kissel, 1994). With the CoSIMA in- strument, a lateral resolution of 5 m with 100 ns primary ion pulse width can be obtained. In a second mode, with 8. TOF-SIMS in space 3 ns pulse width, 20 m spatial resolution is possible. The mass resolution, up to m=Vm = 2000, is su4cient for the For analysis in space, rugged and simpliÿed techniques separation of most hydrocarbon interferences from elemen- are required. Limitations mainly arise from size, mass, en- tal peaks. The CoSIMA results will certainly not reach the 894 T. Stephan / Planetary and Space Science 49 (2001) 859–906 1 ppb 1 ppb 10 ppb 10 ppb 1–100 ppm 1–1% 1–5 ppm : : : ¿ ¿ ¿ ¿ 200–1000 ppm 0 0 a a g g g Yes Yes a a mNo mNo0 mNo mNo 100 nm Yes 1 nm Yes 1nm 1nm ¿ ¿ ¡ ¡ m f m m m –1 b m30 m50 m15 m1 05 2–10 2–10 : : : 0 1–40 d d Yes Yes d d Yes Yes MS (Kovalenko et al., 1992). 2 d d L Yes Yes m for d d 11 Yes12 (Mg) Yes No No No 0 No 1 16 (S) No No No 1 1NoNoNo1 1 Yes4 (Be) Yes No Yes No 0 No 2 1 ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿ Z Z Z Z Elements Isotopes Molecules Organics Z Z Z Z i X-rays i h Ions Neutrals c e m for the CHARISMA instrument (Nicolussi et al., 1998a), 40 Depending on photon energy and .ux density. Synchrotron radiation. Structural modiÿcations possible. With the Cameca Nanosims 50. Only few atomic monolayers. 1 Resonant or non-resonant laser ionization of secondaryResonant neutrals or sputtered non-resonant by laser primary ionization ions. of neutrals after laser desorption. Energy measurement of backscattered protons or heavier particles. a b c d e f g h i TOF-SNMS TOF-SNMS DF-SIMSLaser desorption Ions PhotonsPIXESXRF Neutrals Ions Protons X-rays X-rays RBS Protons Technique Primary particles Secondary particles Information obtained on Spatial resolution Information depth Destructiveness Detection limits TOF-SIMS IonsEMPA Ions Electrons X-rays Table 4 Comparison of micro-analytical techniques used in cosmochemistry T. Stephan / Planetary and Space Science 49 (2001) 859–906 895 same quality as laboratory TOF-SIMS analyses, but this without their complete destruction is required. Here, a wealth cannot be expected from an instrument with mass less than of new discoveries from TOF-SIMS studies can be expected 20 kg and primary power between 8.2 and 19:5 W (techni- in the future. cal data J. Kissel, pers. comm.). However, it is expected, that profound information on the composition of cometary matter will be obtained with CoSIMA. The experience with Acknowledgements TOF-SIMS analysis of extraterrestrial matter gathered here on Earth and reported in the present study will deÿnitely The author is indebted to many people who made this help to interpret the results that will be acquired in space. work possible. A special thank goes to E.K. Jessberger for Especially the surface analysis of IDPs deals with problems his extensive support. Superb working conditions were also as expected for the space experiment, e.g., rough surfaces. provided by A. Benninghoven, D. StQo\er, and J. Kissel. Since complex mixtures of organic material and silicates are Many colleagues and friends in MunsterQ and Heidelberg sup- expected for cometary dust (Kissel and Krueger, 1987b), ported me throughout the past decade and contributed with laboratory TOF-SIMS investigation of organic compounds their work to this study, especially D. Rost, the compan- in analogous material, e.g., chondrites and IDPs is essential ion for more than 6 years of TOF-SIMS research, and C.H. (Heiss et al., 1998). Heiss, T. Henkel, and S. Schirmeyer. Expert sample prepa- ration by U. Heitmann is gratefully acknowledged. I also thank S. Amari, P. Hoppe, F.J. Stadermann, and E.K. Zin- ner for not only providing samples, but also for making their 9. Summary DF-SIMS data available, and for oKering inside informa- tion on the ion microprobe technique. I thank K.P. Jochum Time-of-.ight secondary ion mass spectrometry has been for providing the glass standards used in this study. The successfully introduced into cosmochemistry during the past manuscript beneÿted from critical reviews of two anony- decade and has demonstrated its potential. Its major advan- mous reviewers. This work was supported by the Deutsche tage results from the simultaneous detection of all secondary Forschungsgemeinschaft through grants Je96=6 and Je96=10 ions with one polarity. This in combination with high lat- and a special grant from the Ministerium fur Schule und eral resolution qualiÿes TOF-SIMS for the analysis of small Weiterbildung, Wissenschaft und Forschung des Landes samples, becoming more and more important in the inves- Nordrhein-Westfalen. tigation of extraterrestrial matter. Although mass resolution and quantiÿcation capabili- Appendix ties in TOF-SIMS are often inferior to conventional ion microprobes, the advantages of TOF-SIMS open speciÿc A suite of eight diKerent glass standards was used to cali- ÿelds of application, not or less accessible with DF-SIMS. brate our TOF-SIMS. These were prepared by fusing Consequently, these two SIMS techniques are rather com- samples of basalt, andesite, komatiite, peridotite, tonalite, plementary than in competition with each other. Parallel de- and rhyolite. They have been analyzed by diKerent bulk and tection of all ions expressly does not require a pre-selection microanalytical techniques in various laboratories to deter- of masses to be analyzed. Therefore, TOF-SIMS is predes- mine their composition of major, minor, and trace element. tined to make unexpected discoveries. But then, for quan- A description of the samples, their preparation, and analyt- tiÿcation, subsequent investigations with other techniques ical results are given by Jochum et al. (2000). including DF-SIMS may be required. Secondary ion sensitivities relative to silicon are com- Further technical developments in TOF-SIMS will help prised in the following tables for all eight glass standards. to overcome some limitations: A new generation of liquid Data are given for positive as well as negative secondary metal ion guns allows to increase the primary ion current— ions, with simultaneous argon sputtering and after sputter- and the secondary ion intensity—even at high lateral resolu- ing, respectively. All glass standards were analyzed with tion. Decreasing the primary beam size below 100 nm will the gallium source at ∼600 ps primary pulse width, except extend the application to very small entities. Especially laser StHs6=80-G, which was measured at ∼5 ns pulse width. post-ionization is expected to improve quantiÿcation. Nev- Errors from the uncertainties in compositional data or ertheless, even today TOF-SIMS is a powerful tool for the TOF-SIMS counting statistics are not given, since they are investigation of natural terrestrial and extraterrestrial sam- typically much smaller than the diKerence between values ples. Applications near the borderline between life sciences from individual standards. and cosmochemistry—studying the origin of life in our so- Element abundances in Tables A1–A8 are given accord- lar system—might be one promising ÿeld for TOF-SIMS. ing to Jochum et al. (2000) in wt% for major elements and First results, although negative for ALH 84001, were high- ppm (g=g) for trace elements. Uncertain values are given lighting the potentials of TOF-SIMS. in parentheses. For the chlorine sensitivity in Fig. 22, the Particle analysis is already one of the strong points of minimum value obtained from standard ATHO-G during TOF-SIMS. A maximum of information on single grains sputtering (Table A1) was used. 896 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Table A1 Secondary ion sensitivities for glass KL2-G prepared from a basalt from the Kilauea volcano of Hawaii

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 5.4 6.3 6.5 Be 0.88 1.4 1.1 B 2.6 1.3 0.77 O (%) 45.2 0.00086 0.00079 30 45 Na (%) 1.68 21 20 Mg (%) 4.37 5.8 6.5 Al (%) 6.93 7.6 7.4 Si (%) 23.4 1 1 1 1 P 1080 0.030 0.026 1.9 1.6 Cl 30 (340) (4900) K 3900 34 33 Ca (%) 7.79 12 12 Sc 32.3 7.9 4.1 Ti (%) 1.53 4.7 4.3 V 370 3.6 3.6 Cr 306 2.7 2.6 Mn 1270 3.6 3.9 Fe (%) 8.24 1.9 1.9 Co 41.6 1.3 1.3 Ni 117 0.85 Cu 95 0.90 1.2 Rb 8.9 59 60 Sr 364 12 11 Ba 123 9.0 8.7

Table A2 Secondary ion sensitivities for glass ML3B-G prepared from a basalt from the Mauna Loa volcano of Hawaii

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 4.2 13 23 Be 0.75 1.1 2.4 B 2.2 0.84 1.0 O (%) 45.8 0.0010 0.00089 32 53 Na (%) 1.74 18 29 Mg (%) 3.94 8.1 10 Al (%) 7.04 7.3 7.5 Si (%) 23.7 1 1 1 1 P 960 0.025 3.4 2.6 K 3200 37 56 Ca (%) 7.43 15 15 Sc 31.4 7.7 5.1 Ti (%) 1.26 7.5 6.1 V 260 5.7 6.3 Cr 170 7.6 10.6 Mn 1370 3.8 4.8 Fe (%) 8.39 1.8 2.6 Co 41.4 0.79 2.0 Cu 115 0.33 0.47 Rb 5.78 49 93 Sr 315 14 14 Cs 0.142 65 59 Ba 80.1 10 18 T. Stephan / Planetary and Space Science 49 (2001) 859–906 897

Table A3 Secondary ion sensitivities for glass StHs6=80-G prepared from an andesite from the St. Helens (USA) eruption

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 19 11 14 Be 1.4 1.8 1.5 B 13 0.52 0.71 O (%) 47.4 0.00091 0.00063 23 57 Na (%) 3.39 22 22 Mg (%) 1.19 7.0 8.1 Al (%) 9.33 6.6 7.1 Si (%) 29.8 1 1 1 1 P 700 1.4 Cl 260 (720) K 10700 35.5 26 Ca (%) 3.76 13.6 15 Ti (%) 0.42 6.4 6.1 V 96 4.0 Mn 590 5.8 3.4 Fe (%) 3.39 3.1 3.4 Rb 29.5 58 37 Sr 486 16 15 Cs 1.89 47 Ba 302 14 14

Table A4 Secondary ion sensitivities for glass GOR128-G prepared from a komatiite from the Gorgona Island, Colombia

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 7.3 22 25 Be 0.04 3.8 B 22 0.58 0.63 O (%) 44.6 0.0012 0.0010 24 40 Na (%) 0.417 31 32 Mg (%) 15.6 9.0 9.6 Al (%) 5.2 8.7 8.3 Si (%) 21.5 1 1 1 1 P 120 0.023 2.1 K 300 50 53 Ca (%) 4.39 19 16 Sc 31.1 11 5.5 Ti (%) 0.168 7.5 6.0 V 170 5.9 5.1 Cr 2150 5.8 5.6 Mn 1390 4.2 4.3 Fe (%) 7.58 2.1 2.1 Co 79.5 1.4 1.8 Ni 1070 1.2 0.74 Cu 63 0.90 2.0 898 T. Stephan / Planetary and Space Science 49 (2001) 859–906

Table A5 Secondary ion sensitivities for glass GOR132-G prepared from a komatiite from the Gorgona Island, Colombia

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 7.9 15 23 B 18 0.44 0.62 O (%) 44.1 0.0011 0.0010 26 40 Na (%) 0.592 25 29 Mg (%) 13.5 8.5 10 Al (%) 5.77 8.0 8.4 Si (%) 21.3 1 1 1 1 P 170 1.7 K 270 37 45 Ca (%) 6.07 17 15 Sc 34.6 7.2 Ti (%) 0.18 6.4 5.8 V 189 5.4 5.7 Cr 2410 5.5 6.2 Mn 1180 4.4 5.1 Fe (%) 7.93 1.8 2.6 Co 88.2 1.1 2.1 Ni 1170 0.67 1.3 Cu 180 1.5 Rb 2.14 78 Sr 15.6 13 Cs 8.08 47 56

Table A6 Secondary ion sensitivities for glass BM90=21-G prepared from a peridotite from the Ivrea Zone,

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 1.4 20 15 B 2.8 0.93 1.4 O (%) 45.9 0.00097 0.00092 23 35 Na (%) 0.0838 24 19 Mg (%) 20.6 8.1 8.3 Al (%) 1.23 7.6 6.9 Si (%) 24.9 1 1 1 1 K316559 Ca (%) 1.5 16 11 Sc 11.3 7.5 7.1 Ti (%) 0.037 6.5 4.9 V 40 8.1 6.2 Cr 2100 5.6 5.1 Mn 829 4.5 4.4 Fe (%) 5.24 2.3 2.6 Co 88.5 1.5 1.9 Ni 1880 0.83 1.2 Cs 1.2 45 T. Stephan / Planetary and Space Science 49 (2001) 859–906 899

Table A7 Secondary ion sensitivities for glass T1-G prepared from a tonalite from the Italian Alps

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 19.8 8.5 9.8 Be 2.4 1.3 1.7 B 4.6 0.88 0.79 O (%) 46.6 0.00083 0.00078 33 56 Na (%) 2.32 18 16 Mg (%) 2.25 7.1 7.8 Al (%) 9.05 7.6 6.6 Si (%) 27.3 1 1 1 1 P 772 0.043 0.039 3.0 1.6 Cl 86 (860) (3000) K 15900 34 27 Ca (%) 5.05 14 11 Sc 26.7 8.6 4.2 Ti (%) 0.445 6.7 5.2 V 190 5.6 4.8 Cr 20 6.5 6.2 Mn 1020 4.1 4.3 Fe (%) 5.02 2.2 2.4 Co 18.9 1.3 1.8 Cu 21 0.90 Rb 79.3 40 30 Sr 282 15 11 Cs 2.85 44 37 Ba 393 17 10

Table A8 Secondary ion sensitivities for glass ATHO-G prepared from a rhyolite from Iceland

Element Abundance Positive with Positive after Negative with Negative after Ar-sputtering Ar-sputtering Ar-sputtering Ar-sputtering

Li 27.6 5.9 7.7 Be 3.7 0.78 1.5 B 5.8 0.46 0.75 O (%) 49.0 0.00073 0.00071 30 53 Na (%) 3 11 24 Mg (%) 0.068 4.5 6.9 Al (%) 6.16 5.4 5.5 Si (%) 35.5 1 1 1 1 P 110 0.066 0.040 2.9 Cl 400 140 (450) K 22100 14 24 Ca (%) 1.18 9.5 12 Sc 5.03 6.5 4.1 Ti (%) 0.145 5.6 4.6 V 4.4 3.2 5.1 Cr 5.3 3.6 4.2 Mn 790 3.6 5.3 Fe (%) 2.42 2.5 3.6 Co 2.2 2.0 Rb 63.8 14 25 Sr 95.8 9.7 9.8 Cs 1.31 57 Ba 549 10 8.8 900 T. Stephan / Planetary and Space Science 49 (2001) 859–906

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