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Mass Spectrometry Isotope Geochemistry W. M. White Appendix: Mass Spectometry APPENDIX: MASS SPECTROMETRY A.1 SAMPLE EXTRACTION AND PREPARATION Isotopic analysis can be performed on minerals, rocks, soil, tissue, gases, and liquids – just about any- thing. In view of the broad possibilities, we will only cover a few generalities here. In most instances, the element to be analyzed must be first isolated and purified before it can undergo isotopic analysis in a mass spectrometer. Purification is necessary to avoid isobaric interferences and well as to enhance ionization efficiency, This can be done in a number of ways: • Dissolving the sample and then chemically purifying it (a process that often involves chroma- tography) • Reacting the sample with a reagent to produce a gas. For example, oxygen isotopic analysis of silicates involves reacting the sample with a fluorine compound such as bromine pentafluoride to produce CO2 gas; in the case of carbonates, the process is similar but the reagent is phospho- ric acid. Purification is then achieved either cryogenically, or more commonly, using a gas chromatograph. For organic compounds, pyrolysis (thermal decomposition) may preceed such reactions. • Heating or fusing a sample in vacuum to release a gas. This is the usual method for noble gases. Individual gas components are separated cryogenically. • Noble gases contained in vesicles and inclusions can be released by crushing in vacuum. • If the sample is a liquid, the element of interest can be extracted chromatographically from the solution. With instruments in which the analyte is a gas or a liquid, a chromatograph can be used to select a spe- cific compound, for example the alkenones discussed in Chapter 10. This technique is termed com- pound-specific isotopic analysis. Micro-analytical techniques in which solids can be directly analyzed have become increasingly im- portant in the last several decades. The first of these is secondary ion mass spectrometry (SIMS), in which an ion beam (generally Cs or O) is fired at at a polished surface to produce secondary ions that are then swept into the mass spectrometer. An alternative approach is laser ablation in which a finely focused laser is fired at a polished surface ablating a small pit. The ‘debris’ consists of very fine particles that are swept up by a carrier gas, generally argon and carried into the mass spectrometer. A variation of this technique is laser fluorination, in which a laser is focused on a small spot of the sample in the presence of fluorine gas, producing a reaction that releases the element of interest (for example, oxygen or argon). A.2 THE MASS SPECTROMETER In most cases, isotopic abundances are measured by mass spectrometry. One exception is, as we have seen, short-lived radioactive isotopes, the abundances of which are determined by measuring their de- cay rate, and in fission track dating, where the abundance of 238U is measured, in effect, by inducing fis- sion. Another exception is spectroscopic measurement of isotope ratios in the Sun and stars. Frequen- cies of electromagnetic emissions of the lightest elements are sufficiently dependent on nuclear mass that emissions from different isotopes can be resolved. This approach is useful in astronomy not only because it is the only possibility, but also because isotope ratios stars show very large variations. This technique is not sufficiently precise for most geochemical problems. A mass spectrometer is simply a device that can separate atoms or molecules according to their mass. There are a number of different kinds of mass spectrometers operating on different principles. Undoubtedly, the vast majority of mass spectrometers are used by chemists for qualitative or quantitative analysis of organic compounds. We will focus exclusively, however, on mass spectrometers used for isotope ratio determination. Most iso- 461 December 4, 2013 Isotope Geochemistry W. M. White Appendix: Mass Spectometry tope ratio mass spectrometers are of a similar design, the magnetic-sector, or Nier mass spectrometer*, a schematic of which is shown in Figure A.1. It consists of three essential parts: an ion source, a mass analyzer and a detector. There are, however, several variations on the design of the Nier mass spec- trometer. Some of these modifications relate to the specific task of the instrument; others are evolution- ary improvements. We will first consider the Nier mass spectrometer, and then briefly consider a few Figure A.1. The magnetic sector or Nier mass spectrometer. This instrument uses a 60° magnetic sec- tor, but 90° magnetic sectors are also sometimes used. other kinds of mass spectrometers. A.2.1 The Ion Source As its name implies, the job of the ion source is to provide a stream of energetic ions to the mass ana- lyzer. Ions are most often produced by either thermal ionization, for solid-source mass spectrometers, electron bombardment, for gas-source mass spectrometers, or by inductively exciting a carrier gas into a plasma state in the case of inductively coupled plasma–mass spectrometers (ICP-MS). In thermal ionization ass spectrometry (TIMS), used for metals such as Sr, Nd, and Pb, a solution con- taining the element of interest is dried or electroplated onto a ‘filament’, a think ribbon of high- temperature metal, such as Re, Ta, or W, welded to supports. The ribbon is placed in the instrument and heated under vacuum by passing an electric current of several amperes through it, the sample evaporates, and some fraction of the atoms, depending on the element, ionize. The alkali metals ionize quite easily; the ionization efficiency for Cs, for example, approaches 100%. For some other elements, it can be 0.1% or less. In some cases, multiple filaments are used to achieve higher ionization efficiency. + - Ions produced in this way may be either the pure metal, for example Sr , or a radical such as OsO4 . Ionization efficiency can sometimes be increased by using a suitable substrate with a high work func- tion. The greater energy required to evaporate the atom results in a higher likelihood of its also being ionized. Tantalum oxide, for example, is a good substrate for analysis of Sr. Ionization efficiency can also be increased by altering the chemical form of the element of interest so that its evaporation tem- perature is increased (ionization is more likely at higher temperatures). For example, when a silica gel suspension is loaded along with Pb, the evaporation temperature of Pb is increased by several hundred degrees, and the ionization efficiency improved by orders of magnitude. Finally, the sample may be loaded in a particular chemical form in order to (1) form a positive rather than negative ion (or visa versa) and (2) provide a molecule of high mass, such as cesium metaborate where boron is the element of interest, to minimize mass fractionation, or to promote or inhibit the formation of oxides. * It was developed by Alfred Nier of the University of Minnesota in the 1930's. Nier used his instrument to determine the isotopic abundances of many of the elements. In the course of doing so, however, he observed variations in the ratios of isotopes of a number of stable isotopes as well as Pb isotopes and hence was partly responsible for the fields of stable and radiogenic isotope geochemistry. He also was the first to use a mass spectrometer for geochronology, providing the first radiometric age of the solar system. In the 1980's he was still designing mass spectrometers, this time miniature ones which could fly on spacecraft on interplanetary voyages. These instruments provided meas- urements of the isotopic composition of atmospheric gases of Venus and Mars. Nier died in 1994. 462 December 4, 2013 Isotope Geochemistry W. M. White Appendix: Mass Spectometry Electron bombardment is used when the analyte is a gas, such as CO2, H2, or N2. The gas is flow slowly into the evacuated source through a small orifice. In this case, a current is passed through a filament, normally Re, giving off electrons that strike the analyte molecules, and ionize them by knocking off one + + of the electrons. Except for noble gases, the ion is generally a polyatomic species such as H2 or CO2 . Most thermal ionization mass spectrometers employ a turret source in which a number of samples (typically 6 to 20) can be loaded. The turret is rotated to bring each sample into position for analysis. Gas source mass spectrometers often employ automated gas inlet systems, which allow for automated analysis of many samples. They also have so-called dual inlet systems that allow for rapid switching between a sample and a standard gas, insuring both are analyzed under similar conditions allowing correction for instrumental mass fractionation and other effects. A third type of ion source is an inductively coupled plasma (ICP), which is slowly replacing thermal ionization for many applications. An ICP operates by passing a carrier gas, generally Ar, through an induction coil in which an electric current alternates at radio frequencies. This excites the gas into ~7000°K plasma, in which the gas is completely ionized. The analyte is aspirated, generally as a so- lution, into the plasma and is ionized by the plasma. Alternatively, the sample can be introduced into the gas stream as an aerosol produced by laser ablation. The ions flow through an orifice into mass spectrometer. Initially, quadrupole mass spectrometers, which are commonly used for analysis of or- ganic compounds, were employed for these instruments. However, quadrupoles cannot achieve the same level of accuracy as magnetic sector instruments, although they are widely used for elemental analysis. Magnetic sector ICP-MS instruments came on the market a decade after quadrupole ICP-MS instruments and now achieve accuracies competitive with thermal ionization instruments. Combined with their generally higher ionization efficiency and hence higher sensitivity, they produce results that are superior to thermal instruments for several elements.
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