Philip S.H. Wong The operating principles of linear quadrupoles and quadrupole ion traps Bioanalytical Systems West Lafayette, IN are described, and the performance characteristics of triple quadrupoles 47906-1382 and ion trap instruments are compared. The theoretical basis for mass R. Graham Cooks* analysis using quadrupole fields is also described. The high Department of Chemistry performance of quadrupole ion traps is illustrated by introducing some Purdue University West Lafayette, IN new developments, including mass range extension, high resolution 47907 experiments, MSn experiments, selective ion manipulation techniques, *Corresponding author and non-destructive ion detection. E-mail: [email protected]
Mass spectrometry, the science and particles with electric and/ or mag- Quadrupoles and Ion Traps technology of gaseous ions (1), has netic fields. T1 summarizes the as its basis the measurement of most common mass analyzers and With the above background on mass-to-charge ratios (m/z) of ions. lists some analytical performance ionization and mass analysis, we All atomic and molecular ions are, characteristics by which they can can now introduce a family of mass in principle, accessible by mass be compared (6). As with the vari- analyzers whose operation is based spectrometry, making it a universal ous ionization methods, there is no on ion motion in rf electric fields. method for chemical analysis. Its single right choice — the nature of The quadrupole mass filter (8), or implementation requires suitable the problem and the resources of linear quadrupole, consists of a lin- methods of ion generation, ion the laboratory will dictate which ear array of four symmetrically ar- analysis, and ion detection. We treat mass analyzer is most appropriate. ranged rods (F1) to which rf and dc each of these processes in turn and Most uses of mass spectrome- voltages are supplied. Forces are show below that there are multiple try are made in combination with exerted in a plane normal to the di- methods of accomplishing each. chromatographic separation, princi- rection (z-direction) in which the The first step in recording a pally in the form of the GC/MS or ions drift through the array in their mass spectrum is to convert analyte LC/MS technique. These combina- journey from the ion source to the molecules (or atoms) into gas phase tions have been used, for example, detector. The rf potential gives rise ions. In biological applications, the in organic analysis in the environ- to a field which alternatively rein- most common ionization tech- mental sciences and in charac- forces and then dominates the dc niques are electrospray ionization terization of biological compounds, field, also applied by coupling op- (ES) (2), atmospheric pressure including molecular weight (MW) posite sets of rods. Ions oscillate in chemical ionization (APCI) (3,4), determinations, and sequence the x,y-plane with frequencies and matrix-assisted laser desorption analyses of biopolymers (7). In- which depend on their m/z values ionization (MALDI) (5). These are creasingly important applications and with excursions which depend soft ionization methods in the sense have been found in drug metabo- on the amplitudes of the applied that at least some analyte molecules lism and protein sequencing due to potentials and their initial positions. are converted, intact, into corre- the high sensitivity and chemical If the oscillations of an ion in this sponding ions. Solution phase sam- specificity of mass spectrometry. plane are stable, the ion will con- ples are examined with ES and These advantages apply even when tinue to drift down the rod assem- APCI while MALDI is particularly the samples are presented to the bly and reach the detector. Stable appropriate for solid phase samples. mass spectrometer as mixtures oscillations are only achieved by Having successfully generated gas since the two-stage tandem mass ions of given m/z values for a given phase ions, they must then be mass spectrometry (MS/MS) experiment rod assembly, oscillation frequency, analyzed. There are several differ- serves as a method of separation as rf voltages, and dc voltage. The ent types of mass analyzers, all well as characterization of the sepa- range of values of m/z which corre- based on the interactions of charged rated components. spond to stable motion can be made T1 Quantity Mass/charge Resolution at Dynamic Characteristics of Differ- Method Measured (m/z) range m/z = 1,000 Range ent Mass Analyzers. (Adapted from 4 5 7 reference 6.) Sector Magnet momentum/charge 10 10 10 Time of Flight flight time 106 103 -104 104 Ion Cyclotron cyclotron Resonance frequency 105 106 104 Ion Trap frequency 104 104 104 Quadrupole filters mass filter for m/z 103 -104 103 -104 105
very large (wide band pass) or it F1 Quadrupole Mass Filter can be a single m/z value (narrow Schematic diagram showing the operation band pass). In practice, ions of a of the quadrupole mass particular m/z value are often se- filter. Note that as ions drift through the array lected, and mass scanning is usually of rods they are sub- Ion Source Detector achieved by sweeping the dc and rf jected to forces which cause oscillation in the voltages, keeping their ratio and the x,y-plane (shaded). oscillator frequency constant. The quadrupole ion trap, X (9,10), the subject of this article, is the three dimensional analogue of the linear quadrupole mass filter. In this device too, ions are subjected to Z forces applied by an rf field but the Y forces occur in all three, instead of F2 just two, dimensions. Stable motion The ion trap consists of of ions in the linear quadrupole al- three electrodes with hy- perbolic surfaces, the lowed ions freedom of motion in central ring electrode, one dimension (z-direction); in the and two adjacent end- cap electrodes. The ion trap, stable motion allows no schematic of the assem- degrees of freedom. Hence, ions are bly shows how the elec- trodes are aligned and trapped within the system of three isolated using ceramic Ions in Zo Ions out electrodes-a ring electrode and two spacers and posts. The Ion Source Detector device is radially sym- ro end-cap electrodes of hyperbolic metrical, and ro and zo cross-section (F2). The principal represent its size. advantages of the quadrupole ion trap in chemical analysis can be summarized as follows:
(i) high sensitivity, (ii) compactness and mechanical simplicity in a device which is Endcap Ring Endcap nevertheless capable of high Ceramic post performance, and spacer (iii) tandem mass spectrometry ex- periments are available by per- forming sequential mass analy- sis measurements, (iv) ion/molecule reactions can be studied for mass-selected ions, (v) high resolution (>106 at m/z >1000) is accessible through slow scans, but mass measure- ment accuracy is relatively poor, Endcap Ring Endcap F3 Φ formance characteristics, one im- Potential and field mediately notes that a unique fea- strength in a hypothetical one-dimensional quadru- ture of an ion trap is that MS/MS pole field. The slope of experiments are possible. Even the plot of potential (Φ) against position (x) yields when compared with a triple quad- the field strength E(x). rupole MS/MS instrument, the ion trap can perform multiple stage mass spectrometry (MSn) simply by the use of additional operations which are performed sequentially in time. The triple quadrupole has the advantage of access to parent F=f(x)2 ion and neutral loss scans, and ana- E=f(x) Φ lytically useful versions of the Field = E = - d dx MS/MS experiment; however, MSn experiments can only be performed X in multi-quadrupole instruments. Although ion/molecule reactions can be studied in both instruments, (vi) ions of high mass/charge are Comparisons of the Ion Trap the reaction time can only be varied accessible using resonance ex- with the Triple Quadrupole in the ion trap. This allows the ki- periments, and netics and equilibrium of ion-mole- The differences in operating cule reactions to be studied. On the (vii) non-destructive detection is principles of the linear quadrupole other hand, the triple-quadrupole available using Fourier trans- and the ion trap have just been de- instrument provides good control form techniques. scribed. In comparing their per- over the kinetic energies of the ions
F4 Potential used for trap- ping ions (A) in the radial direction and (B) in the ax- ial direction. An ion in the (A) position shown is acceler- ated away from the trap center in the axial direc- tion at the rf phase V shown in (A) and towards it in (B) (Adapted from ref- erence 13).
-Z o ro Z -r Axial Dimensiono o Radial Dimension
(B)
V
-Z o ro Z -r Axial Dimensiono o Radial Dimension F5 If it were possible to employ a az The Mathieu stability Operating line for system of electrodes and construct a diagram for the quadru- pole ion trap. Ions are mass selective instability one-dimensional quadrupole field, stable in both the r- then the potential distribution and and the z-direction if their Mathieu parame- the field strength would be as ters az and qz fall within 0.4 shown in F3. Ions located an in- the shaded area in the diagram. The common creasing distance from the center mode of mass analysis z stability would be subjected to a force which is the mass-selective in- would increase linearly with dis- stability scan in which 1.0 the rf potential is raised 0.2 0.8 placement and which would tend to to increase the value of 0.7 0.1 q = 0.908 return the ions to the center of the qz to the instability point 0.6 eject qz = 0.908, while az =0. 0.5 0.4 device. Ions could be trapped in such 0.3 a hypothetical field. If the field direc- 0.2 1.0 1.5 tion were reversed, a potential maxi- 0.4 mum would occur and ions would be qz accelerated away from the center. 0.5 In the three-dimensional quad- rupole field present in an actual ion -0.2 0.6 trap, ions are alternatively subjected to stabilizing and destabilizing r stability 0.7 forces and oscillate in both the r- and z-directions. When the phase of -0.4 0.8 the rf signal is positive, the quadru- pole potential surface is saddle- 0.9 Operating line for shaped as shown in F4A. An ion mass selective stability located as shown is on a potential -0.6 downhill in the z-direction and it will be accelerated from the center of the device. As the rf field changes sign, the field inverts and the same ion is accelerated towards the center of the trap (F4B). Similar which are important for thermo- quadrupole ion trap, an increasing considerations apply with respect to chemical studies. number of analyses will be per- an ion displaced in the radial (r) di- A recent, instructive compari- formed with ion traps coupled with rection. If the field inverts at an ap- son of the Finnigan LCQ ion trap ion sources (ES or APCI) which al- propriate rate, the ions will be with the Finnigan TSQ 700 triple low solution analysis. trapped in both the r- and z-direc- quadrupole mass spectrometer was tions, in the volume defined by the made using an LC/APCI/MS assay Operating Principle of ring and the end-cap electrodes for several spinosyns (11). The Quadrupole Ion Traps (13). overall sensitivity of the LCQ in the Mass Analysis Using Quadru- full-scan mode was found to be 5- Quadrupole Fields A quadrupole field is one in pole Fields 10 times greater than the TSQ. In Physically, ion traps are made contrast, in the selected ion moni- which the field strength E varies linearly with displacement x, up of a rotationally symmetrical toring mode, in which a single ion ring electrode of hyperbolic shape is monitored, the TSQ was found to and two endcap electrodes of the Ε=ΕΟ x (1) be 3-5 times more sensitive than the same cross-section. An rf voltage is LCQ. Similar results were obtained The applied potential Φ which es- applied to generate an electric in a comparative study of the LCQ tablishes the electric field must vary quadrupole field. Because the elec- ion trap and the PE/Sciex API 300 quadratically in order that the field tric field is rotationally symmetric, it triple quadrupole instrument using strength vary linearly with x. Hence is convenient to consider only radial LC/MS/MS quantitation of orlistat r= xy22+ and axial (z) displace- in human plasma (12). Clearly, both Φ = 2 (2) Φ f(x) ments. The potential ( r,z)atany instruments have unique strengths. point in this field is given by Given the small size, relatively low and r22 -2z + 2z 2 cost, modest pressure requirements, Φ Φω= (U + Vcos t)(0 ) Ε= d = f (x) (3) r,z 2 2 and experimental flexibility of the dx r+2z0 0 (4) F6 (B) Amplitude of rf signal (A) A simulation of the tra- (A) Amplitude of rf signal jectory of an ion of m/z and supplementary ac signals 100 in a r0 = 1 cm ion trap operated at a rf volt- age of 500 V and a fre- 500 Vrf' 500 Vrf' quency of 1.1 MHz. The 1.1 MHz 12 VAC first three boxes are time plots of the instantaneous Ion Motion in r direction Ion Motion in r direction rf amplitude, the excur- sion of the ion from the center in the r-direction, 7.1 mm, 7.1 mm, and the z-excursion, re- spectively. The last box is 78 kHz a plot of r, z-motion. (B) The same simulation in Ion Motion in z direction Ion Motion in z direction which a supplementary ac voltage is applied at the time indicated to reso- 10 mm 10 mm nantly excite ion motion. 160 MHz (Adapted from reference 9.) 0 Microseconds 100 0Microseconds 100
RingRing ElectrodeElectrode
7.1 mm 7.1 mm
10 mm 10 mm End Cap Electrode
F7 1962.12 8zV 100 1962.36 q=-2q= (6) Zoomscan showing part zr 2 2 2 m(r0 + 2z0 ) ω of the ESI mass spec- 90 +4 IL-8 (Rat) 1962.60 Radial stability, expressed in trum of rat interleukin-8, 80 1961.88 including the isotope en- terms of a and q , must also be velope around around the 70 1962.87 r r [M+4H]4+ ion at m/z maintained simultaneously with 1,962. (Adapted from Fin- ance 60 stability in the z-direction. Note nigan LCQ Operator’s 1961.63 1963.10 Manual, Revision B, July 50 that ions with identical Mathieu pa- 1996.) 40 rameters but different m/z values 1963.34 30 behave identically. Optimum opera- 1963.59 20 1961.35 tion requires the ions have favor- Relative Abund 1963.85 able initial conditions, which is 10 1961.12 1964.09 achieved by using a helium buffer 0 1960 1961 1962 1963 1964 1965 gas (~1 mTorr) to remove kinetic energy from the ions and cause m/z them to occupy the central region where the first term describes its condition is described by a second of the trap. Typically, the ion trap temporal variation and the second order differential equation of the can hold up to about 105 -106 ions its spatial dependence (9). Note Mathieu form. The solutions to this before coulombic repulsions sig- again that r is the internal radius of equation (actually two independent o nificantly affect their trajectories and the ring electrode and zo is the clos- equations which describe the un- est distance from the center to the coupled motion of an ion in the r- greatly reduce the mass resolution. end-cap, while U is the dc potential and z-directions) represent stability Mass spectra are normally re- and V is the rf potential (zero-to- conditions which are readily sum- corded by operating the quadrupole peak) applied between the ring and marized in the form of a stability ion trap in the mass selective insta- end-cap electrodes, ω is its angular diagram (F5) expressed in terms of bility scan mode (9). In this experi- ment, the amplitude V of the ap- frequency, and t is time. Ions of a the Mathieu coordinates az and qz given m/z value may undergo stable (EQ5-6). plied rf is increased so as to “move” motion in the trap for the reasons ions along the qz axis (F5) until -16zU already given qualitatively. The a = -2a = (5) they become unstable at the bound- zr 2 2 ω2 quantitative solution to the stability m(r0 + 2z0 ) ary, where qz = 0.908. As they ap- F8 no dc voltages are applied to the 6 1363 Sequential MS analy- end-cap electrodes. For traps built sis of an oleanolic acid MS (M + TFA)- glycoconjugate per- with the so-called ideal geometry, , formed using a Finni- COO Glc EQ7 ro = 2zo can be simplified gan LCQ ion trap instru- to EQ8 ment showing control Rha Glc Glc O over loss of the sugar Relative 4V monomers, to allow sim- Abundance Glc MW 1250 m/z = 2 ω2 (8) ple, rapid elucidation of qrz 0 a complex structure. (Adapted from Finnigan Trapped ions have charac- LCQ Catalog 1996.) 1249 teristic frequencies of oscillation, MS/MS -TFA known as secular frequencies, again separately in both the r- and z-di- 1363 rections. The principal component 1087 of these secular frequencies is
Relative
Abundance (ω/2)β radian per second, where β is a parameter that varies with the coordinates a and q of which it is a 1087 MS3 -Glc continuing fraction. (At low values β of az and qz, z is approximately 2 given by aq/2z + z . Motion is un- coupled in the r- and z-directions
Relative and the r-frequency is half that in Abundance the z-direction. Because ions have the characteristic frequencies just 925 noted, a supplementary ac potential MS4 -Glc of frequency equal to the secular frequency of motion of the ions will 941 causeionstopickupincreasing -Rha amounts of kinetic energy. If the
Relative
Abundance 779 signal is applied between the end- cap electrodes, ions will be acti- 779 vated in the z-direction. If the reso- MS5 -Glc nant signal is strong enough, these or translationally activated ions can be -Rha ejected from the trap in the z-direc- tion.
Relative This resonance experiment is Abundance extremely valuable in causing par- ticular ions to be excited so that 617 they can be made to dissociate or 6 MS -Glc eject so that the population of ions in the trap can be controlled. F6A shows a simulation of the trajectory -Glc of an ion of m/z = 100 in an ion trap Relative 455 Abundance operated at an rf voltage of 500 V and a frequency of 1.1 MHz. Be- 250 500750 1000 1250 1500 cause of the particular initial condi- m/z tions chosen, the center of the trap proach the region of instability, their ion trap operated in the mass-selec- is not visited; instead, a “donut” of kinetic energies and z-direction ex- tive instability mode is obtained sim- space is accessed. F6B shows the cursions increase and they exit the ply by rearranging the expression for simulated ion trajectory when a supplementary ac potential, in reso- trap through a hole in the end-cap the Mathieu parameter qz (EQ6) electrode and reach an external de- nance with the frequency of ion 8V tector. Ions of increasing m/z are m/z = (7) motion in the z-direction, is applied q (r2 + 2z2 ) ω2 ejected and detected as the rf volt- z 0 0 across the endcap electrodes. It can age V is raised, so yielding a mass This emphasizes the fact that in be seen that there is no effect on ion (actually m/z) spectrum. The mass this mode of operation, ion motion motion in the r-direction. However, the excursion in the z-direction in- analysis equation for a quadrupole is constrained to the az = 0 axis, i.e., creases, and the ion is energized responds to this hole and causes In an ion trap mass spectrome- and ejected through the apertures in resonant ejection. They exit the ion ter, MS/MS is achieved by the use the end-cap electrodes after a few trap in sequence of m/z values but at of an additional sequence of opera- cycles of application of the ac po- a lower rf amplitude than would ordi- tions in the scan function. The scan tential. Other ions of different m/z narily be required for ion ejection. function begins with ionization and values are not affected, so the ex- is followed by selection of a parent periment can be used for selective Slow Scans and High Resolution ion in a step that involves ejecting In the usual mode of operation ejection or activation (see section all other ions from the trap. The (mass selective instability scan) of on Ion Population Control). parent ion is then translationally ex- an ion trap mass spectrometer, ions cited, typically by applying a sup- High Performance and of different m/z values arrive at the plementary rf voltage to the end Biological Applications detector separated in time. Ions of caps. The product ions resulting increasing mass are ejected in turn from collision-induced dissociation Mass Range Extension By as increasing rf voltages are applied of these excited ions with the he- Resonant Ejection to the ring electrode. If the rate at lium buffer gas are recorded by The mass range of an ion trap which the amplitude V of the main scanning the rf voltage to perform a can be calculated by substituting rf is changed is too fast, ions will second mass-analysis scan. The appropriate values into EQ7. Note fail to respond to the instability main advantage of the MS/MS ex- that qz = 0.908 is the qz value at condition completely before ions of periment is its enhanced specificity. which instability occurs in the nor- next value begin to be ejected. Loss This is useful in isomer distinction, mal mass-selective instability mode of resolution will result. By slow- sequencing of biopolymers, and of operation. However, under reso- ing the rate at which V is increased, most particularly in the analysis of nance conditions, qz becomes a an improvement in resolution is ex- complex mixtures. Tandem mass variable provided the supplemen- pected (14, 15). In fact, this type of spectrometry experiments eliminate tary resonance frequency can be experiment has been shown to pro- or greatly reduce signals due to varied. Since it appears in the de- duce extremely high resolution: in other matrix components or instru- nominator of EQ7, it can be de- excess of 106 at m/z 3000 (15). It is mental background (chemical noise). creased to increase m/z. Typical op- most appropriately applied in a A unique feature of the ion trap erating conditions for the Finnigan zoom-scan mode in which ions of is the MSn capability which has un- LCQ are V0-p = 0 - 8500 V, ro = interest of a narrow mass window precedented power in structural elu- ω 0.707 cm, zo = 0.783 cm, = 0.76 (<10 dalton/charge) are examined. cidation. The selectivity of MSn MHz and (qz)eject =0.83.The The zoom-scan mode is able to re- means that a compound can be maximum m/z range that can be solve the isotopic forms of multi- fragmented, and the resulting frag- achieved under these conditions is ply-charged ions observed in elec- ments further isolated and analyzed about 2000 dalton/charge. trospray ionization mass spectra. to yield structural information Decreasing the size of the trap F7 illustrates a zoomscan of the +4 about complex molecules in the or lowering the frequency has been charged state of rat interleukin-8, presence of mixtures. F8 shows a used to extend the mass range of i.e. [M+4H]+4. The resolved iso- sequential MS6 experiment per- the ion trap. However, the most topic cluster reveals a one-fourth formed on an oleanolic acid gly- straightforward method is to lower m/z unit difference between carbon conjugate demonstrating dramatic (q ) by modulating the ion mo- z eject isotopes, which are separated in control over the loss of the sugar tion at a chosen frequency using the mass by one dalton due to the monomers. The experiment allows dipolar electric field applied across 12C/13C difference. Therefore, the simple, rapid elucidation of a com- the end caps. Ions of a particular charge on the ion must be +4. plex molecular structure. m/z value in resonance with the ap- plied frequency then pick up trans- MS/MS and MSn Ion Population Control By Selec- lational energy and are ejected from Mass spectrometry/mass spec- tive Ion Manipulation Techniques the trap. Conceptually, the resonant trometry (MS/MS), or tandem mass A significant weakness of ion ejection experiment creates a spectrometry (16), is a procedure traps is the poor dynamic range due “hole” in the stability region (F5)at for examining individual ions in a to space charge effects, defined as that value of qz which corresponds mixture of ions. The ions of interest changes in ion motion which result to the frequency applied to the end- are isolated by their characteristic from the mutual coulombic interac- cap electrodes. By scanning the am- m/z values, activated by collision, tions of ions. Space charge effects plitude of the main rf voltage in the and allowed to dissociate. The re- can be alleviated by removing ma- normal way, ions of different mass sulting product ions are examined trix ions. Similar reasons for devel- sequentially acquire qz values that in a second mass measurement step. oping a capability for exciting ions give them the frequency which cor- of specified m/z values exist in ion cyclotron resonance instruments. rectly measured using a differential 6. J.B. Lambert, H.F. Shurvell, D.A. preamplifier, filter, and amplifier Lightner and R.G. Cooks, Introduc- The basis for selective excitation is tion to Organic Spectroscopy, the resonance experiment described combination and then Fourier ana- Macmillian Publishing Company, above. It can be used to extend the lyzed to obtain the broad-band fre- New York (1987). m/z range, as already noted, or to quency domain spectra charac- 7. A.L. Burlingame, R.K. Boyd and S.J. teristic of the sample ions. An ad- Gaskell, Anal. Chem. 68 (1996) allow only specified ions to frag- 599R. ment by collision induced dissocia- vantage of non-destructive ion de- 8. P.H. Dawson, Quadrupole Mass tion. Resonance excitation experi- tection is the ability to measure a Spectrometry and Its Applications, ments are most effectively imple- single-ion population multiple New York: Elsevier Scientific Pub- lishing Company, 1976. mented by applying a mixture of times. 9. R.G. Cooks, G.L. Glish, S.A. frequencies so as to manipulate McLuckey and R.E. Kaiser, Chem. ions of different m/z values simul- Conclusions Eng. News 69 (1991) 26. taneously. This can be done using a 10. R.E. March and R.J. Hughes in time domain signal in a technique Ion trap mass spectrometry has “Quadrupole Storage Mass Spec- trometry” Wiley, New York 1989. known as SWIFT (stored waveform recently undergone very rapid de- velopment and is emerging as a 11. J. Gilbert, J. Balcer, L.T. Yeh and inverse Fourier transform) (17). high performance technique which S. Erhardt-Zabik, Qualitative and This was the first procedure applied Quantitative Performance of a show signs of becoming one of the to quadrupole ion traps for the gen- Benchtop LC/MS Ion Trap Verses leading tools in the discipline. Traditional Research-Grade LC/MS eral purpose of ion population con- These instruments allow tandem Systems, Presented at the 45th trol. In trace level analysis, it is de- ASMS Conference on Mass Spec- mass spectrometry experiments sirable to selectively fill the trap trometry and Allied Topics, Palm which are possible only using com- Springs, CA, June 1-5, 1997. with the analyte ions while the ma- binations of multiple quadrupoles 12. R. Wieboldt, D. Campbell and J. trix ions are ejected (18). The abil- such as the highly successful triple Henion, LC/MS/MS Quantitation of ity to selectively store ions is ex- Orlistat in Human Plasma with an quadrupole instrument. Extension Ion Trap and a Triple Quadrupole pected to provide a substantial im- to high mass/charge measurements, Spectrometer: A Comparative provement in limits of detection. Study, Presented at the 45 th the development of high resolution This and related selective ion stor- ASMS Conference on Mass Spec- capabilities and the very recent trometry and Allied Topics, Palm age techniques have been used in demonstration of non-destructive, Springs, CA, June 1-5, 1997 ultra-trace-level analysis of volatile broad-band Fourier transform capa- 13. C. Weil, M. Nappi, C.D. Cleven, H. organic compounds at levels as low bilities, all suggest an increased Wollnik and R.G. Cooks, Rapid as parts-per-quadrillion (pg/L) (19). Commun. Mass Spectrom. 10 role in the future. Limitations occur (1996) 742. Non-Destructive Ion Detection in dynamic range, accurate mass 14. J.D. Williams, K.A. Cox, R.G. A new and simple method of measurement, and quantitative pre- Cooks, R.E. Kaiser, Jr. and J.C. Schwartz, Rapid Commun. Mass ion detection in the quadrupole ion cision. Spectrom. 5 (1992) 327. trap is that in which ions approach 15. J.C. Schwartz, J.E.P. Syka and I. an electrode and polarize it so that a Acknowledgement Jardine, J. Am. Soc. Mass Spec- current flows toward and away trom. 2 (1991) 198. from the electrode through an ex- The work at Purdue is sup- 16. K.L. Busch, G.L. Glish and S.A. ported by the Department of En- McLuckey in “Mass Spectrometry/ ternal conductor in response to the Mass Spectrometry: Techniques oscillating ion motion. The induced ergy, Office of Basic Energy Sciences. and Applications of Tandem Mass current flow has a frequency equal Spectrometry” VCH Publishers, References Inc., New York, 1988. to that of the coherently moving 17. A.G. Marshall, T.-C.L. Wang and ions, which in turn depends on their 1. R.G. Cooks, G. Chen, P. Wong and T.L. Ricca, J. Am. Chem. Soc. 107 m/z value. Since the ions are not H. Wollnik, Encl. Appl. Phys. 19 (1985) 7893. destroyed, they can be remeasured. (1997) 289. 18. L.D. Bowers and D.J. Borts, Clin. A non-destructive method of ion 2. J.B. Fenn, M. Mann, C.K. Meng, Chem. 43 (1997) 1033. S.F. Wong and C.M. Whitehouse, detection (20) is achieved in ion 19. M. Soni, S. Bauer, J.W. Amy, P. Mass Spectrom. Rev. 9 (1990) 37. Wong and R.G. Cooks, Anal. traps by impulsive excitation of a 3. J.R. Chapman, Practical Organic Chem. 34 (1995) 1409. collection of trapped ions, typically Mass Spectrometry, Chichester: 20. M. Soni, V. Frankevich, M. Nappi, of different m/z values. The ion im- Wiley, 1993. R.E. Santini, J.W. Amy and R.G. age currents are induced on a small 4. R.K. Mitchum and W.A. Korfmacher, Cooks, Anal. Chem. 68 (1996) Anal. Chem. 55 (1983) 1485A. 3314. detector electrode embedded in, but 5. M. Karas, U. Bahr and U. Giess- isolated from, the end-cap elec- mann, Mass Spectrom. Rev. 10 trode. The image currents are di- (1991) 335.