Toward the Next Generation of Atomic Mass Spectrometers† Plenary Lecture GARY M. HIEFTJE*, DAVID P. MYERS, GANGQIANG LI, PATRICK P. MAHONEY, THOMAS W. BURGOYNE, STEVEN J. RAY AND JOHN P. GUZOWSKI Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Atomic mass spectrometry, embodied principally as ICP mass Given these capabilities, it is perhaps not surprising that spectrometry (ICP-MS) and glow discharge mass many researchers have turned to ICP-MS or glow-discharge spectrometry (GDMS), has enjoyed rapid growth during the mass spectrometry for routine analysis.However, neither last decade, yet both methods exhibit shortcomings that would method is without its shortcomings. ICP-MS, for instance, still be desirable to reduce or eliminate. Prominent among these suffers from a number of matrix and spectral interferences. shortcomings are drift and limited precision, several During the past several years, a great deal of effort has been troublesome spectral and matrix interferences, and moderate expended in understanding the nature of many matrix interatom- detection efficiency.This last limitation is particularly ferences. As a result of those studies, modifications have been troublesome when ICP-MS, for example, must be interfaced made in the design of the interface between the ICP and mass to analytical systems that deliver extremely small sample spectrometer in order to reduce the incidence of space charge volumes or low flow rates or when extremely limited sample (coulombic repulsion) and the effect it has on analyte signal sizes must be examined.Such situations are projected to be levels. However, enough concern about interferences remains increasingly common in the next decade because of the that most commercial laboratories rely upon standard importance of biotechnology and nanostructured materials. additions (spiking) or matrix matching in order to ensure an Overcoming these limitations will require substantial acceptable level of accuracy.Similarly, spectral overlaps with modifications in both sources and mass-spectrometer designs. polyatomic species continue to be a problem, despite a number Sources will be required that are more efficient at sample of clever schemes to overcome them. In addition, GDMS utilization, aerosol volatilization and atomization and that suffers from a difficulty in using internal standards, in blank provide multidimensional information.Similarly, mass subtraction, and in a susceptibility to contamination. spectrometers of the future must be more atom-efficient, Difficulties are also encountered when there is a need to should measure all elements and isotopes simultaneously, and measure transient or time-dependent samples or signals. In must operate on a time scale that is compatible with ICP-MS such signals are encountered when one employs microsampling and transient-sampling technology. Possible electrothermal atomization, laser ablation, flow injection or alternative systems that meet these criteria will be outlined chromatographic separation.In GDMS, time-dependent and their likely performance assessed. Greatest emphasis is signals must be measured if depth profiling is desired. placed on time-of-flight mass spectrometry coupled with an Unfortunately, virtually all commercial ICP-MS and GDMS ICP source. instruments are scanned devices; that is, they can measure only Keywords: Inductively coupled plasma mass spectrometry; a single mass at a time.As a result, there is a necessary trade time-of-flight mass spectrometry; glow discharge mass off between elemental coverage and signal-to-noise ratio. On spectrometry ; instrumentation ; plasma-source mass the one hand, one can limit the elemental coverage and thereby spectrometry accumulate enough signal to yield low detection limits or high precision. On the other hand, one might prefer more complete elemental coverage but will have to accept a loss in sensitivity Over the past decade, plasma-source mass spectrometry or reproducibility as a result.(PSMS) has emerged as an attractive technique for elemental The limitation of measuring only a single isotopic peak at analysis. Despite its relatively high cost, the method enjoys a a time also constrains the precision that can be realized in a high degree of popularity because of a number of important ratioing mode.It is generally recognized that the dominant attributes. For example, the most widely used of the PSMS source of fluctuations in a PSMS are multiplicative, that is, techniques, ICP-MS, offers sub-part per trillion detection they seem to affect all signals in much the same way. This is limits, a linear range that extends over approximately seven especially true of ICP-MS. Variations in nebulizer or spray- orders of magnitude, isotope analysis and isotope-ratioing chamber performance, flutter in the plasma tail flame and the capability, modest precision (1–5%), excellent performance in sampling of inhomogeneous plasma zones all lead to signal a semi-quantitative (standardless) mode, limited spectral (iso- fluctuations that more or less track each other.That is, when baric) and matrix interferences, virtually complete elemental one elemental signal rises, others tend to follow it. As a result, coverage, high speed on a per-sample and per-isotope basis, precision can best be improved by a ratioing method such as and the convenience of solution-based sampling.This last internal standardization or isotope dilution. Unfortunately, the feature, of course, requires sample dissolution in many cases. signal fluctuations are often rapid enough that ratioing-based However, it also simplifies standardization, blank subtraction, compensation is effective only if the target isotopes or elements and the use of standard additions or internal-standardization are measured within microseconds of each other.Although approaches. this sort of speed is possible with quadrupole-based mass spectrometers, only a few elements or isotopic peaks can then † Presented at the Eighth Biennial National Atomic Spectroscopy Symposium (BNASS), Norwich, UK, July 17–19, 1996. be measured at a time. The problem is even more difficult in Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12 (287–292) 287sector-field instruments, which often cannot be scanned (or limited storage capacity, just as does the IT.In addition, to realize the extremely high resolution of which the FT–ICR is peak-hopped) as rapidly. In recognition of this limitation, most sector-field instruments designed for high-precision isotope capable requires an extended trapping time, rendering the system unsuitable for use with transient-sampling devices. ratioing employ multiple collectors and an extremely stable (thermal ionization) source.Lastly, the FT–ICR instruments available today are extremely expensive, made so by the need for cryogenically cooled From the foregoing comments, the requirements of the nextgeneration PSMS instrument seem evident. It should offer magnets, extremely low operating pressures, and sophisticated signal-processing systems. Overall, it seems unlikely that they virtually complete freedom from matrix and spectral interferences, simultaneous detection of the entire atomic mass will be widely used in the future.The third candidate for a future system is a sector-field mass range of interest, high precision in a ratioing mode and high spectral-acquisition rates in order to be amenable to coupling spectrometer equipped with a focal-plane detector array. Such a system has been in development in our laboratory9,10 and in with transient-sampling devices. Furthermore, such an instrument must not sacrifice any of the important figures of merit others11 for several years and offers substantial promise.This device can be considered to be the mass-spectrometric analog that have become associated with PSMS in general. Lastly, the next-generation device should possess the attributes that of modern emission spectrometers based on charge-transfer device technology. However, unlike an atomic emission spec- all in the field have come to know and love: low cost, high reliability, user-friendliness and a high degree of automation.trum, an atomic mass spectrum consists of a relatively modest number of peaks whose locations are well established and whose ratios, for a given element, do not vary greatly. In THE OPTIONS particular, unit-mass resolution is all that is required for routine atomic mass spectrometry measurements. Also, in most Some of the features to be present in the next-generation of PSMS instruments will no doubt be the result of modification applications it is necessary to detect only the mass range from lithium (7 amu) to uranium (238 amu). Thus, in the best of or re-design of sample-processing or sample-introduction equipment, or from improved plasma sources.For example, circumstances, fewer than 250 detector elements (pixels) would be required to provide full AMS coverage. Thus, it would unless extremely high mass-spectral resolution is employed, the most likely way to overcome residual spectral and matrix seem that PSMS is almost ideally suited for detector-array technology.interferences is through more efficient solvent removal, improved sample-introduction arrangements and modification Unfortunately, the mass-spectral display produced by most magnetic-sector mass spectrometers is quadratic. As a result, of the plasma environment. In large part, these improvements will be possible with only slightly modified glow-discharge or a detector array that serves well on one end of the mass spectrum does not provide adequate resolution on the other.ICP sources. However, it seems likely also that more substantial departures from the conventional systems might be neces- In addition, problems can arise from extremely intense massspectral peaks, such as that produced by Ar+ in the ICP, sary. One such approach, the tandem source, will be described in more detail later. However, to constrain the length of this causing detector saturation, blooming or non-linearity. In recognition of this situation, the system we designed9,10 treatment to a manageable level, the rest of our discussion will focus mainly on mass-spectrometer options for PSMS.is configured to record the full atomic mass-spectral range in two segments that straddle, but avoid, 40Ar+. The low-mass In identifying options for the next-generation mass spectrometer to be used in elemental analysis, one must recognize segment, extending from 7 to 38 amu, is recorded first and the accelerating voltage in the instrument then switched abruptly that the desirable features outlined above do not require that the mass spectrum be recorded in a truly simultaneous fashion.to cover the high-mass segment from 42 to 238 amu. Unfortunately, our current system suffers from problems Rather, it is necessary only that ions be sampled from the plasma source simultaneously. In an ion trap (IT), for example, that have been traced to the detector array. Regrettably, it is difficult to detect atomic ions directly on a linear detector ions can all be accumulated at the same time from a continuously operating ion source, so the stored population represents array of the kind intended for optical sensing.In a sector-field instrument, ions achieve an extremely high kinetic energy, the composition of the plasma at a particular point or period in time. Determining the mass spectrum of the ions stored in sufficient to sputter the surface of the detector. Consequently, it is necessary to turn to several inter-domain conversions in an IT then can be achieved in a much slower, sequential fashion, by expelling ions from the trap one mass at a time, so order to preserve detector integrity.The most common such approach is to employ a microchannel plate to convert each they can be detected by a suitable ion multiplier. Indeed, a substantial amount of success has already been achieved with incoming ion to a shower of electrons. In turn, the electrons are converted to photons by impinging on a phosphor screen.both ICP and GD sources with an IT mass spectrometer.1–4 Unfortunately, ion traps can accommodate populations of only Lastly, the photons are detected with a conventional linear photodetector array. Although cumbersome, this approach has about 106 ions before space charge repulsion becomes a problem. Furthermore, ion traps can contain only approxi- been found successful by a number of earlier workers.12–14 Unfortunately, such an arrangement is extremely susceptible mately 104 ions and remain analytically useful.Thus, at least 100 experiments would have to be performed if the dynamic to fringing fields from the magnetic sector. Unless these fields are reduced or the detector array shielded from them, the range expected of moderate PSMS instruments (106) were to be matched. Furthermore, this multiple-storage requirement consequence is a peak shaped such as that seen in Fig. 1. Apparently, the electrons produced by the microchannel plate will make the system rather unattractive for coupling with transient-atom sources.are affected sufficiently by fringing magnetic fields that they add a broad and almost triangular base to the mass-spectral Another candidate instrument for next-generation systems is the Fourier transform ion-cyclotron resonance (FT–ICR) peaks. As a result, the mass-spectral resolution (defined in terms of peak half-width) is adequate for elemental analysis, mass spectrometer.5–8 Like the ion trap, an FT–ICR instrument accumulates ions from an external source but then reads them although abundance sensitivity (the ‘leakage’ of one mass into the next) is unacceptable.Work continues in an effort to in a truly simultaneous fashion. Furthermore, the tremendous resolution of which the method is capable should completely overcome this problem. A final candidate for the next generation of PSMS instru- eliminate any concerns about isobaric overlaps.Indeed, recent experiments have shown that resolving powers as high as ments is the time-of-flight mass spectrometer (TOFMS). Like the ion trap, the TOFMS accepts all ions at the same time, 1800 000 can be realized.8 Unfortunately, the FT–ICR has 288 Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12OVERCOMING THE LIMITATIONS OF TOFMS The resolution ‘problem’ in TOFMS can be solved by applying a number of technologies developed in the past.For example, an input ion beam of extended width can be collapsed upon itself, and thereby will produce a short detector pulse, by means of a technique termed ‘space focusing’, developed first by Wiley and McLaren.15 The technique involves using a twostage ion-input system, consisting of an ‘extraction zone’ and a subsequent acceleration region. Ions are initially fed into the extraction zone, from which they are pushed by means of a ‘repeller’ voltage into the acceleration region.Ions at the rear of the extraction zone begin farther from the detector and therefore would ordinarily arrive later in time than those ions Fig. 1 Image of a mechanical slit placed in front of a microchannel in the front of the extraction zone. However, ions in the rear plate–CCD detector system situated at the focal plane of a Mattauch– of that zone experience a greater ‘push’ from the repeller Herzog mass spectrograph.10 The measured half-width of the peak is 510 mm, whereas the expected width is 495 mm.The broadened, almost electrode behind them (since they are closer to it) and therefore triangular base of the peak is probably the result of fringing fields catch up with ions ahead of them. Placing a detector at the produced by the magnetic sector. location where the ions meet then results in improved resolution. Similarly, any kinetic-energy variation among analyte ions in the direction of the flight tube can be overcome by means but in an extremely short burst that lasts for only a few of a ‘reflectron’.In essence, the reflectron configuration involves nanoseconds. However, the mass spectrum then requires more the use of an ion mirror which consists of suitably applied time to record, ordinarily of the order of 50 ms or so. Still, this fields that repel ions sent into it. A relatively low-energy ion spectral-acquisition time is considerably shorter than that of penetrates the field for only a short distance before being which any of the other devices discussed above is capable.reflected to a distant detector. In contrast, a higher-energy ion Stated differently, it allows as many as 20000 elemental mass penetrates the reflecting field farther and therefore must travel spectra to be accumulated each second, making it applicable a greater distance before it reaches the detector. Because the to all but the most rapid transient-sampling systems. path traveled by the higher kinetic energy (higher velocity) ion Also, the TOFMS is among the simplest of all mass is greater than that of the low-energy ion, the kinetic-energy spectrometers, consisting basically of an input section, an open variation can be almost completely overcome.16 evacuated tube and a detector.To perform a mass-spectral Raising the duty factor of a TOFMS is also possible with analysis, a packet of ions is accelerated through a fixed voltage well established technology. In the 1960s, workers in the drop, typically on the order of 2000 V or so, imparting to all Bendix Corporation17 realized that an orthogonal acceleration ions the same kinetic energy (KE).The ions therefore achieve (OA) configuration would achieve the desired end. When ions a velocity (v) that is inversely proportional to the square root are sampled from an atmospheric-pressure source, they first of their respective mass-to-charge ratios (m) (i.e., KE=0.5 mv2). form a supersonic expansion, in which they all achieve roughly The arrival time of each ion at the end of a field-free region the same velocities.Obviously, that velocity should correspond (termed the ‘flight tube’) can then be determined and related to their original thermal temperatures which, in the case of to mass-to-charge ratio. The result is an instrument that is the ICP, are of the order of 0.5 eV. In fact, the energies are a rather maintenance-free and relatively inexpensive to produce little higher because of the existence of a plasma offset voltage.and to operate. Furthermore, because of its open structure, the The ions therefore begin their flight into the vacuum system TOFMS offers extremely high efficiency. In some applications, at a relatively low energy (velocity) of from 1 to 5 eV. In such as secondary-ion mass spectrometry, transmission contrast, when the ions are accelerated into the TOFMS flight efficiencies approaching unity have been achieved.This factor tube, they reach velocities that correspond to 2000 eV, if the alone should make the TOFMS as much as 100 times more accelerating voltage used in propelling them into the flight sensitive than competitive quadrupole-filters or sector-field tube is 2000 V. This factor of roughly 1000 difference in devices. velocities enables one to overcome all but about 10% of the Not surprisingly, the TOFMS is also not without its weak- duty factor of the TOFMS. The initial, slowly moving ion nesses.Because ions of a given mass to charge ratio must beam first fills the extraction zone, from which ions are sent arrive at the detector at virtually the same time, the TOFMS in a perpendicular direction down the flight tube. As the mass is often viewed as a low-resolution instrument. Any factor that spectrum of that ion packet is being recorded, the extraction can contribute to a broadened ion packet will degrade reso- zone is slowly refilled by the continuously flowing input beam.lution. Such factors include a spatially broad input ion pulse, The resulting limited duty factor, roughly 10%, is more than a temporally long input ion pulse, a spread in kinetic energies compensated by the high transmission efficiency of the of ions that enter the drift tube, and disparate ion paths TOFMS. In addition, the duty factor can be further improved through the drift tube. Although these problems can be largely either by electrostatically decelerating the input ion beam or overcome through use of established techniques, they offer by shortening the TOFMS flight tube (i.e., so that a higher challenges to instrument design.repetition rate can be employed). The latter approach produces Perhaps even more of a concern, however, is the relatively a loss in resolving power, of course, but not to the point where low duty factor of a TOFMS. In order to achieve high massthe device becomes unsuitable for atomic mass spectrometry.spectral resolution, only an extremely short input pulse of ions Interestingly, the open structure that imparts a high trans- can be used. Ordinarily, that pulse is on the order of 5 ns. It mission efficiency to TOFMS also raises background noise. then requires as long as 50 ms for a full atomic mass spectrum Of course, in an orthogonal acceleration TOFMS arrangement to be recorded; only after this time will the next 5-ns pulse of there should be little difficulty caused by either neutral species ions be introduced.Consequently, only a tiny fraction of the or photons, since the detector can be effectively shielded from input ion beam can be used for the analysis; in this case the them. However, because of the spread of the input-ion beam, fraction is (5×10-9)/(50×10-6)=10-4. In essence, the device will waste 99.99% of a continuously operating input ion beam. ion–atom and ion–ion collisions, and fringing fields in the Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12 289extraction zone, the input ion beam leaks almost continuously from the extraction zone into the acceleration region of the TOFMS.These leakage ions are then accelerated into the flight tube and ultimately reach the detector, where they produce a continuous background-noise level. Fortunately, it is possible to overcome this ion-induced background noise by means of energy discrimination.18 It will be recalled that an input ion pulse is created by ‘pushing’ a segment of the input beam from the extraction zone into the acceleration region.This ‘push’ imparts to the ion packet a kinetic-energy component that is added to the energy the ions receive in the acceleration region. For example, it is common to employ an accelerating voltage of 2000 V and an extraction pulse of 500 V. The analyte ions therefore receive a kinetic Fig. 2 The precision of isotope-ratio measurements improves as the energy between roughly 2150 and 2350 eV, depending upon square root of the number of signal counts that are accumulated.their initial positions in the extraction zone. In contrast, Closed circles represent precision values obtained by ion counting for background ions leak into the acceleration zone without the ICP-TOFMS measurement of the Ag 107:109 isotope ratio. The dashed line indicates the precision expected from counting statistics. experiencing the same ‘push’. As a result, their kinetic energies cannot exceed 2000 eV.A kinetic-energy barrier placed immediately in front of the ion detector then serves to discriminate efficiently against background ions but to pass the analyte Because of its extremely high-speed spectral acquisition, the ICP-TOFMS is also well suited to the measurement of transi- packets with high efficiency. ent signals. In fact, as many as 100 mass spectra can be accumulated during a single laser-ablation event lasting no PERFORMANCE OF AN ICP-TOFMS longer than 10 ms.23 This capability enables the spatially resolvedcomposition of aheterogeneous solid to be determined Because of the high throughput (transmission efficiency) of the by repetitive laser-ablation sampling.In these experiments, as TOFMS and use of the energy-discrimination scheme described in the continuous-nebulization experiments outlined above, above,18 detection limits achieved with a laboratoryprecision can be improved by isotope ratioing or internal constructed ICP-TOFMS instrument lie mostly below 1 ppt, standardization.competitive with commercial instrumentation (see Table 1).18,19 Perhaps the time-resolution capability of an ICP-TOFMS Moreover, these detection limits are all achievable in the same is best exploited when it is used to add dimensionality to an ten-second interval. In contrast, scanning-based ICPMS instruanalysis. 20 This added dimensionality is illustrated, for example, ments require a separate 10 s interval to measure the detection in gas chromatography–mass spectrometry, wherein gas- limit of each isotope of interest.Moreover, our current system chromatographic separation provides information along one is not equipped with an advanced high-efficiency interface such axis (dimension) and mass analysis provides orthogonal (inde- as can be found on most commercial instruments. It seems pendent) information along a second dimension. In ICP- likely, therefore, that detection capabilities should be able to TOFMS, the same capability could be achieved in combination improve appreciably.Indeed, in a recent analysis20 it was with chromatography. Importantly, unlike with scanning mass shown that the simplicity of a TOFMS instrument enables ion spectrometers, no mass-spectral skew will then exist, since all losses to be tracked and possible areas of improvement to be atomic masses of interest will be taken from the same portion identified.That evaluation estimated that between 100 and of the chromatographic peak. Consequently, empirical formu- 1000 atoms of a target element present in the sample should lae should be more reliable to determine and the overlap of ultimately be able to be detected. chromatographic peaks simpler to detect. As was suggested earlier, the virtually simultaneous extrac- An example of this added dimensionality taken from our tion of all ions from a continuous source enables precision to own work24 arises when the ICP-TOFMS is coupled with an be improved by isotope ratioing or by internal standardization.electrothermal atomizer (ETA). In this experiment, the ETA As is shown in Fig. 2, this precision improves as the square receives a 10-ml sample which is subsequently dried and ashed, root of the number of ion counts that are accumulated,21,22 much as is done in atomic absorption spectrometry. The enabling it to be improved to virtually any desired level just furnace temperature is then ramped upward; along that ramp, by employing a sufficiently long integration time.Furthermore, elements are volatilized in accordance with their appearance this high-precision capability is available for all elements or temperatures. That is, the more volatile elements appear earlier isotopes at once, an impossibility with scanned mass than those of lower volatility. This temperature ramp then spectrometers. provides a time axis that enables elements to be separated in part on the basis of their volatility differences.Because a TOF Table 1 Limits of detection obtained with an ICP-TOF mass spectrometer18 mass spectrum can be obtained at virtually every point during the temperature ramp, what results is a two-dimensional plot Detection limit/ with volatilization time (temperature) on the vertical axis and Element ng l-1 a flight time (the mass spectrum) on the horizontal axis. An Li 1.5 example of such a plot is shown in Fig. 3. Mg 4.2 From Fig. 3, it can be seen that all isotopes of a given Mn 1.5 element volatilize at the same time, which would clearly be Co 1.1 expected. However, the locations in the two-dimensional plot, Sr 0.6 where different elements can be found, are separated by a Rh 0.5 Ag 0.9 greater distance than they would be in a simple one- Cs 0.5 dimensional mass-spectral display. This added dimensionality Ho 0.4 allows species to be distinguished that would otherwise overlap Bi 0.6 isobarically.For example, the ArO+ peak (not shown in the U 0.6 particular display of Fig. 3) appears very early in the volatiliz- 290 Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12therefore follow a different trajectory in the perpendicular flight tube, and some miss the detector. The duty factor of the current system is also only about 10% at present. It could be improved, however, by shortening the flight tube, with an acceptable sacrifice in resolving power. The same change would increase the spectral-acquisition rate from the current 20 kHz to roughly twice as fast.Interestingly, several of these problems can be overcome by abandoning the OA geometry in favour of a more traditional but significantly modified on-axis configuration. In this alternative arrangement, the goal is to retain the advantages of the OA geometry (mainly its improved duty factor), but to avoid its limitations (mass bias and low transmission efficiency). The way in which this goal is being achieved is to ‘tag’ (by means of its higher energy) each introduced ion packet before it is accelerated and to use second-order (higher efficiency) space focusing27 to collapse that tagged ion packet upon itself.The Fig. 3 Two-dimensional plot illustrating the ability of ICP-TOFMS packet so tagged then enters an acceleration zone, but travels to resolve atomic isobars (spectral overlaps) on the basis of volatility in the same direction in which it had been moving.Thus, the differences. An electrothermal atomizer is used to provide a tempera- difficulties of the OA geometry are avoided. However, the high ture ramp (vertical axis), along which elements volatilize sequentially, in the order of their appearance temperatures. The high speed of the duty factor is retained because ions are tagged and collapsed TOFMS enables this time of appearance to be followed and the upon themselves while they are moving slowly rather than otherwise overlapping isotopes individually measured (horizontal axis).after they are accelerated to the drift-tube velocity. In this new Taken from reference 24. instrument, the ‘tagging’ is accomplished by means of a voltage pulse applied to an in-line repeller grid, much as has been used ation cycle, whereas the 56Fe+ peak, with which it overlaps, in the past in our OA geometry. As a result, only the tagged appears much later. Similarly, Cd, In and Sn have isotopes ions have the extra velocity (energy) required to penetrate an that mutually overlap.Separating them by mass spectrometry energy barrier placed just before the ion detector and only alone would require resolving powers between 90000–300 000. they contribute to the mass spectrum. The rest of the ion In contrast, they can be resolved with the tandem ETA–ICP- beam, allowed to travel unimpeded from the ion source into TOFMS combination quite readily (see Fig. 3). the TOFMS flight tube, is ignored. Of course, the TOFMS can be used also with alternative Although still in its relative infancy, we feel this new on-axis ion sources. We have already reported its coupling with a glow PS-TOFMS might truly be the next generation of atomic mass discharge (GD) source25 and are pursuing its use with an spectrometers. We will look forward to reporting in greater electrospray ionization (ESI) system.26 With both the GD and detail on its performance in the future.ESI source, the advantages outlined above for ICP use are Supported in part by the National Institutes of Health through once again realized: high precision in a ratioing mode, excellent grant GM 53560. detection limits and applicability to transient-sampling analysis. 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Accepted November 25, 1996 292 Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12