Using High-pressure Ionization Plasma and electrospray ionization sources can be coupled with the ion trap to meet inorganic and organic analytical challenges
G
iven the preceding discussion in Part 1 of this series on the use of ion injection/quadrupole ion trap MS in the July 1 issue (p. 689 A), it should be apparent that the ion trap may or may not be an appropriate mass analyzer for a given analytical problem, depending on the relative importance of the mass analyzer characteristics. Ion sources operated at high pressures, however, are natural candidates for coupling with a quadrupole ion trap because of pumping requirements and because ion injection efficiencies are highest for ions with low kinetic energy. The latter characteristic obviates the electrical isolation challenges associated with certain techniques, such as coupling highpressure ionization sources with sector mass spectrometers. Many high-pressure ion sources are used with MS. In recent years, several of these have been coupled with the ion trap to address analytical challenges in both
Scott A. McLuckey Gary J. Van Berkel Douglas E. Goeringer Oak Ridge National Laboratory
Gary L. Glish University of North Carolina 0003- 270019410366-737A/$04.50/0 01994 American Chemical Society
A 1
aimed at the development of a highly sensitive and specific detector for trace quantities of high explosives (such as nitroaromatic compounds, nitrate esters, and heterocyclic nitramines) in air. The strategy was to use an atmospheric sampling A ionization technique with negative ion tandem MS. Because species commonly found in high explosives often form stable negative ions ( I ) , negative Iion analysis significantly reduces background noise. The quadrupole ion trap seemed to be a particularly attractive analyzer because of its small size, its ability to operate at high pressures, and its high MS/MS efficiencies (2).It was apparent, however, that an in situ ionization method would lack the specificity and sensitivity required for the ion trap to compete with a beam-type scanning technology that uses negative ions. Low part-per-billion by volume (ppbv) detection limits have been demonstrated for in situ positive ionization ion trap MS in direct air-sampling scenarios (3).Lower detection limits are desirable because the saturated headspace roomtemperature vapor pressures of many explosives of interest can be orders of magnitude lower than 1ppbv (4). Negative ionization via electron capture can provide lower detection limits because of
L
T organic and inorganic MS. In this article, we will review this work briefly, drawing from our own experiences with atmospheric sampling glow discharge ionization (ASGDI), a form of ionization that occurs at 1torr, and electrospray (ES), an atmospheric pressure technique. Our discussion is divided into these two pressure regimes and includes descriptions of organic and inorganic applications.
-
Ionization at 1 torr
Organic MS. Our involvement with qriadrupole ion traps and specifically with ion injection experiments grew from a project
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its selectivity, thereby reducing background, and because of the higher rates associated with electron capture relative to ionization by ion-molecule reactions for these species (5).However, efficient negative ion formation, particularly when direct electron capture is desired, is difficult to achieve within the ion-trapping volume because of the wide disparity in mass for electrons and ions ( 6 ) .Conditions appropriate for trapping electrons lead to essentially zero well depths for ions, whereas appropriate conditions for trapping ions lead to values of m/z that are orders of magnitude greater than the m/z of the electron (see Equation 1 in Part 1). Therefore, it is dif6cult to accumulate enough slow electrons within the ion trap to achieve efficient ionization of the analyte. Consequently, we were forced to consider another means of ionization to take advantage of the virtues of the ion trap. In previous work we had already a p plied an ASGDI source (7) with beamtype tandem MS (8)to the detection of explosives in air. This form of ionization is particularly well suited to the problem because it samples directly from the air, which supports the glow discharge. There is, of course, a high density of slow electrons within the discharge that yields high electron capture rates for the explosives
of interest (9). Furthermore, at the pumping speed and analyte number densities associated with the ASGDI source, ionization is under kinetic control-which minimizes matrix effects upon ionizationinstead of thermodynamic control. Given the poor efficiency of anion formation within the ion trap, we interfaced an ASGDI source with the ion trap (10).The glow discharge source can be used to ionize the analyte directly, as in explosives detection, or it can be used to form reagent anions that are injected into the ion trap to effect in situ negative chemical ionization (6). Figure 1 shows a schematic of this combination. The filament assembly normally mounted on the entrance end cap was removed. A three-element lens system was used to focus ions issuing from the ion source onto the entrance endcap aperture during ion accumulation and to prevent ions from reaching the entrance aperture at all other times. The ASGDI/ion trap combination, with air in the vacuum system at 0.1 mtorr and helium added to bring the total pressure to 1mtorr, provides subpicogram and low part-per-trillion detection limits for the organic high explosives of interest and yields MS/MS efficiencies of 40-100% for the precursor ions derived from these species. Figure 2, for example, shows virtuN
Sampling aperture
Figure lD Side-view schematic of an ASGDI source coupled with a quadrupole ion trap.
738 A Analytical Chemistry, Vol. 66, No. 14, July 15, 1994
ally 100%MS/MS efficiency for the precursor ion derived from pentaerythritol tetranitrate (PETN), a plastic explosive. These performance characteristics are highly desirable in an explosives vapor detection system. However, the influence of matrix ions on the ions of interest and the mutual influence of like ions can seriously compromise the actual application of the ion trap in detecting targeted compounds in an automated system. Matrix ions, if permitted to accumulate with the ions of interest, can greatly affect the number of analyte ions that can be accumulated, because of the limited ion storage capacity of the ion trap. This is especially problematic when the analyte ions represent only a small fraction of the accumulated ions. A means to allow for the accumulation of the ions of interest is therefore desirable when the ions of interest represent a small fraction of the ion beam injected into the ion trap. Explosives detection is just such a scenario, and we used it to illustrate this effect and to show that the ejection of matrix ions during ion accumulation could allow for the concentration of the ions of interest in an ion injection experiment (11).We applied a single-frequency largeamplitude signal to the end caps to continuously eject a wide range of low m/z ions formed from the discharge gases while the molecular anion of 2,4,&trinitrotoluene (TNT) accumulated. However, it is much more desirable to use tailored signals that allow for the accumulation or ejection of arbitrary ranges of m/z so that ions of widely different m/z can accumulate simultaneously. Such a capability is necessary for the ion trap to retain its duty cycle advantage. Recently, stored waveform inverse FT (SWIFT) (12) and filtered-noise fields (13) have been used with ion injectiodion trap experiments to provide this capability. We have evaluated filtered-noise fields applied to the ASGDI/ion trap explosives detection experiment for the simultaneous accumulation of the molecular anion of TNT and three anions observed from sulfur, S i , S i , and ST. Figure 3 shows the resulting mass spectra acquired with and without a filtered-noise field that allows for the selective accumulation of the targeted ions. Note that in the absence of the filtered-noise field, the
figure 2. Ion trap MS/MS data for the (M-CH,NNO,)- precursor ion derived from PETN by using ASGDI. Spectrum acquired (a) after precursor ion isolation and before collisional activation and (b) after collisional activation.
more abundant and slightly space-charge broadened matrix ion signals somewhat obscured the sulfur-derived ions. In addition, the matrix ions adversely affected the accumulation of the TNT molecular anion. These and other results have shown that mass-selective ion accumulation techniques can be used to make the full dynamic range of the ion trap available in an ion injection experiment even for ions of low abundance in the injected ion beam. A second potentially troublesome aspect associated with detecting targeted compounds with the ion trap is the need for optimization of collisional activation conditions. The major tuning variables for such experiments are the well depth, the frequency of the dipolar potential applied to the end caps, the amplitude of the potential applied to the end caps, and the duration of resonance excitation. The optimum well depth, amplitude, and duration for a targeted ion are largely independent of the number of ions and therefore can be established in the laboratory and used in the field. However, the electric
field of other ions can affect the frequencies of ion motion, although they are largely determined by the parameters in Equation 4 in Part 1. Ions of similar m/z are most influential because they occupy similar wells (Le., they overlap most in space). Frequency shifts associated with space charge can be large enough to move an entire ion population of a particular m/z out of resonance with a radio frequency (rf) potential tuned to the frequency of a single ion. In explosives detection, for example, it is not possible to predict a priori the quantity of analyte and hence the number of ions likely to be encountered. It is therefore desirable to use an ion activation method that is independent of both m/z and the number of precursor ions. Recently, several approaches that address this space charge problem, such as rapid forward- and reverse-frequency scan to determine optimum frequency and 1.7kHz bandwidth signals (14),have been demonstrated. Random noise (15,16) and a low-frequency dipolar field applied to the end caps (17) have been shown to be effective in dissociating ions independent of m/z and ion number. The latter approach effectively shifts the entire potential well off-center. The ions gain kinetic energy from the trapping field because suddenly they are no longer at the bottom of the well. When the field is reversed, the potential well is shifted off-center again, but in the other direction, once more placing the ions away from the bottom of the well. These techniques, together with mass-selective ion accumulation, should now make direct mixture analysis for targeted compounds with the ion trap practical in the field. The ASGDI source is a form of chemical ionization source. However, conventional filament-based chemical ionization sources have been interfaced with a quadrupole ion trap by Pedder et al. (18)and by Bier et al. (19). (A notable aspect of the work of Pedder et al. is the use of a dc quadrupole to gate the ions into the ion trap. This allows the ion source to be mounted off-axiswith respect to the entrance end cap, as opposed to the lineofsight configuration of Figure 1.)Perhaps the main advantage of an external chemical ionization source over in situ chemical ionization used with the ion trap derives
from the separation of neutral analyte from the mass analysis region. External ion sources greatly reduce the likelihood for ion-molecule reactions and memory effects (19).Furthermore, the pressure regions are much different, which can significantly affect ion chemistry. Inorganic MS.By far, most ion trap work has been directed toward the MS of organic and biological species. However, the unusual characteristics of the ion trap might also prove to be useful in some inorganic MS applications, and inquiries along these lines have begun. For example, high MS/MS efficiencies, which can
Figure 3. Ion trap mass spectra obtained by using ASGDI of the headspace vapors of a mixture of TNT and black powder in air with an ion accumulation time of 1 s. Results obtained (a) without selective ejection of matrix ions and (b) using a filtered-noise field to allow for selective accumulation of the molecular anion of TNT and S;, Si,and S.; (c) Expanded-scaleversion of (b) to show sulfur ions in more detail. (Adapted from Reference 13.)
Analytical Chemistry, Vol. 66, No. 14, July 15, 1994 739 A
be obtained for many organic ions, would be useful in eliminating or reducing the number of polyatomic ions that interfere with the analysis of atomic species. Isobaric interferences can limit some glow discharge MS analyses (20).Two approaches can overcome this problem: use sufficiently high resolving powers to separate the isobars in either sector MS (21) or FT-ICR MS (22),or destroy polyatomic isobars by using collision-induced dissociation instrumentation (23,24). An rf glow discharge ionization source was substituted for the ASGDI source of Figure 1 to explore the strengths and limitations of the ion trap for inorganic analyses (25,26). A particularly promising result was that high MS/MS efficiencies (essentially 100%)were obtainable with metal oxides and hydroxides, as illustrated in Figure 4. These results show that ion trap collisional activation can be used to destroy polyatomic interferences without significantly affecting the signals associated with atomic ions. Another interesting result is that several intense ions associated with the argon used to support the discharge, such as h 2 + , h i , and ArH'' were removed from the ion trap via electron and proton transfer reactions with background gases, yielding H,O+ as the terminal ion. These reactions served to remove the charge from the ion trap because the m/zminused in these experiments exceeded the m/z of H,O+. Glow discharge MS is often used to determine major, minor, and trace atomic components in bulk solids. Beam-type mass spectrometers provide dynamic ranges extending up to lo9 (i.e., trace components present at the part-per-billion by weight level can be determined). As noted in Part 1, dynamic range is a weakness of ion trap techniques. Dynamic range in a glow discharge application would be expected to be poor because argonderived ions typically dominate the spectrum. However, the rapid reactions with background gases tend to remove these ions and mass-selective ion accumulation techniques can provide a dynamic range of at least lo5 (26). The relatively high levels of chemical and background noise in these early experiments precluded greater extension of the dynamic range. Further extension might be possible, but only at the expense
cal applications. Ion-ion interactions complicate the achievement of high performance in these areas for ion trap techniques; however, the limitations of ion traps have not yet been determined nor has much attention been paid to improving their performance in these areas. Given the desirable characteristics of the quadrupole ion trap, such as high MS/MS efficiencies and small size, it is worthwhile to determine the inorganic analysis problems for which the glow discharge-ion trap combination might be suitable.
'
Figure 4- ion trap Ms/ms data derived from the BaOH' precursor ion. (a) The spectrum of isolated BaOH' isotopes prior to collisional activation. (b) The spectrum obtained after collisional activation of '38BaOH'- (Adapted from Reference 25.)
of the mass range of accumulated ions. Ions derived from minor or trace components must be accumulated by allowing ions within only relatively narrow mass ranges to be stored during a given ion injection period. The accumulation of ions over a small mass range is also desirable from the point of view of the measurement precision of isotope abundances. The best ion statistics are obtained by allowing accumulation of the isotopes of interest and excluding all others. There is a tradeoff, therefore, between dynamic range and duty cycle when a variety of e l e ments are to be determined in a given sample, because a series of ion trap experiments is necessary to fully characterize a sample with many components of widely differing concentrations. Dynamic range, abundance sensitivity, and abundance measurement precision are more important in inorganic MS applications than in most organic and biologi-
740 A Analytical Chemistry, Vol. 66, No. 14, July 15, 1994
Atmospheric pressure ionization
Organic MS.ES is the ionization method (27-29) used most often with the quadrupole ion trap. We expect that this combination will be used extensively in both organic and biological applications because the characteristics of ES as an ionization method for organic molecules and biomolecules play directly to the strengths of the ion trap. For example, although the total ion current produced by the ES is as high as 1 pA, 10-"-lO-'o A actually reach the analyzer (28).The current associated with the analyte may be a small fraction of the current injected into the analyzer. Therefore, the potential duty cycle advantage offered by the ion trap is realized because of the long ion accumulation times required. The duty cycle advantage coupled with the transmission advantage in tandem MS experiments allows the ion trap, in favorable cases, to provide detection limits several orders of magnitude lower than those provided in an analogous scanning beam-type instrument experiment. The sensitivity of ESMS to both solution chemistry and gas-phase ion chemistry results in the tendency for researchers to observe the most thermodynamically stable ions. Therefore, ES is particularly prone to matrix effects, and mixture analysis problems are ordinarily addressed by using some form of separation before ionization so that analyte species are subjected to ES one at a time (30).The selectivity of ES for species present as ions in solution usually leads to the concentration of charge in the analyte species. The most intense matrix ions appear at relatively low m/z and can be readily discrimiN
nated against with proper adjustment of m/zmi,. Consequently, ES matrix ions are far less troublesome than those associated with the plasma ion sources described elsewhere in this article. As in other ion injection experiment, the bath gas serves to remove kinetic energy from the ion to facilitate trapping and focuses the ions at the bottom of the potential well. The bath gas plays an additional role that is particularly significant in the ES/ion trap experiment: It provides a mechanism for energy transfer to and from the surroundings. Without ion-ion interactions, trapped ions assume the temperature of the bath gas (i.e., the bath gas thermalizes the ions both internally and kinetically). In contrast to most other forms of MS, ions in the ion trap are not isolated within the analyzer; therefore, the internal energy distribution of the ions that issue from the ion source can change upon storage. Thus, loosely bound species dissociate within the ion trap if they are not kinetically stable at the temperatures they assume upon storage. The significance for ES is that extensive desolvation can occur within the ion trap (31);therefore, the need for desolvation within the ES ion source is eliminated. Indeed, in our laboratory we performed all of the ES/ion trap work with a homemade ES source that does not use dry countercurrent gas flow or heating to assist in desolvation. Although ions derived from relatively small molecules are observed without significant solvation, multiply-charged biomolecules, such as proteins, can be observed as highly solvated ions. Figure 5a, for example, shows the ES ion trap mass spectrum of horse skeletal muscle apomyoglobin acquired without desolvating the ions in the ion source. However, ion trap collisional activation can be used to desolvate the ions once they are trapped, as illustrated in Figure 5b (32).The ability to desolvate ions within the ion trap, as opposed to declustering them in the ion source, is desirable because it allows solvated and loosely bound complexes, such as myoglobin (33) and duplex fragments of DNA (34),to be studied in MS” experiments. Declustering within the ion trap can be accelerated non-mass-selectively during ion accumulation by using broadband excitation without dissociating covalently
bound species. This technique is useful for improving S/N and signal-to-background levels in total ion electropherograms obtained with capillary electrophoresis (CE) coupled with ES ion trap MS (35). Most ES ion trap mass spectrometer configurations have used ion injection through an end-cap electrode, and all have used the commercially available “stretched” electrode geometry (36).(A preliminary report has been published that describes ion injection diagonally along an ion trap asymptote [37].)Most of the recently developed high-performance ion trap techniques have been applied to ions formed by ES, such as high-mass multiply-charged ions. These include extended m/z range via resonance ejection (38), MS”, high mass resolution via slow scanning (39),and high-resolution precursor ion selection (40).Collision-induced dissociation, using both single-frequency resonance excitation and random noise (15),and proton transfer (41, 42) have served as the reactions between MS
Figure 5. Electrospray ion trap mass spectra of horse skeletal muscle apomyoglobin. Spectrum acquired (a) without desolvating the ions in the ion source and (b) using ion trap collisional activation to desolvate the ions once they have been trapped. (Adapted from Reference 32.)
stages. In some cases, both reactions have been used in a single MS” experiment (42).Ion trap collisional activation studies have shown that, via M S n , extensive sequence information can be obtained for peptides (43) and oligonucleotides (44). Analyte quantities required to obtain this information can be as much as 2 orders of magnitude less than those required with a scanning beam-type tandem mass spectrometer. Much of the ES ion trap work has used direct infusion of the analyte-containing solution to introduce the sample. However, ES is particularly well suited as an interface between condensed-phase separations and MS (30).Given the susceptibility of ES to matrix effects ( 4 8 , a separation step is often desirable for mixture analysis. Indeed, many important analytical applications of ES, such as the analysis of tryptic digests, mandate a separation step. Therefore, several studies have evaluated the utility of the ion trap in HPLC (32)and CE (46) applications on line with ESMS and ESMS/MS. The CE study demonstrated excellent S/N in fullscan mass spectra acquired for peptides admitted onto the CE column in quantities > 10 fmol. The duty cycle advantage of the ion trap clearly benefits this application. The high efficiency of ion trap MS/MS can provide even greater sensitivity than that obtained with scanning beam-type mass spectrometers in on-line ESMS/MS applications. However, single-frequency collisional activation is of limited utility in on-line separation applications because of the tuning requirements and potential for frequency shifts due to ion-ion interactions. The development of non-mass-selective ion activation methods can overcome this problem. Alternatively, interface-induced fragmentation or fragmentation induced upon ion injection (47)can be used in on-line separations to obtain structural information. These are not MS/MS experiments and may be compromised in cases of co-eluting mixture components, but they are satisfactory when separations are adequate. On-line separation coupled with ES and ion trap MS has not yet become a routine analytical tool. This is largely because an ion trap system designed for such work is not commercially available. However, the work already done with highly modified
Analytical Chemistry, Vol. 66, No. 14, July 15, 1994 741 A
versions of commercially available ion traps demonstrates that the ion trap will be a powerful tool in many organic and biological applications. Inorganic MS.The inductively coupled plasma (ICP) operated with atmospheric pressure argon as the support gas is the most widely used source of ions for trace elemental analysis (48). Barinaga and Koppenaal have recently reported coupling an ICP mass spectrometer with a quadrupole ion trap (49).The instrument's geometry consists of an ICP ion source followed by a linear quadrupole mass filter mated to a quadrupole ion trap. The linear quadrupole can be operated as a mass filter, as a high-pass filter, or as a notch filter to provide some filtering before ion injection. The sophisticated massselective ion accumulation techniques recently applied in organic analysis applications were not available for this work. Many of the considerations for coupling other forms of ionization for elemental analysis to the ion trap, such as glow discharge and laser ablation (50),also apply to the ICP. The challenges posed by ion-ion interactions are particularly important in ICP/ion trap MS, given the intense ion currents derived from the plasma gas and matrix. Nevertheless, this preliminary work noted solution concentration detection limits in the sub-part-perbillion range. This level of performance is possible because of the discrimination against many normally abundant matrix ions observed in beam-type ICPMS. For example, argon-related ions, such as At-+, Ar;,ArO', and ArN', and the other major matrix ions, such as C10+, were virtually absent from the mass spectrum because of mechanisms for ion destruction, including dissociation and charge transfer reactions. Similar observations were also made in glow discharge/ion trap work. The dramatically reduced abundance of matrix ions also facilitates the analysis of elements that are isobaric with the matrix ions. For example, mass spectra obtained with a beam-type ICPMS system and with the ion trap were compared by using solutions containing iron, arsenic, and vanadium (49).Background mass spectra obtained with the quadrupole mass filter showed intense signals for the matrix ions 35C10+,ArO', and Ar35C1+, among others, which are isobaric with 742 A
the major isotopes of vanadium, iron, and arsenic, respectively. The ion trap background mass spectrum, on the other hand, was essentially devoid of the matrix ions, allowing the spectrum of the analyte to be acquired free of interferences. The elimination of Ar+and ArH' also allowed for the analysis of calcium and potassium. These results, along with those obtained with rf glow discharge ionization, clearly show that the ion trap is potentially useful for the removal of both polyatomic interferences and argon ions and suggest that the ion trap might play a role in ICPMS. Summary
The quadrupole ion trap, operated with a relatively high-pressure bath gas, occupies a unique place among mass analyz-
Characteristics of the ion trap make it attractive for organic and biological applications that use high-pressure ionization. ers. As an ion storage device, it shares many characteristics with the ICR instrument, but the use of a light bath gas and mass-selective instability for mass analysis contrasts with normal ICR operation. The bath gas facilitates the capture of ions injected into the oscillating quadrupole field and cools the ions to the center of the ion trap before mass-selective ejection. The bath gas also provides a conduit for heating ions. Mild collisional activation conditions can dissociate clusters, such as solvated analyte ions, formed by ES. Dissociation of covalently bound ions can be used to obtain structural information as part of a tandem MS experiment or to destroy polyatomic species in elemental analyses.
Analytical Chemistry, Vol. 66, No. 14, July 15, 1994
The moderate background pressure requirements of the ion trap using massselective instability make it a natural mass analyzer to be coupled with high-pressure ionization methods. The ion trap can provide a significant advantage in efficiency over beam-type mass spectrometers, which translates into lower detection limits. Lower detection limits can be realized from superior duty cycle and MS/MS efficiency, although transmission for single-stage MS is comparable. The greatest advantage in efficiency can be obtained with weak ion beams that require long ion accumulation times and therefore high duty cycles. This, in part, makes the ion trap particularly attractive for ES. Bright ion sources can provide similar advantages if major ion beam components can be ejected during ion accumulation to'minimize the deleterious effects caused by ion-ion interactions. The relatively high charge densities found in the ion trap complicate the achievement of high mass accuracy, high abundance sensitivity, and wide dynamic range. Methods to control charge densities, such as mass-selective ion accumulation, are currently being evaluated and show promise for enhancing ion trap performance in these areas. The characteristics of the ion trap already make it particularly attractive for many organic and biological applications that use high-pressure ionization methods. The use of external ion sources, such as ES and chemical ionization, is expected to become widespread. The application of the ion trap as a mass analyzer in inorganic analyses is only now beginning to be evaluated. The results obtained from glow discharge and ICP studies have already demonstrated the capability for discriminating strongly against species that sometimes interfere in analyses performed with beam-type mass analyzers. Additional research will show what role the ion trap might play in inorganic MS analyses. Research performed in our laboratory was sponsored by the US.Department of Energy Offices of Basic Energy Sciences and Safeguards and Securities and the National Institutes of Health under grant GM45372. The Oak Ridge National Laboratory is managed for the US. Department of Energy by Martin Marietta Energy Systems, Inc., under contract DEAC05840R21400.
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Scott A. McLuckey (left) received his B.S. degree from Westminster College (PA) in 1978 and his Ph.D. fiom Purdue University in 1982. He joined the research stafat Oak Ridge National Laboratory (ORNL) where he is currently head of the Analytical Spectroscopy Section in the Chemical and Analytical Sciences Division (Oak Ridge, TN 37831). His research focuses on f i n damental and applied aspects of tandem MS, the chemistry of multiply-charged biomolecules, and quadrupole ion trap MS.
Douglas E. Goeringer (second from right) received his B.S. degree from Southwestern Oklahoma State University in 1973 and his Ph.D. from Purdue University in 1979. He joined the research stafat ORNL in 1979 (Chemical and Analytical Sciences Division, Oak Ridge, TN 37831). His research interests includefindamentals and instrumentation for quadrupole ion trap MS and the application of lasers in analytical MS.
Gary L. Glish (right) is associate profisor of chemistry at the University of North Carolina at Chapel Hill (C348 Kenan LaboraGary J. Van Berkel (second from left) received his B.A. degreefiom Lawrence Univer- tory, CB3290, University of North Carolina, sity (WI) in 1982and his Ph.D.fiom Wash- Chapel Hill, NC27599). He received his B.S. degreefiom Wabash College (IN) in 1976 ington State University in 1987. He then joined O W L and is now a member of the re- and his Ph.D. from Purdue University in 1980. He was a member of the research staff search staffin the Chemical and Analytical at O W L f o r 12years before assuminghis facSciences Division (Oak Ridge, TN 37831). His research interests include development of ulty position at the Universityof North Carolina. His research interests include developinstrumentation and methodsfor MS; sohing new techniques and instrumentation for tion chemistry aspects of ESMS; and ESMS methods for biomedical, environmental, and M s and applying them to the study of gasphase ion chemistry. geochemical applications. Analytical Chemistry, Vol. 66, No. 14, July 15, 1994 743 A
