Time-Gated Pulsed Glow Discharge: Real-Time Chemical Speciation

Anal. Chem. 2003, 75, 1983-1996

Time-Gated Pulsed Glow Discharge: Real-Time Chemical Speciation at the Elemental, Structural, and Molecular Level for Gas Chromatography Time-of-Flight Mass Spectrometry Cris L. Lewis,* Mathew A. Moser,† Don E. Dale Jr., Wei Hang, Christian Hassell, Fred L. King,‡ and Vahid Majidi

Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S. Army Medical Research Institute of Chemical Defense, 3100 Ricketts Point Road, Aberdeen Proving Ground, Maryland 21010-5400, and Department of Chemistry, West Virginia University, Morgantown, West Virginia 26505

A millisecond pulsed glow discharge is used as a versatile ion source for time-gated generation of elemental, structural, and molecular ions. The utility of this ion source for comprehensive chemical analysis of a series of aromatic and halogenated hydrocarbons is illustrated in this manuscript. To highlight the analytical utility of this transient ion source, it was connected to a gas chromatograph for the mass spectrometric determination of mixtures containing benzene, toluene, o-xylene, cymene, tertbutylbenzene, carbon tetrachloride, chloroform, chlorobenzene, tetrachlorethane, and dichlorobenzene. Explicit chemical analysis was accomplished by introducing the GC eluent into a pulsed glow discharge operating at a rate of 100 Hz with a 50% duty cycle. Using three independent digitizers for time-gated acquisition in three separate time regimes, nearly concurrent collection of elemental, structural, and molecular information was accomplished. In general, elemental information was obtained during the first 0.015 ms after the plasma onset; structural information, as ascertained from molecular fragmentation, was obtained during the plateau time regime when the plasma pulse is at a steady state, whereas molecular M+ and MH+ ions were obtained during the afterpeak time regime, that is, after the cessation of the plasma power pulse. For elemental determinations of solutions by mass spectrometric detection, the ion source of choice is inductively coupled plasmas (ICP). This atmospheric pressure argon plasma operates at powers exceeding 1000 W, completely atomizing and ionizing the introduced analytes. Sample introduction for both liquids and gases affords excellent sensitivity (sub-parts-per-billion) across the periodic table. The utility of this source has been demonstrated in several reports as an element-specific detector for different chromatographic techniques.1,2 Despite its excellent elemental * Corresponding author. E-mail: [email protected]. † U.S. Army Medical Research Institute of Chemical Defense. ‡ West Virginia University. (1) Olesik, J. W.; Kinzer, J. A.; Olesik, S. V. Anal. Chem. 1995, 67, 1-12. (2) Waggoner, J. W.; Milstein, L. S.; Belkin, M.; Sutton, K. L.; Caruso, J. A.; Fannin, H. B. J. Anal. At. Spectrom. 2000, 15, 13-18. 10.1021/ac026242u CCC: $25.00 Published on Web 03/19/2003

© 2003 American Chemical Society

sensitivity, this source is limited in its ability to yield chemical speciation because structural and intact molecular ions cannot be readily obtained under normal operating conditions. Researchers have demonstrated, however, that by careful attenuation of the plasma power, one can obtain limited fragmentation information similar to that from electron impact (EI) sources.2 Chemical speciation techniques often rely heavily on separation methods to separate an analyte from a complex matrix. Subsequent detection of the analyte is achieved employing some type of mass spectrometry. One classic example is the analysis of volatile organic compounds, in which chemical identity is deduced from both the separation method and the characteristic elemental composition and molecular fragmentation.3 There are numerous ionization techniques available to provide specific information about particular classes of analytes. The most common ionization method is electron impact, in which the compound is fragmented in consistent patterns, which readily provides structural information and, in most cases, lends itself to identifying the class of compounds, because similar compounds will produce similar fragmentation patterns. In contrast, chemical ionization (CI) predominantly yields molecular weight information, because the molecule often remains intact following the ionization process. It is obvious that plasma, electron, and chemical ionization methods are complementary, each offering unique information regarding the elemental, structural, and molecular constituents in a given sample. To our knowledge, at this time, there is no universal ion source available that is capable of providing chemical data similar to plasma, EI, and CI characteristics simultaneously and independently of one another. Thus, analysts have been forced to sacrifice one or two tiers of information or perform multiple analytical techniques in order to obtain complete analyte identification. Several research groups are investigating low-pressure tunable plasmas in the pursuit of an ion source with both EI and CI characteristics.2,4-7 These ion sources include dc and rf steady state glow discharges, microwave-induced plasmas, inductively (3) Hyver, K. J.; Sandra, P. High-resolution Gas Chromatography, 3rd ed.; HewlettPackard Co., 1989; Chapter 5. (4) Marcus, R. K.; Evans, E. H.; Caruso, J. A. J. Anal. At. Spectrom. 2000, 15, 1-5.

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coupled plasmas, and a switched dc sampling glow discharge. Previous investigations demonstrated that the variation of plasma parameters, such as sampling distance and discharge pressure, could yield the enhancement of signal from intact molecular ions, but not independently from fragment and elemental ions. Because each of these sources operates in the continuous power mode, variation of plasma parameters only enhances or suppresses different ionization mechanisms with the suppressed mechanism still occurring. Of these sources, only the dc sampling glow discharge was operated in a transient mode, that is, a series of ionization pulses at various potentials. During these experiments, collected data resulted from the myriad ion processes equivalent to a continuous source. An alternative is to modulate the ion source and use time-gated detection. Since its introduction by Harrison, the pulsed glow discharge ion source has demonstrated the ability to permit the spatial and temporal separation of different ionization mechanisms, such as electron and Penning ionization.8,9 The possibility of selecting distinct ionization mechanisms grants the analyst greater control in interrogating a chemical compound and performing speciation at several different levels. Electron impact ionization is the standard ionization method for gas chromatography prior to mass spectrometric analysis. This has resulted in the availability of spectral libraries that allow for the rapid identification of most organic compounds via computer matching of the fragmented spectrum with the library spectrum. Chemical ionization is employed to yield principally quasi-molecular (parent) ions from which the molecular weight of a species can be readily determined. Penning ionization is accomplished through the transfer of potential energy from a metastable working gas atom to a target analyte, and thus, is a soft ionization method similar to CI. Using argon as the glow discharge working gas provides metastable argon atoms with potential energy of 11.5 eV, which is sufficient to impart enough ionization energy to convert most compounds to their corresponding molecular ions. Several reports have demonstrated that these ionization processes can be judiciously selected on the basis of operating parameters and selection of the temporal observation window.10,11 There are three main temporal regions in pulsed glow discharge plasmas: the prepeak, plateau, and afterpeak time regimes.12 Ionization during the prepeak, noted for the surge in signal intensity of plasma working gas species upon power initiation, is dominated by electron impact ionization. During the first 5-20 µs of plasma initiation, the most dominate peaks found in the mass spectra are the singly and doubly charged argon ions at 15.8 and 27.7 eV, respectively. Under these harsh ionization conditions, elemental information can be easily obtained, because (5) Belkin, M. A.; Olson, L. K.: Caruso, J. A. Ra J. Anal. At. Spectrom. 1997, 12, 1255-1261. (6) Zapata, A. M.; Robbat, A., Jr. Anal. Chem. 2000, 72, 3102-3108. (7) Guzowski, J. P., Jr.; Hieftje, G. H. Anal. Chem. 2000, 72, 3812-3820. (8) Klingler, J. A.; Savickas, P. J.; Harrison W. W. J. Am. Soc. Mass Spectrom. 1990, 1, 138-143. (9) Lewis, C. L.; Oxley, E. S.; Pan, C.; Steiner, R. E.; King, F. L. Anal. Chem. 1999, 71, 230-234. (10) Lewis, C. L.; Jackson, G. P.; Doorn, S. K.; Majidi, V.; King, F. L. Spectrochim. Acta B 2001, 56, 487-501. (11) Jackson, G. P.; Lewis, C. L.; Doorn, S. K.; Majidi, V.; King, F. L. Spectrochim. Acta B 2001, 56, 2449-2464. (12) Klingler, J. A.; Barshick, C. M.; Harrison, W. W. Anal. Chem. 1991, 63, 2571-2576.

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the plasma energy is sufficient to dissociate species to their constituent atoms and ionize those atoms. Within the next 100 µs, optical emission and mass spectrometric intensities for both working gas and sputtered species reach a steady state, or plateau intensity. Plasma ionization during this time regime arises from a mixture of electron impact, charge exchange and Penning processes. Previous investigations using volatile organics compounds as samples demonstrated that spectra collected during this time resemble EI spectra obtained with electron kinetic energies of 70 eV.13 Once power is terminated, both electron ionization and charge exchange processes halt, and electrons and argon ions recombine to yield metastable argon atoms that result in a significant enhancement in Penning ionization. Data collected through time gated mass spectrometric analysis at this time is devoid of signals arising from species formed through either electron or charge exchange ionization. All ionization occurs exclusively via the Penning process over the following few hundred microseconds. The result is the predominant production of molecular ions without fragmentation. Previous investigations by these laboratories have demonstrated the utility of the pulsed glow discharge approach in the complete elucidation of chemical structure and identity of volatile organics compounds.14,15 In these preliminary experiments, reagents were leaked into the plasma using a needle valve. This often led to chaotic plasma behavior because of the large analyte vapor concentrations. Building on these initial experiments, the next step is to couple a more stable introduction method by which a fixed amount of analyte vapor can be introduced. In this manuscript, we report the first real-time explicit chemical speciation results for a series of aromatic and halogenated mixtures eluting from a gas chromatograph. Chemical speciation is performed in real time, as the eluent is interrogated for elemental, structural, and molecular weight information. EXPERIMENTAL SECTION Reagents. A series of solutions of reagent-grade carbon tetrachloride, tetrachloroethane, chlorobenzene, dichlorobenzene, o-xylene, benzene, toluene, cymene, and tert-butylbenzene (ThetaAccu Kit, AccuStandard, Inc., New Haven, CT) were prepared in optimal grade methanol (Fischer Scientific, Pittsburgh, PA). Stock solutions of each mixture were prepared by diluting 100 µL of pure compounds to a final volume of 1.0 mL using methanol as the solvent. Samples, manually injected using a 5-µL syringe, were introduced into the chromatographic column via a split/splitless injector. Gas Chromatograph. Separations were performed using a gas chromatograph (HP 5980 II, Agilent Technologies, Palo Alto, CA) fitted with a split injector and a 30-m capillary column (DB-1, J & W Scientific, Folsom, CA). During all experiments, the carrier gas was ultrapure argon, which was also used as the working gas for the glow discharge plasma. The carrier gas flow was maintained at 0.80 mL/min throughout all experiments. (13) Steiner, R. E.; Lewis, C. L.; Majidi, V. J. Anal. At. Spectrom. 1999, 14, 15371541. (14) Majidi, V.; Moser, M. A.; Lewis, C. L.; Hang, W.; King, F. L. J. Anal. At. Spectrom. 2000, 15, 19-25. (15) Li, L.; Millay, J. T.; Jackson, G.; Turner, J. P.; King F. L. Energy transfer processes in pulsed glow discharge mass spectrometry of molecular species. Presented at the 2002 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, January 2002.

Figure 1. Pulsed glow discharge time-of-flight mass analyzer system.

Unless otherwise noted, the split injector was operated at a split flow of 25 mL/min for 0.15 min resulting in a spit ratio of 31:1. Manual injections were performed using a 5-µL syringe (Hamilton, Las Vegas, NV) and an injector temperature of 225 °C. The terminal end of the capillary column was introduced into the glow discharge chamber from the gas chromatograph through a 30cm heating jacket consisting of a stainless steel capillary wrapped in electrical heating tape held at 150 °C. The capillary entered the glow discharge chamber (a conflat six-way cross) through a fitting on one of the flanges. The capillary was positioned 1 mm in front and 0.5 mm to the side of the ion exit orifice, as diagrammed in Figure 1. Pulsed Glow Discharge Source. The discharge chamber was a six-way high vacuum stainless steel cross (MDC, Hayward, CA). Side ports on the cross were equipped with a direct insertion probe inlet, ion exit orifice, auxiliary gas inlet, and capillary inlet. The glow discharge cathode was fabricated from a copper rod (SRM 855, NIST, Gaithersburg, MD) into a flat disk measuring 4 mm in diameter and then attached onto the end of the direct insertion probe. The remaining ports were fitted with stainless steel conflat flanges or suprasil optical viewports (Heraeus Quartz, Duluth, GA). The ion exit orifice, measuring 1 mm in diameter, was positioned 180° with respect to the direct insertion probe inlet and 90° with respect to the capillary inlet from the gas chromatograph. Pressure in the discharge chamber was monitored through one of the gas outlets by a Pirani gauge (model APG-M, Edwards High Vacuum Int., Manor Royal, Crawley, West Sussex, England). Pulsed operation of the glow discharge source was accomplished by using a dc remote pulser (model GRX-3.0 K-H, Direct Energy Inc., Fort Collins, CO) to convert a steady state potential from a dc power supply (model 1570, Power Design Inc., Westbury, NY) into a series of high voltage square wave pulses. The glow discharge pulse duration and frequency were controlled by one channel of a two channel digital delay generator (model

PDG-2520, Direct Energy Inc., Fort Collins, CO), denoted “A” in Figure 2. The second delay channel was used for time-gated control of repeller delay gate “1”, which will be discussed in the following section. Throughout all experiments, the glow discharge was operated using a 5-ms power pulse at a repetition rate of 100 Hz. Time-of-Flight Mass Spectrometer. The time-of-flight mass spectrometer was an orthogonal sampling instrument constructed in-house, as shown in Figure 1. Ions produced in the glow discharge source were guided into the time-of-flight acceleration region using a series of electrostatic lenses consisting of a skimmer cone and three stainless steel tubes, Table 1. A roughing pump evacuated the second stage region, located between the skimmer and ion exit plate, to an operating pressure of 5 × 10-3 Torr. In the third stage, two turbomolecular pumps evacuated the accelerator region and flight tube to an operating pressure of 3 × 10-6 Torr. Ion packets were orthogonally extracted via a positive square wave voltage applied to a 5.0-cm stainless steal plate at the end of the accelerating field by a remote pulser (model PVM4210, Direct Energy Inc., Fort Collins, CO). The accelerating lens stack was constructed using a series of five 5.0 -cm stainless steel square plates, each with a 2.5-cm-diameter hole (Kimball Physics Inc., Wilton, NH), with each plate electrically connected by a 1-MΩ resistor. After acceleration, the ions were directed through a stainless steel liner to a dual 25-mm microchannel plate detector (model C-701/25, R.M. Jordan Company Inc., Grass Valley, CA) using a pair of X and Y deflectors. The ion flight path measured 1.25 m. Data were acquired using a pair of digital delay generators and three digitizing oscilloscopes (models TDS 520C and 3012, Tektronix Inc., Beaverton, OR, and model LT342 LeCroy, Chestnut Ridge, NY). The timing sequence for plasma operation and subsequent data collection is shown in Figure 2. The first delay generator of A (model PVM-4210, Direct Energy Inc., Fort Collins, Analytical Chemistry, Vol. 75, No. 9, May 1, 2003

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Figure 2. Block diagram of the time-gated data acquisition system, allowing data to be collected for three separate time regimes.

Table 1. Millisecond Pulsed Glow Discharge Time of Flight Operating Parameters Time-of-Flight Experimental Settings skimmer -800 V lens 1 -110 V lens2 -400 V lens 3 -740 V repeller +140 V bias -2 V width 2 µs accelerator -1800 V liner -1800 V X deflector -1850 V Y deflector -1825 V microchannel plates -2000 V Vacuum Stages first stage second stage third stage

0.3 Torr 5 × 10-3 Torr 3 × 10-6 Torr

CO) has already been discussed as the controller of the glow discharge power frequency and pulse width. Channel two of this unit served as the first delay gate for setting the repeller pulse and observation window 1. The second digital delay generator, B (model DG 535 Stanford Research Systems Inc., Sunnyvale, CA), was triggered externally in sync with delay generator A. The two delay channels of this unit make up the other two repeller pulses and observation windows 2 and 3 for the experiments outlined in this paper. Each repeller pulse was 2 µs long, and all three were tied together to drive the repeller power supply. By triggering each scope with a different delay pulse and splitting signal from the microchannel plate detector, three independent observation windows could be recorded. Data recorded by each oscilloscope was acquired by a 450 MHz Pentium II computer over a GPIB interface using acquisition software (LabView 5.1, National Instru1986


ments, Houston, TX). The data were written to a hard disk after each acquisition set, allowing for a maximum transfer rate of 78 data files/min (∼1.3 Hz). The averaging of each scope was set to 32 triggered events. Ion chromatograms for each separation were constructed by monitoring the highest ion peak intensity over a predetermined m/z domain during the plateau time regime to construct a time vs intensity plot. Typically, this domain was in the m/z range of 70-160, but excluded the m/z ratio of 80 (Ar2+). Data analysis was performed using a separate program in which the three spectra could be analyzed and transposed to an ASCII format and imported into a spreadsheet (Excel XP, Microsoft, Redmond, WA). Using in-house software routines for the data analysis package (LabView 5.1, National Instruments, Houston, TX), specific m/z ratios could be examined for a series of data gates. This allowed for a particular m/z species to be plotted for each time regime throughout the acquired chromatogram as a single ion chromatogram for each ionization regime. RESULTS AND DISCUSSION Initial attempts by this laboratory using the hyphenated GCpulsed glow discharge approach were encouraging, but lacked reproducible molecular results.16,17 During these preliminary experiments, to remain consistent with the normal methodology used for gas chromatography, helium was employed as the GC carrier gas. Sample introduction to the glow discharge plasma (16) Moser, M. A.; Lewis, C. L.; Hang, W.; Hassell, D. C.; King, F. L.; Majidi, V. A pulsed glow discharge time-of-flight mass spectrometer as a detection method for gas chromatography: Concurrent detection of elemental, structural, and molecular information of volatile organic compounds. Presented at the 2002 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, January 2002. (17) Moser, M. A.; Lewis, C. L.; Hang, W.; Hassell, D. C.; King F. L.; Majidi A universal gas chromatograph detector based on low-pressure pulsed plasmas. Presented at the Conference of the Federation of Analytical Chemists and Spectroscopist Societies, Detroit, MI, October 2001.

Figure 3. Energy diagram of different metastable atoms of the noble gases in conjunction with the ionization fragment potentials of common aromatic and halogen compounds. This plot was constructed using molecular and ion fragment data from the NIST database.

with an argon auxiliary gas resulted in a plasma consisting of 50% helium and 50% argon (0.3 Torr He and 0.3 Torr Ar). Unfortunately, the Penning ionization process, which is essential for molecular ion generation, can be significantly quenched by the presence of contaminant gases, even below the 10% level.18,19 Thus, the helium atom population in these preliminary studies, having a much higher concentration and ionization potential (IP) than that of the eluting analyte, quenched the metastable argon atom population, preventing Penning ionization. Realizing this, we changed the GC carrier gas to ultrapure argon. Plasmas produced by helium alone require a much higher pressure (>1.0 Torr) to sustain a stable glow discharge than do those of heavier noble gases, such as argon or krypton. This increased collision density could lead to the unwanted fragmentation of analytes through inelastic collisions as well as a loss in ion transport efficiency. Lowering the pressure in the glow discharge is a more desirable way to mimic electron impact conditions. Using an argon glow discharge, large electron temperatures are found only at very low pressures (0.2-0.5 Torr) and high powers.20 Obtaining the analyte’s molecular parent ion without considerable fragmentation requires a relatively close energy match between the potential energy of the noble gases metastable and the ionization potential of the target analyte. Previous researchers have demonstrated that the degree of fragmentation of aromatic and aliphatic compounds can be controlled by careful selection of the noble gas that was used.21 Using a metastable beam source, it was found that the degree of fragmentation increased with increasing metastable energy. Penning ionization by helium having metastable energies of 20.6 and 19.8 eV led to extensive fragmentation patterns when compared to metastable atoms of argon with metastable energies of 11.7 (18) Smith, R. L.; Serxner, D.; Hess, K. R. Anal. Chem. 1989, 61, 1103-1108. (19) Pan, C.; King, F. L. . J. Am. Soc. Mass Spectrom. 1993, 4, 727-732. (20) Bogaerts, A.; Dokon, Z.; Kutasi, K.; Bano, G.; Pinhao, N.; Pinnheiro, M. Spectrochim. Acta B 2000, 55, 1465-1479. (21) Faubert, D.; Paul, G. J. C.; Giroux, J.; Bertrand, M. J. Int. J. Mass Spectrom. Ion Processes 1993, 124, 69-77.

and 11.5 eV. Because the ionization potential of most common aromatic and halogenated compounds are ∼9-12 eV, using argon as the working gas is a preferred choice. This is further illustrated in Figure 3, the energy plot of organic compounds and metastable atom energy of the noble gases. Each bar represents the energy range of the lowest (ionization of the intact parent ion) to the highest degree of ion fragmentation listed from the NIST database.22 Data Acquisition Rate, Glow Discharge Pulse Rate, and Duty Cycle. Previous investigations employed a millisecond pulsed glow discharge operated at a rate of 50 Hz and a 25% duty cycle. These studies focused on the development of mechanistic models to deduce plasma behavior for both the working gas and sputtered cathode material. The introduction of a transient vapor requires a much faster data collection rate because chromatographic peaks in this study were typically 75 times the data acquisition rate. Each trace collected represented a rolling average of 32 pulse events. The choice of a millisecond pulse width was to ensure plasma stability and limit the amount of material deposited on the cathode surface. Chromatographic Separations. An example of the variety and amount of information obtained from a single experiment is illustrated through the plots and spectra depicted in Figure 4I and II. The ion chromatogram is based on ion fragmentation data taken during the plateau time regime using a pair of data gates positioned over a range of m/z 70-79 and 81-150 (eliminating (22) NIST Chemistry Web Book; NIST: Gaithersburg, MD; http://webbook.nist.gov/chemistry.


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Figure 4. I and II: Glow discharge pulse profile illustrating regions where data is collected and the mass spectrum collected during the plateau region depicting the m/z ratios in the construction of the indexing chromatogram. The chromatogram was collected using a 1-µL sample consisting of 100 µL each of toluene, o-xylene, and dichlorobenzene diluted to 1 mL with methanol. The cathode was positioned at 3 mm from the ion exit plate, and the following spectra represent the background and three analytes during the prepeak, 0.015 ms; plateau, 0.100 ms; and afterpeak, 5.15 ms.

contributions by m/z 80 that is, Ar2+). The range of m/z used for these ion chromatograms are shaded gray area of the middle plot of Figure 4I. The highest peak intensity over this range was recorded, with background subtracted. This intensity value was plotted as a function of time (bottom plot) to provide means for indexing the three digitized oscilloscope traces with elution time. Typical sample injections were performed using 0.5-1.0 µL of solution. The split injector was used to remove the bulk of the methanol solvent and thereby reduce plasma quenching. For each injection, the split injector operated at a temperature of 225°C with a split flow of 25 mL/min for 0.25 min, resulting in a split ration of 32:1. Under these conditions, manual injections of 1.0 µL of a 1:10 analyte to methanol solution would result in the introduction of ∼20 µg of material onto the column. Data Collection. Collection of elemental, structural, and molecular weight information is only possible through time-gated detection of the full mass spectrum during each specific ionization regime. Only a time-of-flight mass spectrometer is capable of performing this task in real time with the eluting analyte stream.23 The following discussion is based on one data point in a chromatogram (Figure 4), which represents three independent mass spectra collected for each eluting species. Background spectra taken during the chromatogram at time A (Figure 4I, bottom plot) are used to illustrate the species and (23) Bings, N. H.; Costa-Fernadez, J. M.; Guzowski, J. P., Jr.; Leach, A. M.; Hieftje, G. M. Spectrochim. Acta B 2000, 55, 767-778.

plasma conditions present before the eluting compound enters the plasma cell. Elemental information can be acquired in both the prepeak and plateau time regime. During the prepeak, high electron temperatures are demonstrated by the large ion intensities of both 40Ar+ (15. 7 eV) and 40Ar2+ (27.6 eV). At this time, molecular analytes that are introduced into the discharge are completely atomized and ionized, and consequently a small abundance of fragments ions are observed. This is illustrated in each prepeak spectrum where the overall ion intensity is weak. Structural information collected during the plateau arises from the mixture of electron impact, charge exchange and Penning ionization processes. Each plateau spectra presented was taken at 0.100 ms after power initiation; at this time the plasma is functioning in a continuous mode similar to steady-state dc glow discharges. Introduction of the organic vapor into the discharge resulted in a fragmentation spectra with excellent correlation to a standard NIST spectrum collected at 70 eV for all three compounds tested. Differences are attributed to processes such as charge exchange and Penning ionization. When compared to the spectra collected during the prepeak, plateau spectra are rich in structural information. Plateau spectra also exhibit the presence of more polyatomic argon-containing species, which may hinder identification of elemental constituents. Molecular weight information consisting of both M+ and MH+ of eluting analytes were collected during the afterpeak time regime, 0.150 ms after power termination. Upon power terminaAnalytical Chemistry, Vol. 75, No. 9, May 1, 2003

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Figure 5. Chemical speciation matrix composed of reconstructed ion chromatograms for different m/z species taken during the prepeak, plateau, and afterpeak for Figure 4.

tion, argon ions and electrons recombine to produce long-lived metastable argon atoms. These metastable species can release their potential energy through inelastic collisions with analytes. Traditional elemental pulsed glow discharge research has focused on this temporal domain as a way of eliminating ions of the residual gases that have IP above the metastable energy.24 Introduction of molecular analytes having IPs below this metastable energy can lead to ionization of the analyte without considerable fragmentation, similar to the situation in chemical ionization. Both elemental and structural ion fragments are not visible in the spectrum at this time, suggesting that the charge exchange and electron impact processes have halted. In each case, the presence of the MH+ ions suggests proton attachment by species such as ArH+. Collisions with incoming analyte molecules would result in proton attachment similar to methane chemical ionization sources. Millisecond pulsed GD plasma reports referenced in this paper all exhibit spectra containing the presence of ArH+ well into the afterpeak time regime. The formation of this species has been elucidated as collisions between argon metastables and elemental hydrogen atoms, whereas the source of hydrogen is most likely from the introduced methanol solvent. This introduction of methanol is analogous to similar effects observed by Ratliff and Harrison when they introduced water into the glow discharge cell.25 Experimentally, the position of this molecular temporal window was found to vary with both pressure and distance. If the temporal window was taken too soon, fragment ions can still be observed. If taken to late, poor molecular sensitivity is obtained. Speciation Matrix. Once data are collected, true chemical speciation can be accomplished using the series of elemental, structural, and molecular weight spectra collected. This is best illustrated in a series of single ion monitoring chromatograms that are constructed post data collection to build a speciation matrix (Figure 5). The indexing chromatogram has been converted from acquisition number, which is the index of the filing system 78 files/min, to the actual experiment time domain, and positioned as the header for each column of the speciation matrix. Each matrix column is denoted by the time domain to which it corresponds, prepeak (column A), plateau (column B), and afterpeak (column C), whereas subsets 1-3 are single ion chromatograms of specific m/z ratios of interest. The indexing chromatogram shows three eluting peaks that have been identified in Figure 4 as toluene, o-xylene, and dichlorobenzene. The absence of the methanol peak is due to the selection of data gates that were used as the basis for the indexing chromatogram (shaded area of Figure 4I), which were judiciously positioned for eluting species with larger m/z. The presence of the eluting methanol can be inferred by spectra collected during the prepeak, plateau, and afterpeak, as evident in the signals obtained from 12C+, 16O+, and 65Cu+, shown in the ion chromatogram subset A1, A2, B1, and C2 in Figure 5. The eluting methanol is verified in both single ion chromatograms taken during the prepeak and plateau for 12C+ and 16O+ by the chromographic peaks present at 100 s. The tailing of the carbon peak in both the prepeak and plateau region is likely a result of carbon deposition on the cathode surface. Note, however, that neither species is present in data collected during

the afterpeak, as seen in inset C1 in Figure 5. The absence of each is attributed to the ionization potential and relative amount in the atomic form at this time. Oxygen, 13.6 eV, is above that of the argon metastable atom, 11.5 eV, and one would not expect it to contribute to the mass spectrum. The IP of carbon, however, is 11.2 eV, so one would expect it to be visible based on energy; however, the absence is a result of the metastables argon atom’s inability to fragment and ionize the parent molecule to the elemental carbon ions. During the afterpeak, the eluted methanol acts as a quenching agent for the metastable ionization of the sputtered material, 65Cu+, shown as inset C2 in Figure 5. The use of the 65Cu+ over more abundant 63Cu+ is because the more abundant isotope saturates the digitizer. As organics are eluted, interaction with the plasma results in sputtered ion intensity decreasing as the metastable population is quenched. This single ion chromatogram demonstrates that the metastable population fully recovers before the next analyte is introduced. As an element specific detector, let us compare spectra taken during the prepeak and plateau. All compounds introduced contain carbon, but because of significant solvent carbon deposition on the cathode surface, it will not be included in this part of the discussion. Methanol was the only source of oxygen for this analysis; the single ion chromatograms of 16O+ taken during the prepeak and plateau demonstrate a different number of eluting peaks, shown as insets A2 and B1. Both demonstrate an eluting peak at 100 s that has been characterized as solvent methanol; however, the plateau also demonstrates a second peak at 260 s (inset B1), which has been previously identified as dichlorobenzene. This contrast in information demonstrates why data must be taken in three time domains. The origin of the second peak in the plateau ion chromatogram is attributed to the combination of ionization and plasma recombination processes that have occurred during the 100 µs prior to data acquisition. By collecting data during the prepeak, 15 µs, these processes are limited to only electron impact ionization, with little time for plasma byproducts to form. This is further illustrated in the single ion chromatograms for the eluting chlorine ions of dichlorobenzene. The prepeak chromatogram demonstrates only one visible peak for 35Cl+ occurring 275 s into the run, seen in inset A3. The plateau intensity is much stronger than the prepeak, allowing for the isotopic determination of both isotopes, seen in inset B2. In contrast, no 35Cl+ is detected in the afterpeak (inset C ). This observation 1 confirms that the ionization mechanism is dominated by Penning ionization, not charge exchange between 40Ar+, IP ) 15.8 eV, and 35Cl+, IP ) 13.0 eV. Structural information collected during the plateau can be used as a quick indicator in identifying classes of eluting compounds. For example, all of the compounds in the run shown in Figure 4 are substituted benzenes, and elution can be tracked by monitoring ion intensity at m/z 91, the tropyllium cation. The tropyllium cation is formed during ionization of carbon-substituted benzenes.26 The single ion chromatogram of m/z 91 demonstrates that the first two eluting species are carbon-substituted benzenes (inset B3). The difference in intensity of these peaks arises from the differences in fragment abundance upon ionization. The intensity of the third species, denoted as dichlorobenzene, which

(24) Duckworth, D. C.; Smith, D. H.; Mcluckey, S. A. J. Anal. Atom Spectrom. 1997, 12, 43-48. (25) Rattliff, P. H.; Harrison, W. W. Spectrochim. Acta B 1994, 49, 1747-1757.

(26) Cooper, J. W. Spectroscopic Techniques for Organic Chemists; John Wiley & Sons: New York, 1980.


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Figure 6. I and II: The chromatogram was collected using a 1-µL sample consisting of 100 µL of benzene, toluene, o-xylene, cumene, and tert-butylbenzene and diluted to 1 mL with methanol. The cathode was positioned at 3 mm from the ion exit plate, and the following spectra represent the background and three analytes during the prepeak, 0.015 ms; plateau, 0.100 ms; and afterpeak, 5.15 ms.

is not carbon-substituted benzene, demonstrates insignificant intensity from m/z 91. The intact molecular ion is the last piece of the puzzle that allows speciation to be validated. We can now look at the intact molecular ions, which are collected in the afterpeak with masses of 93, 107, and 148, shown in C3. However, for analyte identification, we will have to consider ionization by both Penning ionization and protonation by ArH+ and H30+. The first eluting peak can only be toluene: upon fragmentation, it converts to the tropyllium ion and has a parent mass at 92, thus suggesting that the observed molecular is MH+. The second peak is also a substituted benzene with no oxygen and a parent mass of 107, which would suggest it is o-xylene. Further confirmation can be obtained with the comparison of the fragmentation pattern collected during the plateau, Figure 4II, with the NIST EI for o-xylene. From the single ion chromatograms, we know that the third and final peak may be a substituted benzene with at least one chlorine functional

group and having a molecular weight of 148. However, on careful inspection of the fragmentation pattern collected, we see very little intensity at 91; instead, the major ion fragment is found at 111. This would be the loss of chlorine from the parent mass of 146. The fragmentation pattern is not a complete match with the NIST EI spectrum for 1,3-dichlorobenzene, because the ion abundances at m/z 146 and 147 are much lower than those at 111 and 113. However, using the molecular weight from the parent ion spectra, the presence of chlorine, and the fragmentation pattern collected, we are able to ascertain that the compound is dichlorinated benzene. The overall sensitivity of intact parent ion chromatograms was found to be weak, S/N ∼ 12:1. This is attributed to the current interface design and will be addressed in future investigations. Determinations of Mixtures of Aromatics and Chlorinated: Limitations. The following chromatogram and spectra were obtained for an aromatic mixture containing of benzene (IP Analytical Chemistry, Vol. 75, No. 9, May 1, 2003

1993

1994


Figure 7. I and II: The chromatogram was collected using a 1-µL sample consisting of 100 µL of carbon tetrachloride, chloroform, chlorobenzene, 1,1,2,2 tetratchloroethane, and 1,3-dichlorobenzene. The cathode was positioned at 3 mm from the ion exit plate, and the following spectra represent the background and three analytes during the prepeak, 0.015 ms; plateau, 0.100 ms; and afterpeak, 5.15 ms.

) 9.2 eV), toluene (IP ) 8.8 eV), o-xylene (IP ) 8.6 eV), cumene (IP ) 8.7 eV), and tert-butylbenzene (IP ) 8.7 eV), as shown in Figure 6I-6II. The spectra presented for each analyte were background-subtracted using the spectrum at time A to remove background discharge ions. Structural information of each analyte taken during the plateau resulted in a fragmentation pattern similar to that found in conventional NIST EI spectra. Molecular ions produced in the afterpeak resulted in the predominant formation of the MH+ and M+ ions, with the exception of tert-butylbenzene. Using argon metastable atoms (11.5 eV), Penning ionization of tert-butylbenzene resulted in formation of ions at masses 56 and 119; the actual analyte molecular weight is 134. The mass of 119 is the C9H11+ ion resulting from the loss of one methyl group, and the appearance energy of this product was determined to be 10.26 eV using electron impact,27 whereas the m/z ratio at 56 represents the cleaving of the tert-butyl functional group. This (27) Howe, I.; Williams, D. H. J. Am. Chem. Soc. 1969, 91, 7137-7144.

result further illustrates that while we may be able to control the amount of energy in the afterpeak by selection of the reagent gas, we cannot control the molecule’s native partitioning of this energy. Further, using helium as the reagent gas will not work in this pulsed glow discharge approach, because its metastable energy is far too high for most common analytes. The last chromatogram was obtained using a mixture of halogenated compounds containing carbon tetrachloride (IP )11.5 eV), chloroform (IP ) 11.4 eV), chlorobenzene (IP ) 9.1 eV), 1,1,2,2 tetratchloroethane (IP ) 11.1 eV) and 1, 3-dichlorobenzene (IP ) 9.1 eV), shown in Figure 7I and 7II. In each case, the eluting analyte resulted in the presence of elemental chlorine in both the prepeak and plateau. The structural information obtained demonstrates similarities with conventional NIST EI spectra. In most cases, the observed ion fragments were the same, although the abundance patterns differed because of plasma processes, such as charge exchange and Penning ionization. Molecular ions were Analytical Chemistry, Vol. 75, No. 9, May 1, 2003

1995

not observed for both carbon tetrachloride (IP ) 11.5 eV) and 1,1,2,2 tetratchloroethane (IP ) 11.1 eV). In each case, the metastable energy should be more than sufficient to ionize and even surpass the ionization potential of the parent molecule. The absence of carbon tetrachloride M+ and MH+ ions will be addressed in future studies. The absence of the molecular parent of 1,1,2,2 tetrachloroethane is the result of the transfer in potential energy of the metastable argon atom being repartitioned, resulting in product ions. The NIST listed ionization energy of this molecule is 11.6 and 11.1 eV. Using the highest listed value, this energy can be assessed by a 3P0 argon metastable state with an energy of 11.7 eV. This further demonstrates the limitation in this approach for certain analytes, which may fragment during Penning ionization. CONCLUSIONS A pulsed glow discharge ion source is capable of producing EI and CI-like ionization for gas chromatographic eluents. Use of time-gated detection over the prepeak, plateau, and afterpeak time regimes allows for chemical identification, including the elemental, structural, and molecular level. This was demonstrated for a series of aromatic and halogenated compounds. Elemental information was collected during the prepeak time regime independent of signal arising from intact molecular fragments. Structural informa-

1996


tion collected during the plateau time regime resembled 70 eV EI NIST reference spectra. Intact molecular ions were observed independent of fragmentation peaks during the afterpeak time regime, ∼150 µs after power termination. In certain molecules, the energy imparted during the Penning processes was repartitioned, resulting in subsequent fragment product ions formation. Tailoring the discharge parameters in conjunction with time-gated data acquisition allows the analyst unprecedented control over the processes leading to ionization. Future experiments will focus on the optimization of this technique and improvements in sensitivity. ACKNOWLEDGMENT The authors thank Lei Li at West Virginia University for his helpful discussions in setting up the proper plasma parameters. We are grateful to David Wayne, Nuclear Materials and Technology Division at Los Alamos National laboratory, for his invaluable help. F.L.K. gratefully acknowledges support of this work in part from the U.S. Department of Energy, DE-FG02-00ER45837.

Received for review October 17, 2002. Accepted February 14, 2003. AC026242U

Time-Gated Pulsed Glow Discharge: Real-Time Chemical Speciation

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