Time-of-Flight Mass Spectrometer for the

Anal. Chem. 1997, 69, 3780-3790

Development of an Ion Store/Time-of-Flight Mass Spectrometer for the Analysis of Volatile Compounds in Air David M. Chambers,* Louis I. Grace, and Brian D. Andresen

Forensic Science Center, Lawrence Livermore National Laboratory, Livermore, California 94551

A time-of-flight mass spectrometer (TOF-MS) that incorporates an ion storage trap as a pulsed extraction source and sample preconcentrator has been developed for air monitoring. This new instrument configuration is designed for real-time monitoring of changes in trace volatile and semivolatile compound concentrations in air. Typically, detection limits are in the low parts per billion (v/ v) range. This ion store/TOF-MS configuration permits a relatively large volume air flux to be drawn directly into the ion trap cavity, creating a local pressure that exceeds 10-4 Torr. Operating the ion trap at this pressure increases the extent of chemical ionization with the abundant N2+ and O2+ that are formed by electron impact. The air background also serves as a suitable collisional damping matrix, helping to compress the ion cloud to the center of the trap, which is ideal for TOF-MS analysis. Ions are typically accumulated in the trap for 400 ms, at which point they are extracted through the exit end cap and analyzed within 100 µs, thus yielding a duty cycle near 100%. The necessary hardware and electronics configuration needed to obtain optimum resolution (routinely 2000 m/∆m, fwhm), mass accuracy and sensitivity is described.

ionization, electron capture, or mass spectrometric detection.1 Sorbent traps and canisters are well suited for field sampling because they are easy to handle and use simple equipment. Both of these approaches, however, are susceptible to positive and negative errors, which can result from contamination as well as from reaction or decomposition, which some compounds undergo during adsorption and removal. For example, in the analysis of phosgene, glass or metal tubing cannot be used in the sampling device because these materials catalyze decomposition that becomes significant at concentrations below 5 ppm.2 Commercially packaged systems using cryogenic or sorbent traps followed by GC/MS currently yield low- to sub-ppb detection of compounds in air.3-7 Similar detection limits can be achieved without preconcentration, and in near real time, by membrane introduction8,9 and direct introduction mass spectrometry.10-15 The most recently developed direct air monitoring systems demonstrated to achieve these low detection limits have involved the use of ion trap mass spectrometers, which have an ion storage capability that can be used to improve sensitivity. Nevertheless, the storage capability of the trap is limited by certain properties of the resonant excitation and mass-selective instability ion ejection method used for mass analysis. In this process, only half the ion cloud is directed toward the exit end cap, and the process itself is adversely affected by space charge, which causes a decrease in resolution and a shift in apparent mass.16,17 As a result, the

The desire to comply with legislation such as the 1990 amendments to the Clean Air Act, which govern the release of volatile and gaseous compounds, has increased the need for air analysis. Many of the regulated chemicals are classified as volatile organic compounds (VOCs) because they have vapor pressures over 75 mTorr and boiling points between -30 and 300 °C. Although most of these chemicals are essentially unique, a good number have similar physical properties and are closely related in structure and molecular weight. In addition, analysis scenarios can vary widely among applications, which include monitoring the workplace, industrial stack output, vehicle exhaust, the use of pesticides, releases from conflagrant sources, chemical spills, and surveillance. Most often in these applications, real-time and specific identification of trace compounds amid an air background is needed. Such analysis is difficult because the concentrations of the major components of air dwarf those of the analytes and also because the air matrix often contains VOCs in concentrations comparable to or greater than those of the analytes. The most common method for monitoring VOCs in air involves collecting samples with containers (e.g., plastic bags, glass or metal canisters) or solid sorbent traps (e.g., Tenax, XAD, silica gel, or carbon) followed by gas chromatography (GC) with flame

(1) Berezkin, V. G. Gas Chromatography in Air Pollution Analysis; Elsevier: New York, 1991. (2) Methods of Air Sampling and Analysis; Lodge, J. P., Ed.; Lewis: Chelsea, MI, 1989; p 4. (3) Almasi, E.; Kirshen, N.; Kern, H. Int. J. Environ. Anal. Chem. 1993, 52, 39-48. (4) Davoli, E.; Cappellini, L.; Moggi, M.; Fanelli, R. J. Am. Soc. Mass Spectrom. 1994, 5, 1001-1007. (5) McCaffrey, C. A.; Maclachlan, J.; Brookes, B. I. Analyst 1994, 119, 897902. (6) Bianchi, A. P.; Varney, M. S. J. Chromatogr. 1993, 643, 11-23. (7) Oliver, K. D.; Adams, J. R.; Daughtrey, E. H.; McClenny, W. A.; Young, M. I.; Pardee, M. A.; Almasi, E. B.; Kirshen, N. A. Environ Sci. Technol. 1996, 30, 1939-1945. (8) LaPack, M. A.; Tou, J. C.; Enke, C. G. Anal. Chem. 1990, 62, 1265-1271. (9) Cisper, M. E.; Gill, C. G.; Townsend, L. E.; Hemberger, P. H. Anal. Chem. 1995, 67, 1413-1417. (10) McLuckey, S. A.; Glish, G. L.; Asano, K. G. Anal. Chim. Acta 1989, 225, 25-35. (11) Ketkar, S. N.; Dulak, J. G.; Dheandhanoo, S.; Fite, W. L. Anal. Chim. Acta 1991, 245, 267-270. (12) Kelly, T. J.; Kenny, D. V. Atmos. Environ. 1991, 25A, 2155-2160. (13) Wise, M. B.; Thompson, C. V.; Buchanan, M. V.; Merriweather, R.; Guerin, M. Spectroscopy 1993, 8, 14-22. (14) Hart, K. J.; Dindal, A. B.; Smith, R. R. Rapid Commun. Mass Spectrom. 1996, 10, 352-360. (15) Gordon, S. M.; Callahan, P. J.; Kenny, D. V. Rapid Commun. Mass Spectrom. 1996, 10, 1038-1046.

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S0003-2700(97)00102-9 CCC: $14.00

© 1997 American Chemical Society

storage capacity of the ion trap is ∼100 000 ions for optimal operation,18 of which only a small fraction reaches the detector. There have been several approaches taken to fully use the storage capacity of the ion trap. In one, the symmetry of the trapping fields is altered to displace the ion cloud toward the exit end cap, where it is preferentially forced to exit. In this case the signal is enhanced by a factor of 2.19 Another approach uses selective ion storage to resonantly eject high-abundance matrix ions and allow the buildup of target species.20 This selective ion monitoring approach has been demonstrated in certain air monitoring applications to achieve part-per-trillion detection.9,21 Recently, a number of groups have been developing time-offlight mass spectrometers (TOF-MS) for air and other gas analysis applications.22-24 The advantages of TOF-MS are short analysis times (submilliseconds per spectrum acquisition), high analyzer throughput efficiency, and a broad dynamic mass range capability. However, TOF-MS analyzers are best configured with ionization sources that impart the narrowest spatial and kinetic energy distributions, otherwise poor resolution results. Pulsed extraction is one technique that permits the use of continuous-beam ionization sources with time-of-flight mass spectrometers. The most commonly used pulsed method is orthogonal extraction, whereby a focused ion beam traveling in the y direction is extracted perpendicularly, in the z direction, along the flight tube axis.25 Although high resolutions can be achieved because the velocity distribution in the z direction is narrow, the mass range for which one can acquire a single spectrum can be limited, depending on the y-direction velocity.26 Another emerging pulsed extraction method involves the use of ion traps to collect and spatially confine ions from a continuous beam.27-29 This approach uses the ion storage capability of the ion trap to increase the signal as well as the duty cycle, yet it eliminates the less efficient process of resonant excitation with mass-selective instability. Following the ion accumulation step, an extraction pulse is typically applied to the exit end cap, pulling the entire ion population down the flight tube. The use of the ion trap as a pulsed extraction source for TOFMS is particularly attractive for air analysis because the trap storage parameters can be easily set up to exclude abundant air matrix species while other targets are enhanced through accumulation. In addition, both the ion trap, when operated in pulsed extraction mode, and TOF-MS maintain relatively high (16) Franzen, J. Int. J. Mass Spectrom. Ion Processes 1991, 106, 63-78. (17) Wang, X. M.; Bohme, D. K.; March, R. E. Can. J. Appl. Spectrosc. 1993, 38, 55-60. (18) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry; Wiley: New York, 1989; p 182. (19) Marquette, E.; Wang, M. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA, 1993; p 698a. (20) Kenny, D. V.; Callahan, P. J.; Gordon, S. M.; Stiller, S. W. Rapid Commun. Mass Spectrom. 1993, 7, 1086-1089. (21) Goeringer, D. E.; Asano, K. G.; McLuckey, S. A.; Hoekman, D.; Stiller, S. W. Anal. Chem. 1994, 66, 313-318. (22) Sin, C. H.; Lee, E. D.; Lee, M. L. Anal. Chem. 1991, 63, 2897-2900. (23) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403-1407. (24) Franzen, J.; Frey, R.; Nagel, H. J. Mol. Struct. 1995, 347, 143-152. (25) Coles, J.; Guilhaus, M. Trends Anal. Chem. 1993, 12, 203-213. (26) Verentchikov, A. N.; Ens W.; Standing, K. G. Anal. Chem. 1994, 66, 126133. (27) Chien, B. M.; Michael, S. M.; Lubman, D. M. Anal. Chem. 1993, 65, 19161924. (28) Michael, S. M.; Chien, M.; Lubman, D. M. Rev. Sci. Instrum. 1992, 63, 4277-4284. (29) Michael, S. M.; Chien, B. M.; Lubman, D. M. Anal Chem. 1993, 65, 26142620.

Figure 1. Diagram of IS/TOF-MS instrument configured with a Finnigan ITMS EI source and ion trap. L1, L2, and L3 are einzel lens elements. XY1 and XY2 are beam steering plates.

throughput efficiencies. The result is an instrument with low detection limits and short analysis times. The work presented here focuses on the development of the ion trap to improve the overall performance of the IS/TOF-MS used for air monitoring. A number of operating parameters are identified that increase trapping and storage efficiency. Further studies involve determining those conditions that promote the formation and extraction of the ion cloud with the proper spatial and velocity distributions for focusing in the TOF-MS. The importance of these findings is reflected in the instrument performance characteristics, which are described with respect to resolution, mass accuracy, and detection limits. EXPERIMENTAL METHOD The IS/TOF-MS instrument used is similar to the system available from R. M. Jordan Co.27-29 but uses a Finnigan ITMS ion trap assembly instead of the R. M. Jordan trap. In this system (refer to Figure 1), there is no standard acceleration grid behind the exit end cap as described in refs 27 and 28, and the first einzel optic (L1) is located 5 mm from the innermost back surface of the end cap. The ion trap chamber is designed to accommodate a Finnigan ITMS electron impact (EI) source or a laboratoryconstructed atmospheric sampling glow discharge ionization (ASGDI) source.10 Both of these ionization methods were studied for their suitability in this application. The IS/TOF-MS electronic components used in this system are shown in Figure 2. Although many of the components are similar to those available from R. M. Jordan Co.,27-29 several changes were needed to improve spectrum-to-spectrum reproducibility, signal-to-noise ratio, and resolution for lower m/z ions. Most of these modifications involve optimizing the ion trap for pulsed extraction, which is quite different from the normal operating conditions used in conventional ion trap mass spectrometers. Spatial and kinetic energy distribution and density of the ion cloud extracted from the ion trap were optimized for the time-of-flight mass spectrometer, which is unchanged. EI and ASGDI Sources. The ion trap is fitted with a Finnigan ITMS EI source assembly, which is driven by an R. M. Jordan Co. EGUN C-950 power supply. In this configuration, air is introduced directly into the ion trap cavity through a Teflon transfer line, which is fed through a needle valve. Because the Finnigan source has no collector electrode, the EGUN supply is configured by grounding the circuit collector signal. This bypasses the emission regulator circuit in the power supply. The bias potential is allowed to float. The ASGDI source is installed by removing the filament assembly and replacing the EI source head adapter with a 3-mmthick retaining plate. The source ionization chamber slides into Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

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Figure 2. Wiring diagram for ion store/time-of-flight mass spectrometer configured with EI source.

Figure 3. Exploded view of atmospheric sampling glow discharge ionization source.

the front cavity of the IS/TOF-MS through a 10.8-cm-diameter opening and butts up against the retaining plate. This source (Figure 3) is similar in construction to the one previously described.30 It differs, however, in the dimensions of the anode and cathode, which are roughly half the size, and in that the source chamber wall is electrically isolated. Smaller electrodes decrease the collection and sputtering area and, thus, lower power supply current requirements. Connections that allow the wall potential to be biased provide an additional means to alter ion and electron populations. The anode and cathode are supplied by separate 0-3000-V (66 mA maximum) power supplies (Bertan, Model 18253P and -3N, respectively). The source chamber wall is electrically floated with a 0-300-V (100 mA maximum) supply (Hewlett (30) Chambers, D. M.; McLuckey, S. A.; Glish, G. L. Anal. Chem. 1993, 65, 778-783.

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Packard, Model 6209-B). The power supplies are contained within a single unit. During operation, the air sample expands into the glow discharge region through a 0.1-mm-diameter orifice in the anode sampling plate (positive bias) forming a free-jet expansion. The discharge, which is sustained between the sampling plate and cathode skimmer cone (negative bias), self-ignites when the potential difference between the electrodes is ∼350 V at a source pressure under 1 Torr. Ions are formed mainly behind the shockwave structure of the expansion and are sampled through a 0.5-mm orifice in the skimmer cone, which has an outer cone angle of 74.6°. The skimmer orifice is large enough that the ion flux can travel the 3.65 cm to the entrance end cap without any focusing and adequately fills the trap well within a 400-ms storage time. In this particular source, the distance between the cathode sampling plate and anode skimmer cone can be varied from 0 to 20 mm. The ionization process is controlled by adjusting the electrode potentials, the sampling plate/skimmer cone spacing, and the source pressure. Ion Store. The ion trap assembly is bolted to the front of the TOF-MS envelope. Ion storage is performed with both end caps held at ground potential while the rf is applied to the ring electrode. Following storage, ions are injected into the TOF-MS by clamping the rf potential to 0 V at the proper phase angle and pulsing the end cap and the gate optics. The entrance end cap is pulsed with an opposite potential from the exit end cap. Two fast high-voltage transistor switches (Behlke, Model HTS 31-GSM) supply the extraction potentials and achieve a 7.7-ns rise time from 10 to 90%. The rf power supply (R. M. Jordan, Model D-1240) is modified for increased voltage output of 1000 Vp-p, and a clamp has been added that cuts off the rf with a fall time of 21.1 ns from 90 to 10%. A signal processing circuit samples the output monitor on the rf power supply and converts the sine wave into a square wave, which is used to trigger a delay/pulse generator (Stanford Research Systems, Model DG353). Ion storage time and extraction phase angle are controlled by delaying the rf clamp and extraction pulse, which are triggered from the same channel. The extraction pulse is typically held for 300 µs, which allows enough time for the largest ions to reach the detector. A long extraction pulse is necessary in this system because the end cap fields penetrate the “field-free” region of the flight tube, perturbing ion trajectories. TOF. The TOF mass analyzer portion of the instrument (Figure 1) is an R. M. Jordan Co. Model D-850. The flight tube is 8 in. in diameter and ∼1 m in length and is fitted with an angular reflectron, Model C-852. The front of the TOF-MS chamber is separately enclosed by a stainless steel shield that directly seals to the exit end cap with a ceramic spacer. Ions that are extracted from the ion store pass through an einzel lens and then between a pair of beam-steering plates labeled XY1 and XY2. The einzel lens assembly, which consists of three optics, labeled L1, L2, and L3, is located 5.0 mm from the exit end cap apertures. Typically, L1 and L3 are held at the flight tube potential, and L2, XY1, and XY2 are controlled independently by means of an AREF power supply (R. M. Jordan, Model D-803). The ion source is held at ground potential and ions are accelerated into the flight tube, which is held at -1250 V with the use of a liner. The reflectron used in this system has two stages and consists of 21 stacked plates that are 0.045 in. (1.1 mm) thick and spaced 0.15 in. (3.8 mm) apart except for the first two plates, which are 0.41 in. (1.0

cm) apart. The first plate, which is gridded, is electrically attached to the flight tube liner. The second plate, R1, and the last plate in the series, R2, are also gridded. All of the plates between and including R1 and R2 are connected in series by a string of 10MΩ resistors. The AREF power supply also supplies D1, D2, and D3, which power a 40-mm dual-microchannel plate (Galileo, part no. 13303320) detector assembly (R. M. Jordan, part no. C-726). The microchannel plate detector is fitted with an input grid that is electrically connected to the flight tube liner. Vacuum System. The IS/TOF-MS consists of two vacuum chambers that are pumped by separate turbomolecular pumps (Balzers, Model TPU 180 H, 180 L/s). A single rotary vane pump (Edwards, Model E2M30, 38.9 m3/h) is used to back both turbomolecular pumps as well as the additional vacuum stage associated with the ASGDI source. The ASGDI source chamber is maintained by an automated feedback-controlled throttle valve (Edwards, Model 1800) that is in line between the source and the rotary vane pump. For both the EI and ASGDI sources, typical operating pressure in the ion store envelope is dependent on the flow through the sample inlet and is set at ∼10-4 Torr. For the ASGDI, ionization efficiency is controlled by adjusting the firststage pressure. This is accomplished by choosing the appropriate sampling plate and skimmer orifice diameters and by adjusting the throttle valve. In EI operation, flow through the sample inlet is adjusted with a needle valve. An auxiliary inlet provides a means of keeping the ion trap envelope pressure constant as sample flow is varied or of keeping the sample flow constant and varying the envelope pressure. The pressure in the ASGDI source is measured with a capacitance manometer. The pressures in both the ion store and TOF-MS stages are monitored with cold cathode gauges. In the trap chamber, the cold cathode gauge is located 15 cm from the center of the trap cavity. Signal Processing and Data Storage. Data are collected with a LeCroy digital oscilloscope, Model 9360, which uses an 8-bit ADC capable of digitizing 5 gigasamples/s at a 600-MHz bandwidth. Standards and Reagents. Detection limits and response were quantified by sampling a gas standard diluted with a standard compressed air matrix. Gas standards were generated with gas permeation tubes from VICI Metronics by using a gas standards generator, Kin-Tek Model 585. This system is able to generate standards with an accuracy of (5%. RESULTS Source Conditions. For direct monitoring, air is sampled into the ion trap cavity of the IS/TOF-MS, which is maintained at a pressure of at least 10-4 Torr. This is different from conditions in a standard quadrupolar ion trap mass spectrometer, where the background pressure is maintained in the 10-7 Torr range and helium is used as a buffer gas at ∼1 mTorr.31 In this case, helium is a suitable buffer gas because it collisionally cools ions without significantly damping their secular motion, which is needed for resonant excitation. Resonant excitation32 is used in combination with mass-selective instability31 and involves applying an excitation potential to the end caps, which forces ions out from the center of the trap along the z axis toward the end caps. The use of air (31) Stafford, G. C., Jr.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98. (32) Fulford, J. E.; Hoa, D.-N.; Hughes, R. J.; March, R. E.; Bonner, R. F.; Wong, G. J. J. Vac. Sci. Technol. 1980, 17, 829-835.

Figure 4. Effect of trap background pressure on peak height and width at variable and fixed sample flow rates. Peak width data correspond to fixed sample flow rate curve. Fixed sample flow rate, 1.63 L/min. M+ from benzene at 100 ppb (v/v); ion storage time, 0.4 s; maximum flight tube pressure, 2.8 × 10-5 Torr.

as a collisional damping gas33,34 has been most successful in nonlinear ion traps,35 primarily because they excite these resonances at higher frequencies than do linear traps, resulting in faster scan speeds, and thus reducing the number of collisions during scan out. In one study, the air background was increased to 5 × 10-4 Torr, resulting in modest losses in resolution but increased signal.36 In the IS/TOF-MS, ions are introduced into the mass analyzer by pulsed extraction from an ion trap operated in storage mode, that is, at a low constant rf potential. Most of the ions are extracted out of the trap cavity within 200 ns. This short extraction time reduces the opportunity for collisions during pulse out, which makes this approach ideally suited for operation at elevated pressures. The effect of increasing the trap background pressure on resolution and signal is shown in Figure 4 for the M+ from benzene at 100 ppb (v/v). In this experiment, two separate inlets were used to introduce the sample and background air into the store cavity. The first curve was taken by increasing just the sample inlet flow rate to raise the store background pressure. The enhanced response seen as the sample inlet flow is increased is attributed mainly to the growth in ionization rate from the increase in the partial pressure of benzene. The second curve in Figure 4, which is drawn with a dashed line, supports this explanation by showing the effect of raising the background pressure while the inlet flow is fixed. In this case, the sample flow is set to 1.63 L/min and a second inlet is used to vary the background pressure. There is a slight increase in signal that is attributed to greater collisional damping and, to a lesser degree, enhanced chemical ionization at these higher pressures, which is discussed below in more detail. As the ion store pressure exceeds 0.11 mTorr (1.4 × 10-5 Torr corresponding flight tube pressure), increased scattering collisions during TOF analysis are believed to contribute to the drop in signal. Many of these collisions likely occur as ions are extracted from the trap. In (33) Cameron, D.; Hemberger, P. H.; Alarid, J. E.; Leibman, C. P.; Williams, J. D. J. Am. Soc. Mass Spectrom. 1993, 4, 774-781. (34) Lammert, S. A.; Wells, J. M. Rapid Commun. Mass Spectrom. 1996, 10, 361-371. (35) Franzen, J. Int. J. Mass Spectrom. Ion Processes 1994, 130, 15-40. (36) Morand, K.; Schubert, M.; Franzen, J.; Mann, M. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA, 1993; p 706a.


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Figure 5. Unsubtracted spectral profile of M+ from 100 ppb (v/v) benzene standard at an optimal trap pressure of 10-4 Torr. At 10-4 Torr, background accounts for ∼100 mV of the total ion signal. Lower trace serves as a comparison at 4.0 × 10-6 Torr and is of C6H6+ hydrocarbon background when no sample is introduced.

addition, as the flight tube pressure approaches 3.4 × 10-5 Torr, the mean free path (e.g., N2) is on the order of the ion flight path length. Peak width varies between 10 and 12 ns, full width at halfmaximum (fwhm), and is not affected significantly by changes in pressure as is the ion signal. Shown in Figure 5 are profiles that correspond to the variable flow rate curve in Figure 4 at the optimal pressure of 1.0 × 10-4 Torr and at the lower extreme of 4.0 × 10-6 Torr. The curve taken at the lower pressure is of the C6H6+ instrument background with no sample introduced. The peak shape and arrival time are also not affected significantly by changes in ion store pressure. These results are significant because they demonstrate a high degree of freedom from space charge effects, which are typically minimized through increased collisional damping. Ionization Mechanism. Although ionization can occur by EI, chemical ionization with abundantly formed N2+ and O2+ becomes increasingly important at elevated pressures.37 In the ion store, chemical ionization can be carried out at relatively low pressures by increasing ion storage time.38,39 Shown in Figure 6 is the effect of prolonged storage time on M+ ion signals for three different types of compoundssbenzene, chloroethyl ethyl sulfide (CEES) and xenon 129. The signal for benzene, which is typical of nonpolar volatile compounds, levels off at ∼400 ms and remains nearly constant to storage times as long as 10 s. At these long storage times, the capacity of the ion store has been reached and is limited by ion loss processes that can include ion-ion and ionneutral interactions. As can be seen in Figure 7, which shows profiles taken for benzene for accumulation lasting 25 ms and 10 s, the peak shape and arrival time remain relatively unaffected. This is quite significant when compared to the behavior of standard ion trap mass spectrometers, where excessive accumulation at long storage times results in space charge interference during mass analysis. Instrument response for CEES, which is representative of semivolatile compounds, is similar to that for the more volatile (37) Harrison, A. G. Chemical Ionization Mass Spectrometry; CRC Press: Boca Raton, FL, 1983; p 64. (38) Bonner, R. F.; Larson, G.; Todd, J. F. J. J. Chem. Soc., Chem. Commun. 1972, 1179-1180. (39) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry; Wiley: New York, 1989; p 214.


Figure 6. Effect of storage time on signal for 10 ppm xenon 129, 100 ppb benzene, M+, and 1 ppm chloroethyl ethyl sulfide, M+. Inlet flow, 2.5 mL/min; trap pressure, 0.105 mTorr.

Figure 7. Background-subtracted spectral profiles of M+ from 100 ppb (v/v) benzene standard in air. Upper trace was taken at a 10-s storage time. Lower trace was taken at a 25-ms storage time and maintains similar peak shape and arrival time.

species. In Figure 6, the M+ signal from CEES levels off at storage times approaching 400 ms, similar to that of benzene. On the contrary, xenon signal was found to equilibrate much more quickly, within 10 ms, and decreased at longer storage times. This difference in behavior suggests separate ionization and loss mechanisms for xenon. Under these source conditions, xenon is believed to favor electron impact ionization, which is characterized by larger rate coefficients on the order of 10-8 and 10-9 cm3 molecule-1 s-1.40 This larger rate coefficient results primarily from the lower reduced mass of the electron/atom or molecule pair. Following electron impact, xenon ions can be lost at longer storage times that otherwise enhance the formation of organic ions. Because the ionization potential for xenon is relatively high, being 12.13 eV, charge exchange or possibly resonant charge transfer with the air matrix can result. One candidate for charge transfer is O2, which has an ionization potential of 12.06 eV and makes up 20% of the air matrix. Because most organic compounds have IPs below those of the primary constituents N2 (15.6 eV) and O2 (IP ) 12.06 eV), ionization is unaffected by changes within a standard air matrix. (40) Harrison, A. G. Chemical Ionization Mass Spectrometry; CRC Press: Boca Raton, FL, 1983; p 25.

Figure 8. Comparision of response to Freon 11 (M - Cl)+ in a UHP zero grade N2 matrix (9) and in air (O). Average of 50 traces. Error bars represent standard deviation, σ.

This behavior is demonstrated in Figure 8 for the detection of Freon 11 using both an air and an N2 gas matrix. In this figure, a response curve for the (M - Cl)+ ion is plotted for the two operating conditions. At any given analyte concentration, the signal is the same with either gas matrix. Likewise, the presence of water vapor (IP ) 12.6 eV) does not apparently affect instrument response or contribute significantly to the spectral background either directly or through the formation of H+(H2O)x clusters.37,41,42 Evidence of Chemical Ionization-Dominated Mechanism. An ASGDI source was selected as a chemical ionization source for comparison with the EI configuration described above. A comparison of these two ionization approaches provides further evidence as to the importance of the chemical ionization pathway. The ASGDI source has been shown to achieve detection limits similar to those of atmospheric pressure ionization (API) sources.43 The major difference that distinguishes the ASGDI from API is that ionization occurs in the first differentially pumped stage, which is held under 1 Torr, rather than outside the instrument at atmospheric pressure. This arrangement allows for the control of the primary ionization mechanism by altering the number of gas phase collisions. This can be achieved by adjusting the background pressure or separating the anode and cathode plates, promoting either ion-molecule or electron-molecule reactions.30 Another advantage of the ASGDI configuration is that the source region is one stage closer to the analyzer chamber than in the API source, resulting in increased transmission efficiency. Ejection efficiency into an ion trap using an ASGDI source has been reported to be between 50 and 80%.44 In positive mode, ionization in the ASGDI is initiated by electron impact in the negative glow region of the discharge, which forms N2+ and O2+ from air. Electrons that are generated via ion bombardment of the cathode skimmer are accelerated upstream toward the perimeter of the anode sampling plate, ionizing the air sample outside the expansion shock wave structure. Positive ions are accelerated toward the cathode skimmer cone and either strike the surface, resulting in secondary (41) Good, A.; Durden, D. A.; Kebarle, P. J. Chem. Phys. 1970, 52, 212-221. (42) Good, A.; Durden, D. A.; Kebarle, P. J. Chem. Phys. 1970, 52, 222-229. (43) McLuckey, S. A.; Goeringer, D. E.; Asano, K. G.; Vaidyanathan, G.; Stephenson, Jr., J. L. Rapid Commun. Mass Spectrom. 1996, 10, 287-298. (44) Habibi-Goudarzi, S.; McLuckey, S. A. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA, 1993; p 683a.

Figure 9. M+ signal from a chlorobenzene gas standard (a) introduced at three different locations in the ASGDI source, compared with (b) direct introduction into the trap cavity using an EI source. Storage time, 100 ms; rf ) 318 Vp-p, ASGDI pressure, 0.3 Torr; cathode voltage of 729 at 10 mA; anode ground, and other parameters as shown in Table 1.

emission, or pass through the skimmer orifice into the ion store chamber. Ionization efficiency can be enhanced by increasing the potential difference between the anode and cathode. This increases both the energy and the number density of charged species within the discharge region. A comparison of chlorobenzene M+ ion intensity for the ASGDI and EI source configurations is shown in Figure 9. The three overlapping curves in Figure 9a show different responses from introducing a chlorobenzene standard at various locations about the ASGDI source and ion store region. For this experiment, two additional inlets with transfer lines were installed at available flanges in the ion store chamber. The air sample could be introduced through the sampling plate itself, at the back side of the skimmer cone and into the trap cavity. Flow through both of the secondary inlets was controlled by separate needle valves and was held at ∼0.25 mL/min. Conditions for the ASGDI included a cathode potential of -729 V at 10 mA, a grounded anode, a 0.03 Torr source pressure, and the ion store chamber was maintained at 10-4 Torr. As shown in Figure 9a, chlorobenzene signal (m/z 112 and 114) was enhanced when the sample was introduced behind the skimmer cone; however, the greatest signal was achieved by introducing the standard through the inlet leading to the ion trap cavity. The apparent shift of the background peak at m/z 113 was caused by a change in the relative intensities of two different background species that occurred when the auxiliary inlets were opened. The background peak at m/z 115 was unaffected. It is important to note that ionization at the secondary locations behind the skimmer cone and in the ion store occurs by charge exchange with the flux (i.e., N2+ and O2+) from the source region. One explanation for the lower sensitivity that results from introducing the sample directly into the ASGDI source is that Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

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Table 1. Operating Parameters for IS/TOF-MS Instrument Configured with the EI Ionization Source EGUN power supply E. energy filament current rf power supply rf level rf off duration ion storage time pulsers extraction pulse duration entrance end cap pulse voltage exit end cap pulse voltage ion optic lens potentialsa L1 ) L3 ) flight tube L2 XY1 XY2 reflectron front plate back plate pressures ion store chamber TOF-MS chamber

70 eV 2.48 A 775 Vp-p 100 µs 400 ms 100 µs 900 V -900 V -1250 V -300 V -1100 V -1170 -317 V 350 V 1.0 × 10-4 Torr 5.6 × 10-6 Torr

a Einzel optic elements L1 and L3 are the first and third optic. The second einzel element is L2.

sampling the ion flux through the skimmer cone and entrance end cap orifice lowers ion transmission efficiency. Coulombic repulsion, which can scatter the ion flux, is likely the primary contributor. In this system, there are two processes that influence the composition of the ion flux. The first is charge separation that occurs within the glow discharge boundary nearest to the skimmer, which is largely electron deficient.45 In addition, electrons are stripped46 from the ion flux as it passes through the skimmer orifice (i.e., 0.5 mm) as well as through the end cap channels. Electrons, which have a much greater velocity than ions, strike the skimmer and end cap walls at a greater frequency than ions, resulting in the formation of an ion boundary layer. These transmission losses are avoided with the EI source configuration, where the sample can be introduced and ionized directly in the trap cavity. A slightly higher sensitivity was achieved using the EI source as shown in Figure 9b. For this comparison the sample flow rate was maintained at 0.25 mL/min, resulting in a chamber pressure of 1.8 × 10-5 Torr. The second needle valve inlet was used to bring the chamber pressure to 10-4 Torr. The IS/TOF-MS conditions for both source configurations are the same as those shown in Table 1 with the exception of the trap storage time, which was lowered to 100 ms. An added advantage of introducing the sample directly into the ion trap cavity is that local pressures can be maintained at relatively high levels, thus, enhancing EI and CI. Specifically, this configuration allows the highest density gas sample to be introduced into the ion store for ionization and storage before it diffuses away to the rest of the vacuum chamber. In addition, trapping of locally generated ions reduces losses associated with external injection.47 Extracting Ions Out of the Trap for Mass Analysis. Ion extraction is the most important process needing optimization (45) Slevin, P. J.; Harrison, W. W. Appl. Spectrosc. Rev. 1975, 10, 201-255. (46) Manos, D. M.; Dylla, H. F. In Plasma Etching: An Introduction; Manos, D. M., Flamm, D. L., Eds.; Academic Press: Boston, 1989; p 17. (47) Williams, J. D.; Reiser, H.-P.; Kaiser, R. E.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1991, 108, 199-219.


Figure 10. Ion trajectory models comparing (a) unipolar and (b) bipolar extraction. The initial ion starting plane runs diagonally through the trap center extending ∼3.5 mm. Ions are given no initial kinetic energy. Trajectories are drawn with 1-µs time markers.

because it is key in establishing transmission efficiency and resolution in the TOF-MS. Relatively speaking, ion traps can create broad time-dependent spatial and kinetic energy distributions, especially for low molecular weight species. The approach used to minimize these spreads is to extract ions of a particular phase-space characteristic best suited for this TOF-MS configuration. Consequently, most of this work has involved optimizing the ion trap conditions and extraction procedures. The resulting scheme involves phase synchronizing the end cap extraction pulses and clamping of the rf drive potential. The effect of each of these parameters on signal and resolution is discussed below. Bipolar Extraction. Positive ions can be extracted from the ion trap by applying a positive pulse on the entrance end cap or a negative pulse on the exit end cap.48 When an extraction voltage is applied to just one electrode, a parabolic field gradient is formed that focuses ions to a crossover point outside the exit end cap. Alternatively, if both end caps are pulsed simultaneously, a linear accelerating field is created with little focusing of off-axis ions. This is illustrated in the ion trajectory simulations shown in Figure 10, which were obtained by using the Simion program.49 Bipolar extraction at relatively high field strengths of 1333 V/cm was found to yield the best sensitivity and resolution. This behavior is characteristic of focusing an ion packet with a relatively small spatial distribution, yet significant kinetic energy spread. In operation of the IS/TOF-MS, the use of bipolar instead of unipolar (48) Waldron, R. M.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1979, 29, 315-335. (49) Dahl, D. A.; Delmore, J. E.; Appelhans, A. D. Rev. Sci. Instrum. 1990, 61, 607-609.

Figure 11. Cross-sectional view of the ion trap cavity showing boundaries of the ion cloud for high qz ions at phases Ψ ) 90° and Ψ ) 270° of the rf potential applied to the ring.

extraction increased resolution from 750 to 2000 m/∆m, fwhm, for lower molecular weight compounds. For the comparison of both these configurations, an iterative optimization approach was used that involved finding the best combination of extraction and TOF-MS ion optic settings. For bipolar extraction, the exit end cap optimizes at a high potential of -900 V. All other ion optic settings are given in Table 1. Phase Synchronization. Phase synchronization of the extraction pulse with the rf drive potential is performed by triggering the extraction pulse from the same clock used in the rf generator. During the rf cycle the ion cloud undergoes changes in spatial and velocity distribution.50 By the time the rf reaches the maximum positive potential, Ψ ) 90°, the ions are compressed to the center of the trap and are expanded radially as depicted in Figure 11. When the rf voltage reaches its maximum negative potential, Ψ ) 270°, ions are extended the greatest axially. Ions extracted at this phase are closest to the end caps and can be extracted with a low extraction potential;48,50 however, because the axial spread of the ion packet is relatively large, complete sampling and focusing of the ion packet is difficult, resulting in decreased transmission efficiency and resolution. The relationship between rf phase and ion signal is shown in Figure 12 for the m/z 69 peak from perfluorotributylamine (qz ) 0.46). At rf phases between 0 and 90° the ion peaks were the narrowest, like that shown for the trace taken at 0°. Within this range, the ion cloud is compressing axially toward the center of the trap and axial and radial ion velocity is decreasing.51 The peak arrival time between 0 and 90° remains relatively the same because the rf is clamped to ground during the extraction as described below. At phases greater than 90°, the distribution begins to broaden as shown by the trace taken at 97°. As the rf phase reaches 180°, the ion peak shifts to a longer time and exhibits a high-time tail. At 180°, both axial and radial velocities are at a maximum51 and the ion cloud is expanding axially. Shown in Figure 13 is the phase dependency for ions with different m/z and qz values presented in terms of peak widths. Broadening of the ion peak is also accompanied by decreased peak height as shown by the results in Figure 12. Ions with lower qz values were found to be less affected by phase. A comparison of results for m/z 69 and 131 where the qz value was fixed at 0.5 reveals no significant bias as a function of m/z value. In this experiment, qz was maintained by adjusting the rf potential. These results confirm that the best phases at which to extract ions range between Ψ ) 0 and 90°, where peak shape remains unaffected (50) Todd, J. F. J.; Waldren, R. M.; Freer, D. A.; Turner, R. B. Int. J. Mass Spectrom. Ion Processes 1980 35, 107-150. (51) Todd, J. F. J.; Waldren, R. M.; Bonner, R. F. Int. J. Mass Spectrom. Ion Processes 1980 34, 17-36.

Figure 12. Effect of the ring electrode rf phase during which extraction takes place, shown for perfluorotributylamine standard in air at m/z 69 (qz ) 0.46). The rf cycle shown above is marked at those points where extraction occurred. rf ) 634 Vp-p; 100-ms storage; first stage pressure, 5 × 10-5 Torr; second pressure, 10-5 Torr.

Figure 13. Effect of rf phase during extraction on the peak width plotted for several perfluorotributylamine ions. Because broadening of the ion peak is also accompanied by a decrease in peak height, as shown in Figure 12, the best phases for extraction lie between 0 and 100°. Different qz values were obtained by adjusting the rf potential.

across the mass range. At phases between 180 and 360°, the rf potential was not clamped during the extraction as described below, and therefore the phase dependency of the ion extraction for this portion of the rf cycle was not evaluated. rf Clamping. The benefit of gating the rf is the gain in extraction efficiency and elimination of the rf focusing fields, which are strongest at the ion trap boundary.52 The conventional method (52) Jones, J. A.; Yost, R. A. Proceedings of the 42nd ASMS Conference on Mass Spectrometry and Allied Topics; Chicago, IL, 1994; p 222.


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Figure 15. Effect of rf clamping on peak shape for perfluorotributylamine standard in air at m/z 69 (qz ) 0.46). Extraction was performed at an rf phase of 45°.

Figure 14. Comparison of rf waveforms and corresponding extraction pulses resulting from two different rf circuits. Trace a is typical of a parallel driven resonance circuit where the drive is shut off, requiring several cycles to decay. Trace b is achieved by clamping the stored energy in the oscillator.

for gating the rf field that was initially implemented required several cycles to completely extinguish the rf envelope.53 This circuit used a parallel driven resonance oscillator with a low Q, where Q is a measure of stored energy in the rf oscillator. In this circuit, to extinguish the rf field during extraction, the voltage input to the rf oscillator is merely shut off. Dissipation of the stored energy in the oscillator required several cycles. A new design was developed in which the rf oscillator is shorted to ground, thus immediately extinguishing the rf potential. Oscilloscope traces comparing the behavior of the two circuits are shown in Figure 14. With the new clamping arrangement the rf can be cut off in ∼10 ns. Shown in Figure 15 are spectra for the m/z 69 (qz ) 0.46) fragment from PFTBA extracted at an rf phase of 45° (a) with and (b) without clamping of the rf potential on the ring electrode. Clamping the rf lengthens the flight time by 25 ns and narrows the peak width (fwhm) by 4 ns. With no rf clamping, the velocity distribution is broadened and flight time is shortened because the extraction takes place as the rf is driving to a greater positive potential. Similarly, as with phase synchronization, ions with the highest qz values (lowest mass) are more greatly affected by the rf field during extraction. Instrument Performance Characteristics. Resolution is limited by several mass-independent parameters that include stability of the electronics, precision of circuit timing, and focusing of the ion pulse.54 Of the ion trap operating parameters, the rf (53) Fulford, J. E.; March, R. E. Int. J. Mass Spectrom. Ion Phys. 1979, 30, 373378.


Figure 16. Xenon standard in air using a 10-ms storage time and averaging seven traces. Resolution, 2000 m/∆m, fwhm.

extraction phase has the greatest effect on resolving power, as shown in Figure 12. Ripple from the ion optic power supplies is typically less than 0.0015%, and an equally high triggering precision is maintained by initiating the rf-off and end cap pulses with the same signal from the phase synchronization circuit. Ion storage parameters such as background pressure and storage time have relatively little effect on resolution, as shown in Figure 5 and Figure 7, respectively. Resolution achieved on the IS/TOF-MS is routinely 2000 m/∆m, fwhm, as shown in Figure 16 for a xenon gas standard in air. This performance falls within the range of that achieved by orthogonal extraction TOFMS of API-generated ions. With an orthogonal extraction TOFMS system, a resolution of ∼1600 m/∆m, fwhm, has been demonstrated at an m/z of 242.55 Higher resolving powers, of over 5000 m/∆m, fwhm, have been achieved with a similar API/ TOF-MS design (1.2-m flight tube).26 Also included in Figure 16 are the measured relative abundances and the actual natural abundances for the Xe isotopes. The measured abundances are within 5% of the actual, and most of the error results from background chemical interference. Calibration. The advantage of TOF-MS is the ability to perform a full mass analysis without scanning. This approach, in part, helps maintain high mass accuracy. TOF-MS spectra are commonly calibrated by means of the time-of-flight equation m/z ) Rt2 + β, where t is the time of flight, R is a proportionality (54) Coles, J. N.; Guilhaus, M. J. Am. Soc. Mass Spectrom. 1994, 5, 772-778. (55) Kraft, A.; Wollnik, H.; Laiko, V.; Dodonov, A. F. Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics; Atlanta, GA, 1995; p 120a.

Figure 17. Mass calibration curve for perfluorotributylamine standard in air obtained using an average of 50 traces and 100-ms storage time. Arrival times were determined from peak maxima.

Figure 18. Response curves for three types of compoundssvolatile and semivolatile organics and a noble gas. Average of 50 traces. Xenon storage time, 75 ms. Other conditions are as in Table 1.

Table 2. Detection Limits Extrapolated from Response Curves Yielding a S/N of 3 compound

major ion detected

concn

benzene Freon 11 Freon 12 Freon 12B1 Freon 13B1 methyl tert-butyl ether perfluorocyclohexane toluene 1,1,1-trichloroethane chloroethyl ethyl sulfide

M•+ M - Cl+ M - Cl+ M - Br+ M - F+ M - CH3+ CF3+ M - H+ M - Cl+ M•+

2 2 0.8 2 5 2 4 0.9 2 194 Figure 19. Comparison of mass spectrum of M - Cl+ from 3 ppb Freon 11 standard in air (upper trace) with the air background (lower trace). Asterisk corresponds to analyte signal location. Average 50 traces. Other conditions are as in Table 1.

constant that includes the ion charge, accelerating voltage, and the flight path, and β is a systematic constant that results from differences between the extraction pulse time and data acquisition trigger time. The constants R and β are determined empirically by measuring the flight times of at least two ions of known mass. This equation yields good calibration with accuracy of at least 0.01%. Other equations may also be used with similar results.56 The most precise calibration is achieved when the greatest time resolution is used. A typical mass calibration plot using a perfluorotributylamine standard in air is shown in Figure 17. The points in this curve correspond to peak maxima obtained using a 50-trace average and a 100-ms storage time. Sensitivity and Detection Limits. Table 2 lists detection limits for several compounds of environmental interest. These compounds were selected because they either could be generated with a high degree of certainty at low concentrations with a gas standards generator or served as standards for in-house applications. Ideal compounds were those that did not stick to or decompose on the glass and stainless steel surfaces of the gas standards generator. Most of the gas permeation tubes allowed us to generate concentrations in the low ppb range. Detection limits were extrapolated from response curves (see Figure 18) where the closest measurement to the detection limit was within at least a factor of 10. The limit was determined by extrapolating the response curve to the point at which the signal-to-noise ratio equaled 3. CEES is representative of semivolatile compounds that can stick to the sampling inlet or decompose on metal surfaces.

Although the detection limit for this type of compound is not yet at the same level as for more volatile compounds, the lower sensitivity is not believed to be related to sample loss at the inlet with this direct sampling configuration. Future work is directed at achieving similar detection limits for both types of compounds. Currently, we can achieve low-ppb detection limits for many volatile organic compounds. However, absolute instrument detection limits cannot be determined because of the presence of a high chemical background. This background is most likely caused by the ionization and detection of ppb and pptr concentrations of anthropogenic compounds in the ambient air drawn into the instrument.57 At this time we are relying on background subtraction to compensate for the presence of these contaminants. Shown in Figure 19 are two offset spectra that demonstrate the addition of 3 ppb Freon 11 (top) amid a relatively high background (bottom). For many of the compounds given in Table 2, background interferences were primarily responsible for raising the detection limit.

(56) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Anal. Chem. 1994, 66, 4366-4369.

(57) Air Monitoring by Spectroscopic Techniques; Sigrist, M. W., Ed.; Wiley: New York, 1994.

CONCLUSION The instrument described in this paper is a laboratory prototype, the purpose of which is to test and refine the IS/TOFMS design to explore its potential usefulness in air monitoring


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applications. This requires examination of the various aspects of ion formation, trapping, and extraction discussed above. This work has shown (1) that it is more efficient to form ions inside the quadrupole ion store than to inject them from outside, (2) that bipolar extraction offers advantages for ion focusing over unipolar extraction, (3) that phase synchronization is needed to extract ions with a broad range of qz values, and (4) that to minimize peak width, the rf must be clamped quickly to zero at the proper phase during extraction. These experiments have also offered insight regarding the interplay of electron ionization and chemical ionization in the ion store, as described above. The lowppb and high-ppt detection limits and mass resolution of ∼2000 m/∆m, fwhm, achieved with the laboratory prototype indicate that the IS/TOF-MS combination should be very useful for air monitoring. Ultimately, the goal of this work is to apply the results from the laboratory prototype to the design and construction of a smaller, transportable instrument to be used in the field. Such an instrument has been constructed in our laboratory, and preliminary results obtained with this instrument are promising.

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In a future paper, this instrument, its design, and its operation will be described, and representative data from it will be shown. ACKNOWLEDGMENT We gratefully acknowledge Stan Thomas for design and modification of electronics as well as Russ Jordan and Michael Weifel for specialized design and component fabrication. We thank Dr. David Lubman for the introduction to the ion storage/time-of flight mass spectrometer. Reference herein to any specific commercial product does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.

Received for review January 27, 1997. Accepted June 16, 1997.X AC970102G X

Abstract published in Advance ACS Abstracts, August 1, 1997.

Time-of-Flight Mass Spectrometer for the

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