Improved Signal-to-noise Ratio in Glow Discharge Ion Trap Mass Spectrometry via Pulsed Discharge Operation†‡ DOUGLAS C. DUCKWORTH, DAVID H. SMITH AND SCOTT A. MCLUCKEY Chemical and Analytical Sciences Division, Oak Ridge National L aboratory, Oak Ridge, TN 37831-6375, USA An improvement in the S/N ratio is reported for the analysis ence of intense matrix and discharge gas (Ar) ion beams.9 This of trace elements in brass by glow discharge ion trap mass observation was important because it allowed the dynamic spectrometry.This was achieved by synchronizing the pulsed range of the trap, which has an ion volume limited to 104–105 discharge voltage with the ion injection and acquisition events; ions, to be extended through a variety of selective ion accumu- ‘on’ during ion injection and ‘off’ during data acquisition. Two lation methodologies. Several tactics were shown to eliminate modes of operation were evaluated: (1) a high duty cycle matrix ions, which would normally fill the trapping volume pulse, which allowed a continuous injection over the duration and limit the dynamic range to #102 (due to space-charge of the pulse; and (2) a low duty cycle pulse with multiple data effects), allowing unimpeded accumulation of ions of minor gates, which allowed gated injections of ions at selected regions constituents. In effect, the dynamic range was extended to 105 of the pulse profile.The latter afforded a means of selective ion (i.e., 10 ppm–100%).injection since discharge and residual gas species are formed at Having realizeda substantial improvement in signal intensity different times in the pulse event than analyte ions. for trace species, attention was given to the reduction of noise Improvements in S/N ratios greater than 40-fold were for further improvement in the S/N ratio. This was driven by observed, primarily due to a reduction in background and the following observations: (1) the primary source of noise is background noise after the discharge was extinguished.charged species extracted from the glow discharge; (2) most of Evidence is presented which suggests that electrons emanating the noise arises from outside of the ion trap volume (charged from the ion source are the precursors of most of the noise. particles from the ion gauge contribute substantial noise to Detection limits for various elements were 0.2–0.5 ppm. the detector); and (3) background and background noise increase in the presence of the He buffer gas (1–4 mTorr).Keywords: Glow discharge ion trap mass spectrometry ; (Whether it is detector dark current or non-mass resolved elemental analysis; direct solids analysis ; noise; pulsed signal, background refers to the average signal from the discharge; gated ion injection detector; noise refers to the deviations from that average value.) Shielding the detector resulted in only modest improvement. Quadrupole ion traps have realized wide acceptance as mass In its simplest form, glow discharge analysis using an ion analyzers in organic applications, primarily as a mass analyzer trap is a two-step process, comprised of an ion injection period and tandem mass spectrometer in GC–MS.1 More recently and a data acquisition period, in which ions are ejected from they have been employed as tandem mass spectrometers the trap and detected.Ion injection periods typically occur coupled with liquid chromatography and capillary electrophor- over a time frame of 1 ms–1 s, and mass analysis requires of esis.2 Ion traps are rapidly being extended into the inorganic the order of 20 ms.To provide acceptable ion statistics for mass spectrometry domain with application in the elemental accurate abundance measurements and to improve S/N ratios, analysis of solids and solutions. Such applications include laser a series of scans is typically acquired and averaged for display. ablation,3–7 glow discharge,8,9 secondary ion10 and inductively Because most of the noise originates from the glow discharge, coupled plasma11–13 mass spectrometries.Reported here is the it was reasonable to expect the noise to be largely eliminated use of pulsed glow discharges for substantial improvements in by turning the discharge off during the acquisition period. The S/N ratios in glow discharge ion trap mass spectrometry relatively long time frames associated with ion injection and (GD-ITMS).mass analysis are easily compatible with pulsed discharge In a prior report the interface and operational characteristics operation. of a glow discharge ion trap mass spectrometer for bulk solid Pulsed glow discharges have been used in conjunction with elemental analysis were described.8 Several promising charac- linear quadrupole14–18 and time-of-flight19 mass spectrometers. teristics of the ion trap were identified, including MS–MS Typically, the pulsed discharge operates at 50 Hz with 25% capabilities, ‘spontaneous’ dissociation of some polyatomic duty cycle. There are advantages to operation in the pulsed ions, and the reduction of argon-related species through charge mode.Atomization and ionization are increased relative to exchange processes with adventitious water in the vacuum continuous discharges because higher instantaneous power is system. These observations held promise for the generation of required to maintain the same average power.14 Furthermore, purely atomic spectra of most elements.with gated data collection, one can preferentially sample Also noted in early experiments was the ability to trap ionized sputtered species over discharge gas species by sam- selectively and analyze minor sample constituents in the pres- pling ions in the so-called pre-peak and after-peak portions of the ion signal profile.14,15,18 Descriptions of the mechanisms † Paper presented at the 44th ASMS Conference on Mass giving the observed temporal characteristics have been Spectrometry and Allied Topics, May 12–16, 1996, Portland, OR, USA.‡ The submitted manuscript has been authored by a contractor of given.15–18 the US Government under contract No. DE-AC05-96OR22464. Two modes of operation are reported and characterized Accordingly, the US Government retains a paid-up, nonexclusive, here. The first employed a single continuous injection over the irrevocable, worldwide license to publish or reproduce the published discharge ‘on’ period.The injection time employed is inversely form of this contribution, prepare derivative works, distribute copies proportional to the analyte concentration, resulting in high to the public, and perform publicly and display publicly, or allow others to do so, for US Government purposes. duty cycle, low frequency pulses for trace species. (Duty cycle, Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (43–48) 43as used here, refers to the fraction of the period the discharge the entire elemental mass range.This mode of operation was effectively a continuous discharge (single continuous injection pulse is on. It does not refer to the sample consumption, which only takes place during the discharge. As long as the discharge period) which was turned off after the injection period. A trigger pulse, generated by the 3DQ electronics at the beginning pulse is fully encompassed by the time allotted for ion accumulation, the duty cycle, in terms of sample consumption, is 100%.of each scan, signals a square-wave pulse generator (Model 8010, Berkeley Nucleonics Corporation, Richmond, CA, USA). This is, of course, a significant advantage to pulsed discharge operation.) Alternatively, multiple injections were made using The pulse generator controlled the pulse delay, width and amplitude. The pulse signal drove an operational power supply several pulses at a higher frequency.Used in combination with appropriately timed injection periods, the multiple injection (Model OPS 3500, Kepco, Flushing, NY, USA). Experiments were monitored and traces (glow discharge and rf trapping mode can be used to generate a temporally resolved spectrum for the selective accumulation of analyte ions. potentials) were stored on a digital oscilloscope (Phillips, Model PM3382). Because the ion source was turned off after the injection period, the gate voltage was not necessary; in EXPERIMENTAL some experiments, both half-plates (L2) were operated at a common and optimal potential (#-325 V).The GD-ITMS system employed in this study is schematically The second operational mode used multiple injections in a presented in Fig. 1. The instrument is a Teledyne 3DQ ion single scan function. A discharge pulse of fixed frequency and trap-based mass spectrometer (Teledyne Electronic duty cycle was used in association with a data gate, positioned Technologies, Mountain View, CA, USA), modified for injecover the pulse region of interest.This is analogous to the tion of externally generated glow discharge ions. The glow typical mode of pulse discharge operation.14–19 Data gates discharge was operated in the pulsed-direct current mode. were 1.5 ms long. The discharge was pulsed at 36 Hz and had Samples were introduced through a vacuum interlock on a a 33% duty cycle. To accumulate a sufficient number of ions, direct insertion probe.20 Argon was used as a support gas, and multiple gated injections (n=4) per scan (one per pulse) were pressures ranging from 200–500 mTorr were optimized for made before the acquisition period.This was accomplished by analyte signal intensity and discharge stability. Discharge including multiple triggers in the scan function. Pulses and ion voltage was maintained at 1.5 kV for all modes of operation. gates were synchronized by the use of appropriate delays in Ion focussing and injection were effected using a simple the scan functions and pulse generator.The scan function is three element lens system described previously (see Fig. 1).21 described in more detail later. Ions were vacuum-extracted from the discharge through a Two brass standard reference materials were used: NIST SRM 250 mm orifice. Lens 2, comprised of two half-plates, was used 1101 Cartridge Brass B and NIST SRM 1102 Cartridge Brass to gate ions into the trap.Positive ions transmitted through C. The trace element concentrations of interest are presented in lens 1 were either deflected from, or passed to, lens 3 by the the relevant parts of Results and Discussion. application of +200 or -200 V, respectively. The 200 V potential was supplied by the modified (i.e., voltage inverted) electron gate voltage of the 3DQ electronics. RESULTS AND DISCUSSION Ions were injected through an aperture in the center of an Noise Characterization endcap electrode, which was operated at 0 Vdc during injection.Trapping efficiency was improved by the addition of helium A series of mass spectra, acquired under differing analyzer as a buffer gas (pressure, 2–4 mTorr).3,21 Ions were measured conditions and shown in Fig. 2 (ion gauge off ), illustrates the directly on a Channeltron electron multiplier (Model 4773G, origin of background and background noise in the GD-ITMS. Galileo Electro-optics, Sturbridge, MA, USA), mounted off No scan averaging was made and a steady-state discharge was axis to reduce photon-induced noise.Mass spectra were used. Fig. 2(a) shows a mass scan of brass with the glow acquired and displayed using Teledyne Electronic discharge on, multiplier voltage on (1900 V), but with no He Technologies’ 3DQ Discovery and Sequel Data System buffer gas added to the analyzer. Argon pressure in the analyzer software, respectively. was 3×10-5 Torr (base pressure was 5×10-7 Torr). In the Two operational modes were employed: single continuous absence of buffer gas, no analyte signal is noted with a 5 ms injection, whose duration fully encompassed the glow discharge injection.Noise arising with the initiation of the glow discharge pulse, and multiple injections. The single continuous injection is the first noise of any consequence above the baseline of 83 mode used a high duty cycle, low frequency discharge voltage counts (arbitrary units). Baseline counts are attributed to the pulse.The ranges of duty cycles and frequencies were 5–90% detector dark current; approximately 5 counts are attributed and 4.5–45 Hz, respectively. Both frequency and duty cycle to rf pick-up from the trap electrodes. Noise, 83–658 counts, were dependent on the required injection period, with trace is eliminated when the discharge is turned off. This indicates species sometimes requiring injection times in excess of 150 that the glow discharge is the source of the noise: charged ms; acquisition time (pulse ‘off ’ period) was about 20 ms for species, photons or fast neutrals.With the addition of He buffer gas (5 mTorr) to the ion trap vacuum manifold [Fig. 2(b)], an increase in ion trapping efficiency, background and background noise is observed. Improved efficiency in the injection of externally generated ions through the use of a buffer gas is well known3,20 and is now common practice in ITMS. The background increased from 83 counts to 1600 counts with the addition of He.Background noise increased from 3 to 33% of background. Charged species are responsible for most of the background and background noise. A laboratory magnet, positioned above the exit orifice and outside the vacuum housing, was used to reduce noise on the detector [Fig. 2(c)]. Peak noise, with the magnet in place, ranges from 90–700 counts per channel; this count level approaches the noise characteristics in Fig. 2(a). Since a small laboratory magnet, positioned outside the Fig. 1 Pulsed glow discharge ion trap-based mass spectrometer system. vacuum chamber, is capable of reducing the noise by more 44 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Fig. 2 S/N ratio comparison of steady-state glow discharge mass spectra of brass (1.5 kV, 1.5 mA, 330 mTorr Ar, single scan, ion gauge off) with (a) no He buffer, no magnetic deflection of electrons; (b) He buffer (5 mTorr), no magnetic deflection of electrons; and (c) He buffer (5 mTorr), with magnetic deflection of electrons. than 80%, electrons are believed to be the precursor of most voltage is pulsed at 5.5 Hz, has an 80% duty cycle and an of the noise (see below).amplitude of -1.5 kV. The rf amplitude trace consists of an Most of the noise arising from charged species is believed injection period of 158 ms and a subsequent acquisition ramp. to come from outside of the trapping volume. This is because The rf levels correspond to a low mass cutoff of m/z 130 (qz= lens and ion trap potentials have little effect on background 0.57 for 208Pb) during the injection period and an acquisition and background noise.It is possible that electrons promote ramp of m/z 40–290. The single injection period corresponds electron impact ionization of Ar and He, which can lead to to the duration of the pulse (146 ms) as the ion gate (L2) is the presence of ions in the detector volume via charge exchange ‘open’ over 158 ms injection period.In this manner a single processes. This is supported by the observation that the ion injection is made per scan which corresponds to the pulse gauge, when on during operation (data not shown), contributes period in length (146 ms here). significant amounts of background and noise. Charge mobility A comparison of steady-state and pulsed glow discharge via symmetric charge exchange processes is known to be very operation is presented in Fig. 4, which shows the mass spectra high for both Ar (cross section #60×10-16 cm2)22 and He of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B. The (cross section #30×10-16 cm2).23 steady-state discharge [Fig. 4(a)] was generated by gating ions with L2 during a 183 ms injection period. The pulsed-discharge spectrum [Fig. 4(b)] was obtained as described above. Each Pulsed Operation spectrum is the average of 25 scans. The improvement in S/N Single injection is most evident in the detection of 204Pb (7 ppm), which is not discernible above the noise from the steady-state discharge.The voltage traces in Fig. 3 show the synchronized glow Background is reduced by about 100 counts (from #180 to discharge voltage pulse and the rf signal on the ring electrode during the injection and acquisition periods. The discharge 83 counts). Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 45Fig. 3 Voltage traces showing the synchronization of the rf trapping Fig. 5 Voltage traces showing the rf trapping voltage and the glow voltage with the glow discharge voltage.discharge voltage, which is extinguished after the acquisition ramp is initiated to illustrate noise reduction. Fig. 4 Glow discharge ion trap mass spectra of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B (150 ms injection, 25 scans averaged). (a) Steady-state discharge; and (b) pulsed discharge. Fig. 6 Pulsed glow discharge ion trap mass spectra of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B (250 ms injection, single scan).The low m/z values illustrate background and noise contributions from the discharge. The improvement in S/N is more clearly illustrated by the results of the experiment shown in the voltage traces in Fig. 5 and the resulting spectrum of Fig. 6. The discharge pulse remains on during a portion of the acquisition ramp (Fig. 5); The 20-fold improvement in the noise characteristics of the the discharge voltage is turned off at an rf level (time) during GD-ITMS spectra should result in proportional improvements the ramp corresponding to m/z 130.In this case, L2 deflects in limits of detection. Examples illustrating the detection limits (gates) ions from the trap so that noise is not from ions being are shown in Figs. 7 and 8. Fig. 7 is the GD-ITMS spectrum injected into the trap during acquisition; magnetic deflection of Pb (200 ppm) and Bi (5 ppm) in NIST SRM 1102 Cartridge of electrons was employed.During the discharge ‘on’ portion Brass C. The spectrum is the average of 32 scans acquired of the mass scan (low m/z, Fig. 6), 163 background counts are using a 950 ms pulse (950 ms single injection/scan), 0.95 Hz measured (dark current count subtracted), while only 5 counts pulse, 1.5 kV, #3mA current, and 0.4 Torr Ar. A factor of two are observed during the discharge ‘off’ period (a more than in sensitivity was gained by optimizing both L2 half-plates at 30-fold improvement).With 11 scans averaged, there is a -325 V. Detection limits (3s) for Pb and Bi are 0.2 ppm and 20-fold improvement in the average peak-to-peak back- 0.5 ppm, respectively. Scale expansion [Fig. 7(b)] shows back- ground noise. ground and background noise levels consistent with the dark Even though the discharge is turned off at a time during the current in Fig. 2(a). Fig. 8 shows a number of other elements acquisition which corresponds to m/z 130, noise is observed in the low ppm concentration level in NIST SRM 1102 up to mass m/z 160.This results from the decay period required Cartridge Brass C5Cd (45 ppm), Ag (30 ppm), Sn (60 ppm), for charged species to be lost from the sampled volume of the Sb (50 ppm). Detection limits for each of these is <1 ppm. discharge after it is extinguished. From the ramp rate, this These detection limits are better than the #10–50 ppm limits time is calculated to be#1.5 ms, consistent with that reported in the literature for pulsed glow discharges.15 of detection reported previously,8,9 and are consistent with the 46 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12sampling) without the detector ‘seeing’ the discharge during acquisition (see below). This scan function is generated in the software’s scan function editor. The overlap of the pulse period and the injection period determines the total injection time per pulse. In the experiment described in Fig. 9, a 36 Hz pulse with a 33% duty cycle is used. Only ions formed during the last 1.5 ms of the pulse period and the after-peak ions, formed through ionization by Ar metastables,15–18 are injected. Using the pulse set-up described, four pulses were sampled (n=4) for a total injection time of 6 ms to generate the spectrum shown in Fig. 10(a). The primary peaks in the spectrum are from the isotopes of Cu (72.85%) and Zn (27.10%) in NIST SRM 1102 Cartridge Brass C. Argon and ArH, usually ten times more intense than the matrix, are present at only about 5% of the base peak.Only small peaks of water, argon dimer and other residual Fig. 7 Pulsed glow discharge ion trap mass spectrum of Pb (200 ppm) gases are observed. This demonstrates the analyte ion selec- and Bi (5 ppm) in NIST SRM 1102 Cartridge Brass C (950 ms injection, 32 scans averaged). Limits of detection (3s) for Pb and Bi tivity that is possible by selective sampling of the pulse profile.are 0.2 and 0.5 ppm, respectively. (a) Scale: 1×; (b) scale: 16×. If ions are sampled over the initial 1.5 ms of the pulse, primarily discharge and residual gas species are observed [Fig. 10(b)]. The pulse-injection gate overlap was adjusted by 20-fold reduction in noise and two-fold improvement in increasing the pulse delay after the trigger pulse. The observed signal intensity. preferential sampling of gases in the discharge volume is consistent with that observed on both linear quadrupole14,15,18 Multiple injections and time-of-flight mass analyzers.19 In sampling the early pulse ions, Cu+ intensity is reduced by a factor of three, while argon An alternative method for operation of the pulsed discharge is and related species increase nearly two orders of magnitude to make multiple (n) injections over n pulse periods prior to [note the difference in relative counts shown in Figs. 10(a) and data acquisition. An ion gate is used to sample a portion of (b)].Also noted is continuum noise between H2O+ and Ar+ each pulse. In this manner, constant frequency and duty cycle arising from charge exchange processes during the period of can be maintained; ion intensity increases with n. This should acquisition from low to high m/z values. have several advantages: (1) reduction in detector noise; (2) higher instantaneous powers;14 and (3) temporal selectivity over the ion population.14,15,18 The voltage traces in Fig. 9 indicate the sequence of events associated with the experiment.Two voltage traces are shown: the glow discharge voltage and the rf trapping voltage. The trapping voltage trace consists of a repetitive set of scan periods: a delay period, an injection period, and a cool period. Each region is shown with a different rf amplitude for clarity (delay, rf: m/z 25; inject, rf: m/z 20; and cool, m/z 15). This set is repeated n times for n injections and is followed by an acquisition ramp of m/z 15–150.The delay period and glow discharge pulse are initiated by a software-initiated trigger Fig. 9 Voltage traces showing the synchronization of the rf trapping pulse, which originates in the Teledyne emission control card voltage with the glow discharge voltage during a multiple injection (Fig. 1). The injection period determines when the ion gate is experiment (36 Hz, 33% duty cycle). Overlap of the pulse profile and ‘open’. The cool period allows the pulse to be positioned over the injection period result in selective sampling of ions formed in the post-peak period.the final milliseconds of the injection period (i.e., post-pulse Fig. 8 Pulsed glow discharge ion trap mass spectrum of Cd (45 ppm), Ag (30 ppm), Sn (60 ppm), and Sb (50 ppm) in NIST SRM 1102 Cartridge Brass C. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 47minor and trace species relative to the matrix element(s), consistent with traditional quantification methodology used in GDMS.These issues are the subject of ongoing investigations. Research sponsored by the Office of Basic Energy Sciences, US Department of Energy, under contract number DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. REFERENCES 1 Yates, N. A., Booth, M. M., Stephenson, J. L., Jr., and Yost, R. A., in Practical Aspects of Ion T rap Mass Spectrometry, Vol. 3, Chemical, Environmental, and Biomedical Applications, ed.March, R. E., and Todd, J. F. J., CRC Press, New York, 1995, ch. 4. 2 Kleintop, B. L., Eades, D. M., Jones, J. A., and Yost, R. A., in Practical Aspects of Ion T rapMass Spectrometry, Vol. 3, Chemical, Environmental, and Biomedical Applications, ed. March, R. E., and Todd, J. F. J., CRC Press, New York, 1995, ch. 5. 3 Louris, J. 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Chem., 1993, 65, 3187. advantage in preferentially injecting analyte ions over discharge 19 Steiner, R. E., Lewis, C.L., and King, F. L., Proceedings of the gas ions as a means of selective ion accumulation. Present 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 21–26, 1995, p. 42. methodologies use resonance24–26 and chemically8,27 selective 20 Duckworth, D. C., and Marcus, R. K., J. Anal. At. Spectrom., means of mass discrimination of ions in the trap. There could 1992, 7, 711. be some benefit in minimizing interaction between analyte ions 21 McLuckey, S. A., Glish, G. L., and Asano, K. G., Anal. Chim. with a high abundance of gas ions or with the resonance Acta, 1989, 225, 25. frequencies employed to remove these abundant species. 22 Hergerberg, R., Elford, M. T., and Skullerud, H. R., J. Phys. B: Using multiple injections holds promise for quantification At. Mol. Phys., 1982, 15, 797. 23 Helm, H., J. Phys. B: At. Mol. Phys., 1977, 10, 3683. methodologies in GD-ITMS. It is reasonable to assume that 24 McLuckey, S. A., Goeringer, D. E., and Glish, G. L., J. Am. Soc. the use of a constant frequency and constant duty cycle pulse Mass Spectrom., 1991, 2, 11. will improve the quantitative characteristics of the trap over 25 Julian, R. K., Cox, K. A., and Cooks, R. G., Anal. Chem., 1993, the use of single injections of variable pulse length (i.e., pulse 65, 1827. time is inversely proportional to concentration). Major, minor 26 Goeringer, D. E., Asano, K. G., McLuckey, S. A., Hoekman, D., and trace constituents sampled in this manner are extracted and Stiller, S. W., Anal. Chem., 1994, 66, 313. 27 Eiden, G. C., Barinaga, C. J., and Koppenaal, D. W., J. Anal. At. from a discharge with identical current and voltage character- Spectrom., 1996, 11, 317. istics. Additionally, the use of multiple (n) pulses should allow n data gates to be used to accumulate minor constituents, Paper 6/05312B followed by a single gate (e.g., a 10 ms pulse) over one pulse Received July 29, 1996 for sampling matrix ions. This may allow quantification of Accpeted October 16, 1996 48 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12