Anal. Chem. 1992, 64, 1606-1609
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Radio-Frequency Glow Discharge Ion Trap Mass Spectrometry Scott A. McLuckey' and Gary L. Glish Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6365
Douglas C. Duckwortht and R. Kenneth Marcus* Department of Chemistry, Clemson University, Clemson, South Carolina 29634-1905
A radio-frequency glow dbcharge Ion 8ource has been Interfacedwith a threedhnenslonai quadrupole ion trap. Data are described for ions derived from bra88 and muiticomponent gla88 standards. These data indicate that the Ion trap ha8 m e interesting features as a mass analyzer for Ions formed in a glow dbcharge. MWMS efficienciesapproaching 100% for oxides and hydroxides of all metal ions studled were observed. Ions derived from the argon wpport gas, such as AM+, Afi+, and Ar2*+, were depleted in the ion trap by r a w electron-and proton-tramfer reactionsto background gases in the lon trap. The masdchargeratios of the terminal Ions in the sequence of reactlons fell below the lower malimit of the ion trap and were quickly ejected. The us8 of mass-eelectiveion InJectiontechnique8permilledthe analysls of components present at levels as low as a few tens of parts per million. However, chemlcal ndse due to relatlvely high background preswres In the Ion 8ource precludedanalysls of components of lesser abundance.
INTRODUCTION Atomization and ionization by direct current (dc) glow discharge has proved to be useful in the analysis of conducting solids by mass spectrometry1-3 for major, minor, and trace components. Radio-frequency (rf) glow discharges extend the sample types to nonconducting solids: circumventing the need for mixing nonconducting samples with a conducting host matrix. A significant advantage of glow discharge mass spectrometry (GDMS) is the greatly reduced spectral complexity relative to atomic emission spectrometry. Furthermore, the relatively stable glow dischargeand relatively narrow ion kinetic energy distribution that it provides makes it an attractive alternative to spark source mass spectrometryss6in the analysis of bulk solids. For these reasons, and others, commercial instruments based on GDMS have been introduced. Although GDMS is much less prone to spectral interferences than optical techniques, isobaric interferences pose limitations in some applications. The severity of this problem depends upon the mass resolving power of the mass spectrometer. For this reason, the quadrupole mass filter, despite its simplicity and low cost, may not be the analyzer of choice in many analytical settings. Instruments with much higher t Present address: Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831-6365. (1)King, F. L.; Harrison, W. W. Mass Spectrom. Reu. 1990,9, 285. (2)Harrison, W. W.; Hess, K. R.; Marcus, R. K.; King, F. L. Anal. Chem. 1986,58,341A. (3)Harrison, W. W.;Bentz, B. L. R o g . Anal. Spectrosc. 1988,11,53. (4)Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989,61,1879. (5)Ahearn, A. J. In Trace Analysis by Mass Spectrometry; Ahearn, A. J., Ed.; Academic Press: New York, 1972;Chapter 1. (6)Ramendik, G.; Verlinden, J.; Gijbels, R. In Inorganic Mass Spectrometry;Adams, F.; Gijbels, R., Van Grieken, R., Eds.; John Wiley and Sons: New York, 1988; Chapter 2.
0003-2700/92/0364-1608$03.00/0
resolving powers, Le., greater than a few thousand, have therefore been used as analyzers in GDMS. For example, a double-focusing sector instrument of reverse Nier-Johnson geometry is commerciallyavailable' and is capable of routine operation at resolving powers of up to 8000. Recently, glow dischargesources have been interfaced with Fourier transform mass spectrometer^^^^ with a resolving power in one instance demonstrated to be as high as 37 000. Data at such high resolving power, however, have thus far been presented only for major components. In most cases, isobaric interferences arise from polyatomic species such as argides, metal dimers, and oxides. Dissociation of these species is, therefore, an alternative to higher resolving power in dealing with polyatomic interferences. Collisioninduced dissociation (CID),one of several candidate means of dissociation of polyatomicinterferences in GDMS, has been shown to be useful using triple-quadrupolelOJ1and tandemquadrupole instruments.12 With beam-type instruments, such as the multiple-quadrupole devices, all ions are subjected to collision with the target gas. While the atomic species do not undergo CID, ion loss via scattering out of the acceptance aperture of the final analyzer can occur. Therefore, absolute signals decrease for both atomic and molecular species in CID experiments in beam-type instruments, with the polyatomic species decreasing in abundance a t a faster rate due to the contribution of CID. There exists a trade-off, therefore, in the efficiency with which polyatomic species are depleted and the absolute analyte ion signals in beam-type tandem mass spectrometers. In contrast, the conventional means of collisional activation (CA) in ion-trapping instruments, such as the quadrupole ion trap13and the ion cyclotron resonance mass spectrometer,14is mass-selectivesuch that only ions of particular madcharge values undergo collisional activation. It has been shown for many polyatomic organic ions that the efficiency of convertingparent ions to product ions approaches 100% 13,15in the quadrupole ion trap. This is significant in the present context in that it suggests that it may be possible to completely remove a polyatomic interference without significantly reducing the number of isobaric atomic ions. In addition to its attractive features as a tandem mass spectrometer, the quadrupole ion trap is readily interfaced (7)VG 9000 Glow Discharge Mass Spectrometer; VG Isotopes: Cheshire, England, 1986. (8)Barshick, C. M.; Eyler, J. R. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 1991;p 214. (9)Marcus, R. K.; Duckworth, D. C.; Buchanan, M. V.; Pochkowski, J. M.; Weller, R. R. Unpublished results. (10)King, F. L.; McCormack,A. L.; Harrison, W. W. J.Anal. At. Spectrom. 1988,3, 883. (11)King,F. L.; Harrison, W. W.1nt.J. MassSpectrom.IonRocesses 1989,89,171. (12)Duckworth, D. C.; hlarcus, R. K. Appl. Spectrosc. 1990,44,649. (13)Louris, J. N.;Cooks, R. G.; Syka, J. E. P.; Kelley, P. E.; Stafford Jr., G. C.; Todd, J. F. J. Anal. Chem. 1987,59, 1677. (14)Cody, R. B.;Freiser, B. S. Anal. Chem. 1982,54, 96. (15)Johnson, J. V.; Yost, R. A.; Kelley, P. E.; Bradford, D. C. Anal. Chem. 1990,62,2162. 0 1992 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992 Baratran G w g e
I
Argon Inlet ,
Roughirg Port
Flgure 1. Side view
combination.
I Ian Entrance Endcap
T Ion Exit Endcap
J . 350 Usec
schematic diagram of the RFGD/ion trap
with external ion sources1G19 and typically operates at relatively high background pressures.20 The latter feature is particularly important when relatively high pressure external ion sources are coupled with the ion trap. For these reasons, and others, we have interfaced a radio-frequency GD ion source to a quadrupole ion trap to explore the possibilities of this combination for analytical inorganic mass spectrometry. In parallel with our work, Bier has investigated combined dc GD/ion trap mass spectrometry.21 We relate here our findings with rf GD/ion trap mass spectrometry with particular emphasis on the behavior of polyatomic ions under ion trap collisional activation conditions.
EXPERIMENTAL SECTION Instrumentation. We have already interfaced an atmospheric sampling glow discharge ionization (ASGDI)source2zto a Finnigan ion trap mass spectrometer (ITMS) for the detection of trace organic compounds in air.17 A slightly modified version Detailed of this instrument has also been used with e1ectro~pray.l~ descriptions of the modifications to the commercial ITMS have been given.17J9 Modifying the ASGDI/ITMS system for rf GD studies was relatively straightforward. A schematic diagram of the instrument is shown in Figure 1. Briefly, a home-built rfpowered direct insertion probez was admitted into the GD cavity, comprised of a 23/4-in.Conflat four-way cross, via a l/Z-in. Cajon fitting. Side arms of the cross were used for an argon inlet, a connection for a Baratron capacitance manometer, and a pumpout port, as indicated. The four-way cross was mated with a 23/4-6-in.Conflat adapter flange which was attached to a 6-in. cube that houses the ion trap. The GD cavity was separated from the ion trap vacuum housing by a plate attached to the ion trap housing side of the adapter flange which contained an aperture of 500 pm in ita center. When the probe tip is admitted into the GD cavity, it is in line with this aperture and the aperture in the center of the entrance end-cap of the ion trap. The major experimental parameters for sputtering and ionization were the distance between the sample tip and the exit aperture, the argon pressure in the GD cavity, and the rf power. Some rf GD mass spectra were acquired with this ion source (up to and including the adapter flange) mated to a quadrupole mass filter.24 These data were acquired to indicate the ionic (16) Louris, J. N.; Amy, J. W.; Ridley, T. Y.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1989, 88, 97. (17) McLuckev, S. A.; Glish, G. L.; Asano, K. G. Anal. Chim. Acta 1989,225, 25. (18) Pedder. R. E.: Yost. R. A.: Weber-Grabau. M. Proceedings of the 37th Conference on M a s s Spectrometry and Allied Topics, Miam: Beach, FL, May 1989; p 468. (19) VanBerke1,G. J.;Glish,G.L.;McLuckey,S.A.Anal. Chem. 1990, 62,1284. (20) 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. (21) Bier, M. E. Presented at the 32nd ORNL-DOE Conference on Analytical Chemistry, Gatlinburg, TN, October 1991. (22) McLuckey, S. A.; Glish, G. L.; Asano, K. G.; Grant, B. C. Anal. Chem. 1988, 60, 2220. (23) Duckworth, D. C.; Marcus,R. K. J. Anal. At. Spectrom., in press. (24) Glish. G. L.: McLuckey, S. A.; McKown, H. S. Anal. Instrum.
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distribution produced by the ion source without any changes that might occur during ion trapping. Conditions and Procedures. Helium was admitted directly into the vacuum chamber of the ion trap to a pressure of 0.5-1 mTorr. The background pressure in the ion trap housing during rf GD operation was 0.1 mTorr due to ion source gases entering through the ion source exit aperture. All mass spectra were acquired using argon as the support gas for the discharge at a pressure of 0.3-0.5 Torr, a spacing between sample and exit aperture of 1-2 cm, and an rf power of 30-4OW. The background pressure in the GD cavity ranged from 1-20 mTorr in these experiments. The ion source was operated in a continuous fashion while ions were admitted into the ion trap with a gating lens. All ion trap experiments began with an ion accumulation period of variable duration. Ion injection times varied with the relative number of analyte ions of interest. For example, metal ions derived from the major components were admitted for up to a few tens of milliseconds,at which time the ion trap would have accumulated roughly lo6 ions. (Ions derived from argon, on the other hand, required less than 1ms to accumulate 105-106ions.) Ions from components of concentration less than 0.1 % required injection times of up to 1 s. In the injection of ions of low abundance,one or more of avariety of techniques were employed to prevent the ion trap from 'filling up" with uninteresting ions (e.g., ions derived from argon and the sample matrix). These techniques included the judicious choice of the ring electrode rf amplitude during ion injection, which determines the low mass/ charge cutoff in the ITMS, the use of resonance ejection during ion injecti0n,2~and the use of dc potentials on either the entrance end-cap or the ring electrode during ion injection.z6 MS/MS experiments involved the isolation of a narrow region of mass/charge that encompassed the oxides or hydroxides of all of the isotopes of the metal of interest. Ions were isolated by a ramp of the amplitude of the main trapping rf signal to eject lower massicharge ions and a resonance ejection ramp (i.e., a ramp of the amplitude of the main trapping rf signal in conjunction with a supplementary rf signal applied to the endcapsz5)to eject higher mass/charge ions. Parent ions were collisionallyactivated by excitingthe ion at ita fundamental secular frequency in the z - d i m e n s i ~ n . The ~ ~ ! ~ions ~ were subjected to CA at qz values of 0.4-0.6 for 40-150 ms. The product ions and ions not subjected to CA were then mass-analyzed via massselective instabilityz0in conjunction with resonance ejection.% All data described herein were derived either from NIST brass 1103[nominalweight percent composition: Cu (59.27),Zn (35.72), Pb (3.73), Fe (0.26),Sn (0.88),and Ni (0.1511 or NIST SRM 1412 multicomponent glass.
RESULTS AND DISCUSSION The high background pressure in the ion source (1-20 mTorr), relative to those normally present in rf GD studies, resulted in the appearance of intense signals due to protonated water and the molecular ions of water, nitrogen, and oxygen in the mass spectra obtained with the rf GD ion source attached to the quadrupole mass filter. These ions were observed along with intense signals due to argon. Argonrelated ions in the quadrupole mass spectrum were primarily ArH+ and Ar2+. Although analyte ions appeared primarily as the bare metal ion, significant intensities of ionized metal oxides or hydroxides were also observed. The experiments with the quadrupole mass filter were performed to determine if the oxides and hydroxides observed in the ion trap mass spectra (see below) were formed in the ion source or in the vacuum system via either ionimolecule reactions or ion/surface collisions. The quadrupole data confirmed unambiguously (25) McLuckey, S. A.; Goeringer, D. E.; Glish, G. L. J. Am. SOC.Mass Spectrom. 1990, 2, 11. (26) McLuckey, S. A.; Glish, G. L.; Van Berkel, G. J. Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Topics, Tuscon, AZ, June 1990; p 512. (27) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry; John Wiley and Sons: New York, 1989. (28) Weber-Grabau, M.; Kelley, P. E.; Bradshaw, S. C.; Hoekman, D. J. Proceedings of the 36th ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, June 1988; p 1106.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
that the oxides observed in the ion trap mass spectra were formed in the ion source whereas the hydroxides were formed in the ion trap. The only exception observed was with tantalum, a well-known oxygen-getter.29 The latter results, not described in detail here, showed that oxides of tantalum increased in abundance with trapping time. No such behavior was observed with any of the other metal ions involved in this study. A second consequence of the relatively high air and water background pressure in the ion source is a relatively high background pressure in the ion trap (1 X lo4 Torr). Such a high backgroundpressure of oxygen and water in the vacuum system leads to significant differences in the low madcharge region of the ion trap mass spectrum as compared with the quadrupole mass spectrum. These differences stem primarily from rapid ion/moleculereactions involving ions derived from argon and background gases with lower ionization potentials or higher proton affinities. The ion trap mass spectra are most like the quadrupole mass spectra when short ion injection times and minimal delays prior to mass analysis are employed. At long ion injection times or with long delays before the mass scan, the ions derived from argon, e.g. ArH+,Ar*+,and Arz*+, are absent in the ion trap mass spectra. Proton transfer from ArH+to Nz,02, and HzO are all exothermic and electron transfer from 02 and HzO to Are+ are highly exothermic. (Electrontransfer from Nz to Ar*+is only slightly exothermic.) The ionization potential of Arz is 14.5eV, which is significantly higher than those of water (12.6eV) and oxygen (12.07eV),3O so that loss of Arz*+ due to electron transfer can also occur with high background pressures of air. Of the atmospheric gases, water has the lowest ionization potential so that consecutive charge-transfer reactions tend to shift charge rapidly to HzO*+. Ionized water then undergoes the wellknown reaction
HzO'+ + HzO
-
100 2csPb+
a a
s2
v
I
I
200
,
,
,
,
I
,
,
,
I
20s
I
,
,
,
,
21s
210
Flgurr 2. RFGD ion trap mass spectrum of NIST brass 1103 showing the lead isotopes.
lo01
miZ
Flgure 3. RFGD Ion trap mass spectrum of NIST brass 1103 showing the tin Isotopes.
H30++ OH'
to give protonated water as the terminal ion of the sequence of ion/molecule reactions. For these studies, the relatively high background pressure of air in the ion trap served a useful purpose in that the charge initially carried by ions derived from argon was quickly shifted to HzO*+during and shortly after ion injection. When analyte ions were accumulated in the ion trap, the main trapping rf signal amplitude was always maintained such that the low mass/charge cutoff was greater than 25. Under these conditions, the charge initially introduced into the ion trap as argon-derived ions was quickly "shuttled out" of the ion trap by ion/molecule reactions that terminate in ions derived from water. This phenomenon works to minimize deleterious effects due to space charge that might otherwise limit the dynamic range of the ion trap. It also allows for the analysis of analyte ions that might fall a t or near m/z 40,41,or 80. The use of ion/moleculereactions to remove ions generated from the discharge support gas, therefore, might be useful in calcium analysis, for example. Ion trap mass spectra derived from rf GD applied to a NIST brass 1103 sample illustrate most of our observations. It was observed that the best results, in terms of accurate isotope ratios, were obtained by injecting ions over a narrow mass/charge range encompassing the isotopic distribution of the sample component of interest. In so doing, the analyte ions of interest comprise the most abundant species in the ion trap, yielding optimal statistics. The mass spectra derived from lead (Figure 2),tin (Figure 3), and nickel (Figure 4)are shown here. Note that the Fe and Ni ions could be injected into the ion trap without accumulating the much more abundant ions from the major components Cu and Zn. Figure (29) Mei, Y.; Harrison, W. W. Spectrochim. Acta 1991, 46B, 175. (30)Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988,17, Suppl. No. 1.
40
80
60
100
miZ
Flgurr 4. RFGD Ion trap mass spectrum of NIST brass 1103 showing
the iron and nickel isotopes.
80
100
120 mlz
140
Flgwr 5. RFGD ion trap mass spectrum showing the tin isotopes with
the vertical scale expanded to polnt out signals due to isotopes of lesser abundance.
5 shows an expanded portion of the mass spectrum of the tin isotopes obtained with an injection time of 1.0 s and with a dc voltage (+16 V) applied to the entrance end-cap which eliminated ions of madcharge greater than 122. The tin isotopes at m/z 112,114,and 115 are clearly apparent and are present in the brass sample a t levels of 85,58, and 31 ppm, respectively. Note also that at long ion injection times, signals
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
appear at almost every mass. This chemical noise is due, in part, to the relatively high background pressure in the ion source and, perhaps, to contamination on the surfaces of the GD region. Whatever the source of this chemical noise, the signals at every mass prevented detection of components at lower concentrations. Matrix-related molecular species (e.g., M2+, MAr+) are typically observed at abundances of 0.1-3% of the major atomic analyte ions in rf GDMS. No argides were noted in the ion trap mass spectra in this study. By far, the most abundant polyatomic species observed in the ion trap mass spectra were metal hydroxide ions and, in a few cases, metal oxide ions. Interestingly, metal oxide ions were much more abundant than metal hydroxide ions in the quadrupole mass spectra whereas the opposite was noted in the ion trap mass spectra. Apparently, metal oxides were converted to hydroxides via reactions with adventitious water in the vacuum system via the reaction MO+ + H 2 0
-
MOH'
+ OH
-
Ba+
+ OH
'"BaOH+
100 1
135
145
155
165
155
165
m/Z
I
lO0l
lug,+
(2)
This reaction is nearly 0.8-eV exothermic for BaO+, for example, on the basis of the heats of formationof the reactants and products.30 This reaction, however, could not be monitored under the conditions used in this study due to the relatively long ion injection times required (ca. 1 s) to accumulate the oxidized metal ions. It is this long ion accumulation time that may allow the oxide ions to convert to hydroxides before mass analysis. The fact that a metal oxide ion could not be explicitly shown to react to form the metal hydroxide ion in the gas phase leaves open the possibility that metal oxide ions might be converted to metal hydroxide ions via an ion/electrode collision during ion injection. Support for this possibility comes from the observed pick-up of a hydrogen atom by organic ions in collisions with a ~urface.3~~3~ It was found in every case investigated that the hydroxide or oxide could be dissociated to yield the bare metal ion with nearly 100% efficiency. Figure 6 illustrates this observation with the collisional activation results for 138BaOH+derived from SRM 1412 multicomponent glass. Figure 6a shows the spectrum acquired after ejecting all ions except those of the hydroxides of the barium isotopes. Figure 6b shows the spectrum after the 138BaOH+parent ion was collisionally activated at a qz value of 0.54 for 100 ms. The energy requirement for the reaction BaOH'
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(3)
is roughly 5.5 eV. This is the lowest energy dissociation reaction of BaOH+. Hydrogen loss to form BaO+is endothermic by 5.9 eV. It is, at first glance, remarkable that such high efficiences can be observed for a reaction so highly endothermic in a quadrupole ion trap MWMS experiment. However, these experiments were carried out with relatively deep trapping well depths (50-100 V), due to the relatively large qz values empl~yed,~' and long collisional activation times. Provided the parent ion cannot escape the trapping field duringkinetic excitation and cannot dissipate its excess energy at a rate competitive with the rate of activation, sufficient energy can be added to cause any polyatomic ion to fragment. Similar results were also obtained for the ionized hydroxides of copper, zinc, tin, lead, tantalum, and iron and for ionized iron oxide. It is apparent from the results obtained to date that the quadrupole ion trap has some interesting features as a mass (31) Winger, B. E.; Julian, R. K., Jr.; Cooks, R. G.; Chidsey, C. E. D. J. Am. Chem. SOC.1991,113, 8967. (32)Wysocki, V. H.; Jones, J. L.; Ding, J.-M. J. Am. Chem. SOC.1991, 113, 9869.
135
145
miz Figure 6. R F W ion trap mass spectrum acquired from SRM 1412 multicomponent glass after election of all ions except those of the hydroxides of the barium isotopes (a, top). RFW ion trap MSlMS spectrum after 13eBaOH+was subjected to colilsional acthratlon (b, bottom).
analyzer for ions formed from GD. These include MS/MS efficiencies approaching 1005% for oxides and hydroxides, depletion of ions derived from argon by iodmolecule reactions, and the ability to analyze minor components using selective ion injection techniques. The lowest analyte levels that can be measured with combined rf GD/ITMS, however, could not be determined due to relatively high levels of chemical noise arising,presumably, from the high background preeaurea in the ion source and vacuum system. Nevertheless, these results indicate that further exploration of this combination is worthwhile. For example, detection limits with a clean and dry ion source should be evaluated and the interesting ion chemistry noted here should be studied in detail. Furthermore, high mass resolution experimentspossible with slow ~ c a n n i n gmay ~ ~ also ~ ~ prove to be useful in this application.
ACKNOWLEDGMENT Research sponsored by the United States Department of Energy, Office of Basic Energy Sciences, under Contract DEAC05-MOR21400with Martin Marietta Energy Systems,Inc. This paper is based upon work presented at the 32nd DOEORNL Conference on Analytical Chemistry, October 1991, Gatlinburg, TN. RECEIVED for review February 10, 1992. Accepted April 20, 1992. Registry No. Brass,12597-71-6. (33) Schwartz, J. C.; Syka, J. E. P.;Jardine, I. J. Am. SOC.Mass Spectrom. 1991, 3, 198. (34)Williams,J.D.;Cox,K.A.;Cooks,R.G.;Kai.ger,R.A., Jr.;Schwartz, J. C. Rapid. Commun. Mass Spectrom. 1991, 5, 327. (35) Goeringer, D.E.; Whitten, W. B.; Ramsey, J. M.; McLuckey, S. A.; Glish, G. L. Anal. Chem., in press.
