Anal. Chem. 1907, 59, 1240-1242
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Registry No. Hydroxylamine, 7803-49-8; sodium acetate, 127-09-3;acetone, 67-64-1.
(1)
1
2
4
6
TIME frnln)
Figure 1. Sample gas chromatogram for analysis of hydroxylamine as acetone oxime. Column is 10% Carbowax 1540 on Chromosorb W at condiiions outlined in text. Key: I, injection; A, ether and acetone peak; 6,residual water peak; C, acetone oxime; D, acetone impurity; E, 1-heptanol internal standard. Injection size was 1.4 gL, thermal conductivity detector. Sample is 1.0 mg of NH,OH/mL.
This improved procedure is an accurate, forgiving gas chromatographic approach for routine hydroxylamine determination over a range of concentrations in a reasonable time period.
LITERATURE CITED Langer, S . H.; Pate, K. T. Ind. Eng. Chem. Process Des. Dev. 1983,
22, 264. (2) Pate, K. T.; Langer, S. H. Environ. Sci. Techno/. 1985, 19, 371. (3) Bathias, M. L.; Watkinson, A. P. Can. J . Chem. Eng. 1979, 5 7 , 631. (4) Kirk-Othmer Encycbpsdia of Chemical Technolow; _. Wiley: New York. 1966; Vol. 11, p 4 9 3 . (5) Cason, J.; Harris, E. R. J . Org. Chem. 1959, 2 4 , 676. (6) Vogh, J. Anal. Chem. 1971,-43, 1616. ( 7 ) Streuli, C. A.; Averell, P. R. The Analytical Chemistry of Nitrogen and Its Compounds; Wiley: New York, 1970; pp 72-76. (8) Snell, F. D.; Snell, C. T. Cobrimetric Methods of Analysis; Van Nostrand: New York, 1954; Vol. IV, pp 54-55. (9) Darke, D. J. J . Chromatogr. 1980, 181, 449-462. (10) von Breymann, M. T.; de Angeleis, M. A.; Gordon, L. I. Anal. Chem. 1982, 54, 1209-1210. (11) Jencks, W. P. J . Am. Chem. SOC. 1959, 81, 475. (12) Lohr, L. J.; Warren, R. W. J . Chromatogr. 1962, 8 , 127.
RECEIVED for review October 6,1986. Accepted December 16, 1986. This work was supported by the National Science Foundation and the University of Wisconsin.
Inductively Coupled Helium Plasma as an Ion Source for Mass Spectrometry Akbar Montaser* and Shi-Kit Chan Department of Chemistry, T h e George Washington University, Washington, D.C. 20052
David W. Koppenaal Mineral Studies Laboratory, Bureau of Economic Geology, T h e University of Texas at Austin, Austin, Texas 78712 In earlier communications (1,2), we reported the generation of various types of helium inductively coupled plasmas (He (ICPs) operated at atmospheric pressure. Preliminary studies (1-4) showed that the annular He ICP was an efficient excitation source for atomic emission spectrometry (AES). In addition, observation of prominent emission lines of chlorine ion at 479.54,481.00, and 481.95 nm revealed (4) the potential of the annular He ICP as an ion source for mass spectrometry
(MS). The origin, development, and analytical applications of atmospheric-pressure plasmas as ion sources for MS have been reviewed (5-7). Gray (8-10) demonstrated that useful mass spectra of elemental constituents in solutions could be obtained from a capillary arc Ar plasma, while Douglas and French (11) reported the analytical performance of a microwave induced Ar plasma as an ion source for MS. Analytical mass spectra obtained from an Ar ICP were first reported by Houk et al. (12). Among the plasmas cited above (8-12), the Ar ICP offers the best analytical performance due to its relatively high gas and ionization temperatures and its annular configuration. Because the ionization energy of He (24.6 eV) is higher than that of Ar (15.8 eV), the use of a He ICP as an ion source for MS should, in principle, enhance the degree of ionization for every element, in particular for the non-metals possessing high ionization energies. Also, certain mass spectral interferences arising from the presence of Ar and various polyatomic species in the Ar ICP could be avoided if a He ICP is used as an ion source. To cite a specific example, the 40Ar35C1+ ion presents a major problem in the trace measurement of 75As+because arsenic is monoisotopic and the 40Ar35C1+is quite intense in the Ar ICP when HCl is present in the sample (13).
In this report, we describe the first successful coupling of an atmospheric-pressure He ICP to a commercial ICP-MS system. Minor modifications of the load coil and the plasma impedance matching network were made to sustain the annular He ICP in a low-gas-flow torch (2) at a forward power of 500-900 W and a total gas flow of 8 L/min. The objectives of this work were to investigate the feasibility of using a mass spectrometer for fundamental studies of He ICPs and to explore the analytical capabilities of the He ICP-MS system for the determination of halogens and sulfur. Except for this report on the He ICP and an earlier study with an Ar-N2 ICP (14),all previous investigations of ICP mass spectrometry have been concerned with Ar discharges.
EXPERIMENTAL SECTION The PlasmaQuad ICP-MS (VG Isotopes, Ltd., Winsford, Chesire, England) was used for these experiments. The other commercial instrument available,the ELAN system (SCTEX, Inc., Thornhill, Ontario, Canada) was unsuitable for these studies because the high vacuum of the mass analyzer chamber is maintained by a He-cooled cryogenic pump. The sampling depth of the plasma, defined as the distance between the tip of the sampler cone and the top turn of the load coil, was fiied at 18 mm. The sampler cone and the skimmer cone of the ion extraction interface were made from titanium nitride coated nickel and had orifices with diameter of 300 and 750 km, respectively. Under these conditions, typical pressure readings (calibrated for N2gas) of the ion extraction interface, the ion focusing stage, and the mass analyzing stage corresponded to and 6 X torr, respectively. Orifice approximately 6, 1 X diameters greater than 300 pm were not used for the sampler cone in this preliminary study because the high apparent pressure in the ion extraction interface activated the vacuum safety interlocks, thus preventing ion sampling by the mass spectrometer. No other
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Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
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Mass/chargs
Figure 1. Background spectra of an annular He ICP operated at 700-W forward power, 7 L/mln plasma-gas-flow rate, and 1 L/mln injector-gas-flow rate. Full scale counts = 320 000.
modifications of the mass spectrometer were necessary. The operating conditions of the ion lenses, the quadrupole mass analyzer, and the ion detector were set at values originally optimized for the Ar ICP. Mass spectra of the He ICP background were obtained with zero grade He gas (99.998%, Big Three Co., Austin, TX). For introduction of gaseous sample, a certified gas mixture of 99 p L L-' of CClzFz,96 pL L-' of CBrF,, and 105 pL L-l of SF6 in high-purity He gas (Matheson Gas Products, East Rutherford, NJ) was diluted 10-fold with the He injector gas.
RESULTS AND DISCUSSION Plasma Formation and Plasma Appearance. The annular He ICP was sustained in a low-gas-flow torch described recently (2). A 26-mm-i.d., 51/2-turn load coil was used to generate the ICP discharge at a plasma gas flow of 7 L min-I and an injector gas flow of 1 L m i d . The He ICP was self-ignited at a forward power of approximately 350 W. To stabilize the plasma and to reduce the reflected power, the capacitances of the impedance matching network were varied between 15 and 100 pF for the series capacitor and between 450 and 525 pF for the shunt capacitor. The capacitors were force cooled by air to prevent excessive heating and consequent arcing inside the impedance matching box. The reflected power remained less than 10 W when the forward power was changed from 500 to 900 W. The appearance of the He ICP was similar to that previously described (2). As the torch was moved toward the ion extraction interface of the mass spectrometer, plasma stability was enhanced, especially when the mechanical pump for the ion extraction interface was activated. In addition, a very bright orange-yellow discharge of about 2 mm in length was formed a t or around the tip of the sampler cone. This discharge was probably due to the presence of a low-pressure region created at the orifice of the sampler cone by the vacuum inside the ion extraction intetface. Formation of this lowpressure discharge may complicate the determination of ion populations and electron number densities (14) in atmospheric-pressure plasmas. Optimization of the ion extraction interface and minimization of plasma potential should, as in Ar ICP-mass spectrometry (15,16), reduce the extent of the "pinch" discharge. Mass Spectral Background of the He ICP. Figure 1 shows the mass spectrum of the major positive ions from the annular He ICP when pure He injector gas was introduced into the plasma operated at 700-W forward power. The most intense peaks occurred a t 14 amu (N+), 16 amu (O+), and 28 amu (N2+,CO+),in contrast to the observation for the Ar ICP (13) where the most intense peaks corresponded to O+ (16 amu), Ar+ (40 arnu), and ArH+ (41 amu). The other major peaks appeared, in the order of decreasing intensity, a t amu
Mass/charge
Flgure 2. Mass spectra of helium species from a He ICP operated at (A) 500 W, (B) 700 W, and (C) 900 W. The plasma and the Injector gas flows were 7 and 1 L/min, respectively. Full scales counts =
32 000 (left plots) and 3200 (right plots).
of 40 (Ar+), 32 (02+), 30 (NO'), 17 (OH+, H3N+),18 (H20+, H,N+, "e+), 8 (He2+),58 (Ni+),60 (Ni+),12 (C+),29 (N2H+, COH+), and 20 (Ne+). The occurrence of peaks for Ar+ and Ne+ may be traced to the impurities of the helium gas supply (3),while the Ni+ peaks originate from the gradual erosion of the sampler cone. Mass spectral signals for the nitrogen- and oxygen-containing species either arise from air entrainn;ent into the plasma, especially a t the ion extraction interface, or are due to impurities of the helium gas supply, as noted in our previous atomic emission studies (3). In general, above mass 40, the spectrum of the He ICP is largely free of background peaks. In contrast, background features of the Ar ICP extend to mass 80 (13). Optimization of the ion extraction interface and the use of a He sheath gas around the He ICP should further diminish the level of polyatomic ions and certain background peaks. Mass Spectra of Helium Ion Species. Close scrutiny of the mass spectra of the He ICP reveals unique information that is especially useful for fundamental characterization of this discharge. Figure 2 shows intensified mass spectra of the He ICP at increasing forward power for the mass range 2-10 amu. The presence of He+, HeH+, and He2+is clearly documented. At higher forward power, the concentrations of HeH+ and He2+are reduced while that of He+ is enhanced. The existence and the proportion of the He ion species are significant because they are presumed to be important energy carriers responsible for the excitation of the hard-to-excite emission lines observed in our previous studies (1-4). One should realize, however, that the mass spectra observed may not totally represent the ionic composition of the plasma stream because of ion-electron combination, charge exchange, and clustering reactions between ions and neutral species that may occur during the ion extraction process (17). Mass Spectra of F, S, C1, and Br. Figure 3 shows the background-subtracted spectra of F, S, C1, and Br in their respective mass ranges obtained when a certified mixture of freons and SF6sample was injected into the He ICP operated at 700-W forward power. Except for S, the relative intensities of the peaks decrease with increasing ionization energy (ionization energies = 17.4, 10.4, 12.9, and 11.8 eV, respectively). The anomaly for S may be due to the large background cor-
Anal. Chem. 1987, 59, 1242-1243
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is obtained for gaseous sample injection. Because our previous AES studies ( I , 2, 4 ) indicate that He ICPs, in the present state of development, are less immune to solvent loading than the commonly used Ar ICPs, further investigations are in progress in our laboratories to assess the potentials of He ICP mass spectrometry for determination of halogens in aqueous samples. ' I
16
17
18
! 19
20
In
C
3
0
CONCLUSIONS A low-gas-flow helium inductively coupled plasma is used as an ion source for a mass spectrometer. The modification of the impedance matching network of a commerical ICP-MS system is described for forming the He ICP at a forward power of 500-900 W with helium gas flow of 8 L/min. Observation of important plasma background species such as He+, Hez+, and HeH' is reported along with the sensitive detection of Br+, C1+, S+,and F+ for gaseous sample injection. ACKNOWLEDGMENT We express our gratitude to Leslie Quinton and Steven Tweedy of the University of Texas a t Austin for their assistance during the course of this work. Registry No. He, 7440-59-7;He+, 14234-48-1;He2+,12184-99-5; HeH', 17009-49-3;Brp,7726-95-6;FP,7782-41-4;Cl,, 7782-50-5; S, 7704-34-9. LITERATURE CITED
L .
78
79
80
81
I
82
Masdcharge
Figure 3. Background subtracted mass spectra of F, S,CI, and Br obtained when a certified mixture of freons and SF, was injected in the He ICP operated at 700-W forward power; see Experimental
Section for gas composition and concentrations. Full scale counts = 160 000.
rection for 02+ a t 32 amu. Examination of the spectrum a t 17.5, 18.5, 39.5, and 40.5 amu indicated no detectable peaks from doubly ionized C1 or Br. In addition to the singly charged ion (M+),residual contributions to the mass spectra were noted for certain monoxide ions (MO+). The BrO+/Br+ count ratio was approximately 0.06%, being the most significant. Higher levels of oxides are anticipated when aqueous samples are injected into this He ICP. The high integrated count rates for the halogens and sulfur clearly indicate the potential of He ICP-MS for determination of these elements. For example, considering the peak and background area count rates for *lBr+ (9.7 X lo6 and 100 counts/s, respectively), an estimated detection limit of 0.2 pg/s
Chan, S.; Montaser, A. Spectrochim. Acta, Part B 1985, 408, 1467-1472. Chan, S.;Van Hoven, R. L.; Montaser, A. Anal. Chem. 1986, 58, 2342-2343. Chan, S.; Montaser, A. Appl. Spectrosc., in press. Chan, S.; Montaser, A. Spectrochim, Acta, Part B , in press. Horlick, G.; Tan, S.H.; Vaughan, M. A,; Shao, Y. I n Inductively Coupled Plasmas In Analytical Atomic Spectrometry: Montaser, A., Golightly, D. W., Eds.; VCH Publishers; New York, 1987. Gray, A. L. Spectrochlm. Acta, Part B 1985, 408, 152551537, Houk, R. S. Anal. Chem. 1988, 5 8 , 97A-105A. Gray, A. L. Proc. SOC.Anal. Chem. 1974, 1 1 , 182-183. Gray, A. L. Analyst (London) 1975, 100, 289-299. Gray, A. L. Anal. Chem. 1975, 47,600-601. Douglas, D. J.; French, J. B. Anal. Chem. 1981, 53,37-41. Houk, R. S.;Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L. ; Taylor, C. E. Anal. Chem. 1980, 52, 2283-2289. Tan, S.H.; Horlick, G. Appl. Spectrosc. 1986. 40,445-460. Houk, R. S.; Montaser, A,; Fassei, V. A. Appl. Spectrosc. 1983, 37, 425-428. Douglas, D. J.: French, J. B. Spectrochim. Acta, Part B 1986, 418, 197-204, and references therein. Gray, A. L. J . Anal. At. Spectrom. 1986, 1 , 247-249, and references therein. Olivares, J. A.; Houk, R. S. Anal. Chem. 1985, 57,2674-2679.
RECEIVED for review October 13, 1986. Accepted December 19, 1986. This research was sponsored in part by the U S . Department of Energy under Contract No. DE-AS05-84-ER13172 and Grant No. DE-FG05-87ER13659 and by the University of Texas at Austin. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.
Interchangeable Insert Thermospray Probe Steve E. Unger,* Terry J. McCormick, Mark S. Bolgar, and John B. Hunt The Squibb Institute for Medical Research, P.O. Box 4000, Princeton, New Jersey 08540 Thermospray, as an ionization method and an liquid chromatography/mass spectrometry (LC/MS) interface, has undergone extensive development since its introduction (1-3). Several instrument manufacturers offer a variety of source designs and options, including the use of a repeller, electron filament, and discharge assembly, as well as control of the
vaporizer temperature to compensate for gradient elution. However, little has been done to improve the performance and durability of the thermospray probe. These probes are expensive and often are easily plugged ( 4 ) by the introduction of particulate matter and the precipitation or decomposition of material at the probe tip. We would like to present a
0003-2700/87/0359-1242$01.50/0 t? 1987 American Chemical Society
