Anal. Chem. 1987, 59, 1242-1243
1242
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
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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
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82
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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 = Figure 3.
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
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987 in
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Flgure 1. Interchangeable insert thermospray probe. The vaporizer tip is sealed with a 1/,6-in. Swagelok fitting silver-sobred to the 'l4-1n. outer tubing.
modification which allows rapid interchange of capillary inserts and thermocouples for instruments using a probe for eluent introduction.
INTERCHANGEABLE INSERT PROBE DESIGN The thermospray probe may be fabricated from new materials by using 1/4-in.-o.d. stainless-steel tubing, ironconstantan thermocouples, and 1/16-in.-o.d. stainless-steel capillary tubing with 0.005- or 0.007-in. i.d. (Bodman Chemicals, Media, PA) or from clogged probes. For clogged probes, the thermospray probe is cut approximately 1/2 in. from the vaporizer end, allowing the capillary and thermocouples to be withdrawn from the probe. A new 1/16-in.stainless-steel capillary may be introduced or the old capillary may be reused provided flow is established by cutting a small section of the tubing containing the plug. A standard '/16-in. Swagelok fitting whose internal diameter has been opened to allow passage of the 1/16-in.-o.d.tubing is silver-soldered to the 1/4 in. outer tube. A complete seal is needed here as this junction maintains the vacuum-atmosphere interface. The outer tube and soldered fitting is trimmed to 1/4-in. 0.d. with a lathe. Similarly, the 1/16-in. Swagelok nut is trimmed to 1/4 in. 0.d. Figure 1 illustrates the interchangeable insert thermospray probe. Assembly of the complete thermospray probe is straightforward. The thermocouples are spot-welded to the 1/16-in. capillary tubing while 1/8-in.-o.d. Teflon tubing is used for electrical insulation from the outer tubing. The capillary should protude beyond the end of the probe so that a ferrule and the trimmed nut make a tight vacuum-atmosphere seal. This point also provides electrical contact between the smaller capillary tubing and its larger outer tube. RESULTS AND DISCUSSION Based upon results from two different mass spectrometers, we have found no difference between these interchangeable insert probes and those supplied commercially. Over a variety of flow rates (0.5-2.0 mL/min) and temperatures (150-250 "C), sprays generated from the interchangeable insert probe are visually identical with those from commercial probes. Mass spectral performance as evident from peak stability and
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388
Figure 2. Positive thermospray mass spectrum of 20 KL of a 0.1 % solution of poly(ethy1ene glycols) obtained while using this probe.
detection sensitivity are also identical with commercial probes. Heat conduction across the stainless steel ferrule is good as apparent from electrical resistance measurements. When a four-wire resistance measurement technique and a Kiethly 195A multimeter were used, the interchangeable insert probe gave a resistance of 0.137 R, while a commercial probe gave 0.174 R. These probes yield two advantages. First, clogged probes and probes whose performance is degraded by narrowing or opening of the capillary tip may be restored. Alternatively, new inserts are easily made and may be on-hand for fast repairs. Second, capillary diameters other than those used in commercial thermospray probes are available, allowing maximum sensitivity to be achieved over a wider variety of flow rates and vaporizer temperatures. Figure 2 illustrates the positive thermospray mass spectrum of 20 KL of a 0.1% aqueous solution (0.1 M ammonium acetate) of poly(ethy1ene glycols) (MW 200-600) obtained while using an interchangeable insert probe. This spectrum is identical with that generated from a commercial probe and was acquired by using a Vestec thermospray source (Vestec Corp., Houston, TX) and a home-built quadrupole mass spectrometer comprised of a Finnigan 4500 quadrupole (Finnigan Corp., San Jose, CA) and Extranuclear radio frequency generator (Extrel Corp., Pittsburgh, PA). Similar results have been obtained when using this source on a Finnigan TSQ-4600 mass spectrometer.
LITERATURE CITED (1) Blakley, C. R.; Carmody, J. J.; Vestal, M. L. Anal. Chem. 1980, 52, 1636-1841. (2) Blakley, C. R.; Carmody, J. J.; Vestal, M. L. J. A m . Chem. SOC. 1980, 702, 5931-5933. (3) Blakley, C. R.; Vestal, M. L. Anal. Chem. 1983, 55, 750-754. (4) Hsu, F. F.; Edmonds, C. G. Vestec Thermospray News/. 1985, 7 , 4.
RECEIVED for review November 3,1986. Accepted December 18, 1986.
