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Anal. Chem. 1986, 58,972-974 Morgenthaler, L. N.; Eyler, J. R. Int. J . Mass Spectrom. Ion Phys. 1981. 97. 153. Jaslnskl, J. M.; Rosenfeid, R. N.; Meyer, F. K.; Brauman, J. I . J . Am. Chem. SOC. 1982, 104, 652. Rosenfeld, R. N.; Jasinski, J. M.; Brauman, J. I . J . Am. Chem. SOC. 1982, 104, 658. Thorne, L. R.; Beauchamp, J. L. I n "Gas-Phase Ion Chemistry, Volume 3"; Bowers, M. T., Ed.; Academic Press: New York and London, 1984; Chapter 18. Bowers, W. D.; Delbert, S. S.; McIver, R. T., Jr. J . Am. Chem. SOC. 1984, 106, 7288-7289. McIver, R. T., Jr.; Bowers, W. D. In "Tandem Mass Spectrometry"; McLafferty, F. W., Ed.; Wlley: New York, 1983; Chapter 14. Cody, R. 6.; Burnler, R. C.; Cassady, C. J.; Freiser, B. S. Anal. Chem. 1982. 54. 2225-2228. Amster, I:J.; Baldwin, M. A.; Cheng, M. T.; Procter, C. J.; McLafferty, F. W. J . Am. Chem. SOC. 1983, 105, 1654. Neuman, 0. M.; Derrick, P. J. Org. Mass Spectrom. 1984, 19, 165. Shell, M. M.; Derrick, P. J. Org. Mass Spectrom. 1985, 2 0 , 430. Commlsarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 2 5 , 282. McIver, R. T. Jr. Am. Lab. (Falrfleld, Conn.) 1880, 12(11), 18. Wilkins, C. L.; Qross, M. L. Anal. Chem. 1981, 53, 1661A. Bowers, W. D.; Hunter, R. L.; McIver, R. T.. Jr. Ind. Res. D e v . 1983. 25(11), 124-128. Dunbar, R. T.; Fu, E. W. J . Phys. Chem. 1977, 81, 1531-1536. Van Veizen, P. N. T.; Van Der Hart, W. J. Chem. Phys. 1881, 61, 325-334.
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William D. Bowers Sherri-Sue Delbert Robert T. McIver, Jr.* Department of Chemistry University of California Irvine, California 92717
RECEIVED for review October 7,1985. Accepted December 23, 1985. This work was presented at the 33rd Annual Conference on Mass Spectrometry and Allied Topics, May 26-31, 1985, San Diego, CA. We acknowledge support by Grant CHE8024269 from the National Science Foundation and Grant GM34327 from the National Institutes of Health.
Hollow Cathode Plume as an Atomization/ Ionization Source for Solids Mass Spectrometry Sir: The development of glow discharge atomization/ionization sources for solids analysis by mass spectrometry has been of interest in this laboratory for some years (1-4). The qualities of relatively uniform atomization and ionization, low ion energies, and source stability make glow discharge mass spectrometry (GDMS) an attractive alternative to the thermal (5) and high voltage-spark ionization techniques (6). The acceptance of the technique as an analytical tool has become evidenced by the commercial availability of a glow discharge mass spectrometer system (7). Many types and configurations of glow discharges have been employed as ion sources dating back to the Aston discharge source (8)used in early isotope ratio experiments. The most common electrode configurations have been the planar diode (9) and hollow cathode geometries (10). While most glow discharges are operated in a dc potential mode, Coburn et al. have demonstrated the utility of ratio frequency (rf) powered discharges to sputter atomize and ionize nonconducting samples such as lathanide oxides (11). We have recently described (12,13)the use of a unique type of glow discharge, the hollow cathode plume, as an atomic emission source for direct solids analysis. The plume phenomenon arises from a hollow cathode discharge confined in the orifice of a s m d sample disk. Adjustment of the discharge pressure and current causes extrusion from the orifice of a plasma plume, which contains a high density of sputtered atoms. These atoms are collisionally excited in the intense plasma allowing for elemental analysis by atomic emission. Emission from ionic species is also observed, indicating that a significant ionic population exists and that the hollow cathode plume might be advantageously utilized as a mass spectrometric source. We report here the use of the hollow cathode plume (HCP) as an atomization/ionization source for solids analysis. The source appears to offer certain advantages over other glow discharge devices, notably in the ability to generate a large atomic population and to energy-discriminate against backgrdund gas ions.
EXPERIMENTAL SECTION Figure 1 illustrates the components employed in the mass spectrometricsamplingof the HCP. The quadrupole system used in these studies has been described previously (14). A Bessel box energy analyzer, a three-element lens with a central stop for photons and neutrals, is incorporated in the system as a means of allowing only ions of a narrow kinetic energy spread to enter the quadrupole, improving its resolution (15). The value of this energy window may be varied in order to maximize analytical signal or to study the kinetic energy of ionic species that exit the plasma. Mass spectral data may be taken in either an analog or digital format. A DEC MINC-11microcomputer is employed to control the maw spectrometer,accumulatedata, and store spectra. The HCP source is housed in a stainless steel six-way cross (Nor-CalProducts, Yreka, CA), which allows mounting of optical windows and gas inlets. After an initial evacuation and an argon flushing to reduce residual water vapors and other background gases, the source is operated in an argon atmosphere at pressures of 1-10 torr. The discharge is maintained by a Kepco Model BHK power supply operating in a constant current mode up to 200 mA. The HCP samples take the form of disks, which are 1.5 mm thick and 4.5 mm in diameter with a 1.5 mm orifice drilled through the center. The sample is held in a graphite hollow cathode holder (25.4 mm long, 4.5 mm i.d., and 6 mm 0.d.). RESULTS AND DISCUSSION There are inherent properties of the hollow cathode plume source that distinguish it from other glow discharge ion sources: these are a high sputter atomization rate and a large energy disparity between the sputtered and gaseous ion species. As a glow discharge device, the HCP utilizes sputtering as its means of sample atomization. The discharge occurring in the orifice of the disk sample, where the majority of the discharge current is being delivered, exhibits the characteristics of hollow cathode discharges (16). Among these is the ability to operate at relatively high currents with low discharge voltages, typically 100-200 mA and less than 350 V. These values are in contrast to those a t which the coaxial pin cathode source ( 1 7 ) employed in this laboratory is operated, 1-5 mA and 1kV. Glow discharges operating in the 1-10
0003-2700/66/0358-0972$01.50/00 1986 American Chemical Society
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