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correction. If iodine is present, S20at 858.560 nm may be substituted for If C1 is present, the most serious interference encountered with this spectrograph must be dealt with directly. No good alternative sulfur emissions are available to avoid C1 interference. Figure 4 shows Sz0,at 858.560 nm, to be closely overlapped by C122at 858.599 nm. The S:C1 selectivity ratio at the Szo channel center is only 0.751 for this spectrograph. exhibits a more favorable S:C1 selectivity ratio (3:l)and should be used if preliminary inspection of the Cl19emission reveals the presence of chlorine in the sample. When a compound contains both S and C1, quantitative analysis is still possible by use of an interelement correction method similar to that commonly employed by manufacturers of ultraviolet-visible emission spectrometersbased on multiple exit slits and photomultiplier tubes. First, the C1 content of the compound should be computed from 754.709 nm (Cl,) or 837.597 nm (Cllg)emissions. The measured C1 content and the known S:Cl selectivity ratio for the S8-10channel center (3:l)can next be used to evaluate and subtract the contribution of C1 from the total emission at the Ssl0 wavelength. This yields the net sulfur content of the unknown compound. In the unlikely event that S, C1, and I are all present in a compound formula, the above chlorine-corrected sulfur measurement at Ssl0 can be further refined with a similar interelement correction scheme for iodine. Alternatively, the SZ0emission could be used without iodine correction, but the magnitude of the interelement correction for C1 will be greater. The number of organic compounds that include all three elements S, C1, and I in their formula are extremely small, so the need for the above interelement correction scheme will be rare. The sulfur lines S40 and S41, at 941.346 and 942.193 nm, respectively, are subject to major interferences from both CI3
and C13*emissions. The S:C and SC1 selectivity ratios at the S41 channel center are 1.5:l and 0.5:1,respectively. Since C is much more abundant in organic compounds than S, the use of this line for S determination is not recommended with a low-resolution photodiode array spectrograph. Szoor Sslo will give a much better result. As with the previous photodiode array study involving C, H,N, and 0,the figures of the present work may be obtained in the form of Xerox transparent overlays by writing to the authors. The overlays can serve as labeled master reference spectra for qualitative identification involving the elements F, C1, Er, I, and S. Registry No. F,, 7782-41-4; CI,, 7782-50-5; Br,, 7726-95-6; 12, 7553-56-2; S,7704-34-9; SFs, 2551-62-4; CBrzFz,75-61-6; C2F5Cl, 76-15-3; CFJ, 2314-97-8.
LITERATURE CITED (1) Keane, J. M.; Brown, D. C.; Fry, R. C. Anal. Chem. 1985, 57, 2526-2533. (2) Hughes, S. K.; Brown, R. M., Jr.; Fry, R. C. Appl. Spectrosc. 1981, 35, 396-400. (3) Fry, R. C.; Northway, S. J.; Brown, R. M.; Hughes, S. K. Anal. Chem. 1980, 52, 1716-1722. (4) Hughes, S.K.; Fry, R. C. Anal. Chem. 1981, 5 3 , 1111-1117. (5) Hughes, S. K.; Fry, R . C. Appl. Spectrosc. 1981, 35, 493-497. (6) Blades, M. W.; Hauser, P. Anal. Chim. Acta 1984, 157, 163-169. (7) Zaidel', A. N.; Prokof'ev, V. K.; Raiskil, S. M.; Slavnyi. V. A,; Shrelder, E. Ye. "Tables of Spectral Lines"; IFI/Plenum: New York, 1970. (8) McLean, W. R.; Stanton, D. L.; Penketh, G. E. Analyst (London) 1973, 98, 432-442. (9) Keane, J. M. Ph.D. Dissertation, Kansas State University, 1985.
RECEIVED for review August 7,1985. Accepted October 17, 1985. This work was supported in part by NSF Grant CHE-8219256 and in part by the Kansas State University Foundation. This work was taken from the Ph.D. dissertation of J. M. Keane (9) and was presented in part at the 1985 Pittsburgh Conference (paper 079).
Hollow Cathode Plume as an Atomic Emission Source for Elemental Analysis of Metal Alloys R. Kenneth Marcus and W. W. Harrison* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Sputter atomlzatlon of sample material Into a constricted, energetic plasma yields a stable, low-cost source for solids analysis by atomlc emission spectrometry. The hollow cathode plume (HCP) operates In a reduced pressure (1-5 torr) argon atmosphere at currents up to 200 mA and powers less than 70 W. The plume emerges through a small orlflce (1.5 mm) In a dlsk that forms the analytical sample. Sample atoms are ejected dlrectly Into the base of the plume where subsequent excitation occurs. Characterlstlcs of the HCP relative to Its analytical use for bulk solids analysis, Including stablllty and reproduclblllty, are presented. Short- and longterm stability of the source, as well as Its intersample reproduclblllty, are shown In the analysis of National Bureau of Standards steel and zinc-base alloys.
We describe here the development of a new type of atomic emission source for the direct analysis of metal samples. Spectrometric elemental analysis of bulk metal alloys has had a rich history dating back to spark emission studies by Talbot
in 1826 (1). Until the mid-19609, arcs and sparks were the major sources for such analyses and are still in quite active use (2-4). While both techniques have been characterized extensively, their relatively high costs along with experimental and instrumental complexity can be prohibitive. Laser-based techniques have also been used for atomic emission analysis of solids ( 5 ) ,but cost and complexity are negative factors. The introduction of the inductively coupled plasma (ICP) atomic emission systems and the widespread use of atomic absorption spectrometry have turned the analytical emphasis to solution techniques for trace-elementanalysis, even for solid samples. These techniques have many advantages including ease of standardization and high sample throughput. Problems exist in that the sample dissolution required for aspiration into the plasmas (flames) is time-consuming and results in a loss of physical information about the solid. The dissolution step also normally lowers, by orders of magnitude, the concentration of the analyte in the resulting solution sample. The nebulization process, although the subject of considerable research, is still generally less than 10% efficient (6). Introduction of an aqueous sample into an atmospheric pressure
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diagram of lhe HCP emissbn system.
Flgum 1. HCP source in a disk sample hfgwation.
Figurn 2. Schematic
discharge can also lead to spectral interferences arising from broad band emission of molecular species (e.& OH, NH) (7). Although solution techniques are in widespread use, there still exist many applications that benefit from a direct solids sampling mode. Arc and spark emission are the techniques of choice for many industrial applications because of their ability to analyze solid samples directly. Much interest has recently been directed toward the atomization of solids into the ICP by such means as laser ablation (8),dc and rf arc8 (9,10),high-voltage spark ( 1 0 , and direct insertion (12). All of these methods are based on the thermal vaporization of the samule followed hv transuort into the plasma in a n inert carrier gas. Glow discharee devices have lone " been known to orovide efficient sputter atomization of solid samples, bringing the sample constituents into a plasma for excitation and ionization. Low-power discharges struck in inert atmospheres have been used as emission sources for many years (13). Glow discharges offer certain advantages over the previously mentioned techniques. Because the discharges are operated at reduced pressure and in inert atmospheres, there is minimal molecular background emission. Sputter atomization also has a significant advantage over vaporization in that the relative sputter yields of the elements generally vary by only a factor of 3-5, while elemental volatilities can vary hy orders of magnitude. Analytical studies have been published for Grimm-type glow discharge systems aa well ~ a afor hollow cathode devices (14-16). The interest in glow discharge devices in analytical chemistry is exemplified by the current availability of both glow discharge atomic emission and mass spectrometry devices. This laboratory has been interested in various aspecta of glow discharges for many years, including hollow cathodes aa excitation sources (13),and mass spectrometric sources for solution residues (17) and bulk solid samples (18). We have also used a coaxial cathode geometry as a glow discharge ion source for mass analysis (19) and as an atom source for resonance ionization mass spectrometry (RIMS) (20).A hollow cathode device was also developed as a RIMS atom source by allowing effusive flow of sputtered species from a n orifice in the base of the cathode. During these studies, we came upon prensure/current conditions that cause ejection from the orifice of a flamelike plasma plume which exhibits intense emission characteristics. This hollow cathode plume (HCP), the subject of this report, offers promise as an atomic emission murce for solids (21) and may also prove useful as an ion source for elemental analysis (22). We describe here the examination of a series of National Bureau of Standards (NBS) steel and zinc-base alloys, using the HCP in an atomic emission mode.
disk is inserted hy a simple press-fit into the end of a graphite cylindrical holder. A conventional hollow cathode discharge is struck between an adjacent anode and the graphite cavity. Current and pressure conditions are optimized to enhance the plume formation, which extends from the orifice of the disk. As shown in Figure 2, the HCP source, housed in a standard six-way cross (NorCal Products Co., Yreka, CA) for flexibility in accessing the discharge, is mounted on a 2.75-in.-o.d. flange for convenient extraction and sample replacement. The plume emission is monitored through quartz windows mounted on the other cross arms. Vacuum and gas inlet connections are made through appropriate flange feedthroughs.. The hollow cathode is mounted in a glass-ceramic (Macor) shield and water cooled by means of stainless-steel tubing wound around the shield. The anode connection is made through an isolated MHV connection. A Kepco Model BHK power supply operating in the constant current mode powers the HCP at currents up to 200 mA at pressures of 1-5 torr of argon. A 5M) L/min rough pump (Sargent Welch Scientific Co., Skokie, IL) evacuates the source to about 0.025 torr before introduction of the discharge gas. A Jarrell-Ash 0.5m Ebert monochromator is used with a R955 Hamamatsu photomultiplier detector. The resulting signal is fed to a Pacific Instruments Model 110 photometer-amplifier. Emission spectra are recorded on a Houston Instruments Model Zoo0 X-Y recorder. A 1-in. quartz lens focuses the plume image onto the monochromator extrance slit. After sample introduction the source is evacuated to 0.025 torr and then backfilled to 2 torr with argon. The discharge is struck at the desired current. 100 mA in these studies. and allowed to sputter clean and equilibrate for IO min. and the analysis is begun. Analysis times will depend on the number of elements analyzed for and the spectromeer employed. typically 5 min. SampletOsample turnaround times are less than 20 min.
I
EXPERIMENTAL SECTION A representation of the hollow plume is &- in Figure 1. The analytical sample is normally in the form of a thin disk, 4-5 mm in diameter, 2 mm thick, with a 1-2-mm orifice. This
RESULTS AND DISCUSSION Glow Discharge Processes. The hollow cathode plume is part of the family of glow discharge devices and relies on fundamental glow discharge phenomena for its operation. The principal aspect of the HCP appears to be its very efficient sputtering in a constricted discharge channel. An intense plasma plume is extruded from the hollow cathode and serves as the excitation/ionization volume for the atoms sputtered from the adjacent cathode base. While we are not sure of the origin of all the observed HCP processes, reasonably clear analogies can be drawn to standard glow discharge operation. Figure 3 is a representation of how some of these processes may occur in the HCP. Atomization. Sample atomization in the HCP is the result of cathodic sputtering. In a two-electrode glow discharge configuration, discharge gas ions are accelerated acrosa the cathode dark space (CDS) and strike the cathode sample. The energy of the incoming ion, up to a large fraction of the applied potential (23). is dissipated in the cathode material matrix throuch - a collisional cascade. If the cascade urouaeates . . _ with proper direaionality and sufficient energy, ejection of cathode material as atoms and ions will result. The negative potential of the cathode with respect to the anode dictates that ejected
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rate over this period of 0.146mg/min. It should be noted that the first few minutes of sputtering are not always efficient because of the possible presence of oxide layers on the disk surface. Oxides sputter poorly, inhibiting the sputter process (25).At the pressures involved, significant collisional redeposition oceura (26) so that an atom may be resputtered several times before a fortuitous diffusion path leads it away from the cathode. Weight loss measurements thus produce a net or effective sputter yield, which for this study was 0.112 atoms per incident ion, a value that agrees quite well with results of similar studies in this laboratory using argon as the discharge gas in conventional glow discharge configurations (24, 273.
3. Atomization. excbtion. and ionization processes In the hollow camode plume (shaded area: camode dark space). -re
(a)
Ib)
Figwe 4. Elecbm mlcrogaphs of HCP e!sclr& (a) before and (b) alter sputtering showing localized sample erosion (copper camode. 2 torr. 60 mA. l-h spuner time).
positively charged species will return to the cathode surface, while negatively charged species will be aecelerated away. Of interest in the HCP are the neutral atoms that comprise the great majority (>90%)of the sputtered species. These atoms diffuse through the cathode dark space into the negative glow. which in this case is the hollow cathode plume (see Figure 3). Even though all the cathode is at the same potential, sputter erosion is concentrated in the orifice and particularly at the plume end of the orifice channel. During normal analytical time scales, the erosion proma is not significant, but a t longer times. it becomes more apparent where the sample is being removed. Figure 4 shows scanning electron micrographs of split hollow cathodes, (a) before running and (b) after 1 h of operation at 60 mA (2 torr argon). Micrographs taken a t other areas of the cathode show no or minimal evidence of sputtering. These results show that a great fraction of the discharge power is being dissipated in the cathode orifice. Even a t shorter discharge periods (15 min). the sputter action in the orifice can be seen with an optical microscope as sharp, well-defined spires or cones that are typical sputter products (24).
We have taken advantage of this localized sputtering by using cathode disk samples set in disposable graphite hollow cathode holders (see Figure 2) in order to reduce sample size and preparation time. Comparisons of samples prepared as complete hollow cathodes vs. insert disks showed that comparable results were obtained. To determine the rate of HCP atomization, a sputter weight loss study of a copper disk in this configuration was performed, with the same discharge conditions and sputter time as the SEM study. The resultant m&ps logs after 1 h of sputtering was 8.8 mg, an average sputter
Excitation. Sputtered atom diffuse away from the cathode and are excited in the negative glow, a region that is highly visible as a manifestation of the excitation processes. As indicated in Figure 3. two types of collisional excitation processes are thought to dominate in glow discharge plasmas (28). Collisions of the 'first kind" involve inelastic collisions between electrons and neutral gas and metal atoms. Collisions of the 'secund kind", Penning-type, occur between metastable discharge gas atoms and sputtered neutrals. The metastable energy levels of argon are 11.55 and 11.72 eV, energies sufficient to populate high-lying atomic and ionic states. Both collision processes can amount for the observed HCP emission. Studies are currently under way to define more clearly which of these processes may be dominant in this plasma. Hollow Cathode Plume. The HCP is baaed on the physical properties that are inherent to hollow cathode discharges (28). The source can be thought of as tandem hollow cathodes: the main cathode body, which serves for discharge initiation, and the disk orifice, where a second hollow cathode discharge forms, this of a constricted, intense nature. Mechanisms involved in the plume formation may be best considered by analogies drawn from corresponding hollow cathode characteristics. At low currents (
