Anal. Chem. 1983, 55, 1523-1526
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Simultaneous Analysis of an Abnormal Glow Discharge by Atomic Absorption Spectrometry and Mass Spectrometry 1.J. Loving and W. W. Harrlson' Department of Chemlstty, University of Virginia, Charlottesville, Virginia 2290 1
A method for the simultaneous atomic absorptlon and mass spectrometric anlaiysls of an abnormal glow discharge is described. By cornbination of these two anaiytlcal methods, useful information is gained concernlng the effects of dlscharge parameters, source design, and electrode posltloning. A stainless steel coaxial cathode was used to define an Fe atom population lprofiie, with the greatest atom density being located about the front and center of the cathode. The cathode-anode separation whlch results in the best 5eFe+ signal Is different than the distance at whlch optimum Fe atom signals are obtained. Slmultaneous measurement of "Fe' and Fe atom absorbance about the ion exit orifice shows a slgnlficant difference wlth source pressure.
The glow discharge has long been known as a relatively simple way of producing atoms and ions by means of its energetic sputtering and plasma processes. In recent years, it has found increased interest for a variety of analytical uses, in particular as an atomization (1-4), excitation (5-8), and ionization (9-13) source. The fundamental processes occurring in the glow dischiarge have been described in detail (14-15). From an analytical standpoint, of prime importance is the cathodic !sputtering step, wherein the sample (cathode) constitutenta are sputter-injected, primarily as neutral atoms, into the discharge plasma. A fraction of these atoms is ionized by electron imp,act (161,Penning and perhaps other mechanisms (18-20). Mass spectrometric sampling of the discharge yields spectra representative of the cathode material. By coupling quartz windows to the glow discharge cell, at is possible to optically probe the atomic populations simultaneously with mass spectrometric sampling. The ability to monitor both atlnms and ions is useful in determining the effects of discharge parameters and constitutes the main t h t of this study. 'While to our knowledge no simultaneous atomization/ioniization studies have been made, atomic absorbance profile8 of glow discharges have been reported. Sterling and Westwood (21, 22), in studying the origin of sputtered material, measured absorbance profiles for aluminum, iron, and nickel atoms in a glow discharge. Elbern (23) used laser fluorescence methods to plot atomic populations of iron in a similar discharge.
(In,
EXPERIMENTAL SECTION A schematic diagram of the source developed for simultaneous atomic abscrption/mass spectrometric analysis is shown in Figure 1. The radiation from a hollow cathode lamp [A] passes through a chopper [B] prior to entrance into the stainless steel discharge chamber-anode [C] through a quartz window [D]. The light beam then passes through the glow region where it may be partially absorbed by neutral atoms sputtered from the cathode surface [E]. On passing out of the chamber through a second quartz window into a monochromator [HI, the intensity of characteristic wavelengthia reachling the photomultiplier detector [I] is recorded [J]. Samplnng of ions is accomplished by extraction of a portion of the plasma through a small orifice within an anode plate [F]. A fraction of these positive ions is mass analyzed by a quadrupole
mass filter in the subsequent mass spectrometer vacuum chamber [GI. The focused radiation from the hollow cathode lamp is modulated at 260 Hz with a P.A.R. Model 125 mechanical light chopper before its passage through the glow discharge and subsequent entrance into a Jarrel Ash Model 82-516 single-beam atomic absorption spectrophotometer. The radiation intensity of the Characteristic absorption wavelength determined by a 0.5-m Ebert monochromator is detected with an R106 photomultiplier tube. The output from the detector is fed into a P A R . Model 122 lock-in amplifier. A Linear Instruments dual channel recorder is used to present not only a mass spectrum, but simultaneously the appropriate atomic absorption as well. Aside from offering simultaneous AA/MS analysis of the glow discharge, the source chamber was designed to be versatile in accepting various cathode geometries and modes of sample introduction. A large center port (3.06 cm id.) coaxial to the quadrupole chamber incorporates a cathode manipulator-holder assembly. A smaller upper port [K] entering the source at an angle of 60° allows solution sample introduction onto a cathode for solution residue analysis (24). The source volume was kept small to minimize evacuation time after sample turnover, while still allowing a sufficient discharge-to-window distance for minimizing sputter deposition. The quadrupole mass spectrometer assembly has been previously described (IO),with the exception that the ion source is no longer in an interior chamber of the quadrupole housing, but rather moved outside t o permit the optical measurements. Moving the source housing exterior to the mass spectrometer vacuum envelope provides visual inspection and spectroscopic analysis of the glow discharge and also allows the installation of a vacuum shut off valve between the vacuum chamber and source chamber. A thin vacuum slide valve has been designed which allows samples to be changed without venting the vacuum envelope of the mass spectrometer. Incorporated into the entrance flange of the mass spectrometer chamber, the valve consists of a 1.0 mm thickness brass slide 15 cm in length and 6.0 cm wide containing a hole the same diameter as the entrance orifice into the mass spectrometer (3.0 cm). This slide rides on a track between two stainless steel plates and over two O-rings inserted into each plate. The thickness of the entire assembly is only 1cm. Plans are available from the authors (W.W.H.). For ion transport between the ion source and the quadrupole, an electrostatic lens extraction-transport system was employed, consisting of a simple three-tube einzel lens which focuses the beam without changing the ion energies (25). Such a lens is used frequently for the collecting of ion beams with high divergence (26),as in the case of our flow stream with an expanding jet of gas molecules, ions, etc. issuing downstream from the source orifice. The focused probe beam was made mobile in order to measure atomic populations in various regions of the discharge, thus allowing atomic profiling of the discharge plasma. The entire lens assembly is mounted on an optical rail positioned beneath the source chamber and may be removed as a single unit. A variable movement platform was constructed to support the optical rail and to permit translational and vertical movement as required for the profile studies. The monochromator rests on a similar platform to allow tracking of the probe beam. In this manner, the atomic population of the entire region within the discharge chamber may be profiled.
RESULTS AND DISCUSSION Atomic Absorbance Profiles. During the glow discharge sputter operation, ejection of surface atoms into the discharge plasma proceeds at a rate determined by the incoming particle
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Flgure 1. lonizationlatomization source. See text for details.
Flpure 2. Iron atomic absorbance profile of a coaxlai cathode glow discharge at 248.3 nm. current density striking the cathode and by the particle mass and energy, the latter of which is controlled by discharge pressure. The discharge preasure alw aff& discharge voltage: the higher the pressure, the lower the voltages. At the 1torr pressure normally used, there occurs significant collisional redeposition, but a net diffusion of atoms away from the cathode leads to a steady-state concentration gradient throughout the glow discharge source. Detailed information on such atomic distributions can be valuable in sowce design, selection of optimum discharge parameters, and electrode positioning. From atomic absorption data obtained by probing the various regions of the coaxial cathode glow discharge, an atomic absorbance (population) profile can be developed as shown in Figure 2. This plot represents the relative Fe neutral atom population profile which results from sputtering a stainless steel 304 cathode at 1.0 torr Ar and 30 mA constant current. Aa Seen in Figure 2, the atomic population is greatest about the front and center of the cathode; it falls off in direct proportion to decreasing negative glow intensity. In as much as the negative glow molds itself to the shape of the tube containing it, so does the apparent atomic population profde. The similarity in negative glow-anode container geometry arises because cross sections for electron excitation are maximum at the intersection of the cathode dark space-negative glow regions (27) and fall off with decreasing electron energies as the electrons traverse the chamber. Since the negative glow plasma is considered to be an essentially field free region (20), motion of electrons is diffusion controlled. In this sense the
DISTANCE FROM CATHOOE lmnl Flgure 3. Copper atomic absorbance (324.7 nm) measured as a
function of distance from the front of the coaxial cathcde. copper cathode. anode chamber housing acts as a sink, drawing electrons to its surface, thereby partially depleting electrons from the area next to these surfaces as reflected by a decrease in AI excitation and thus a relative decrease in negative glow intensity. The movement of neutral sputtered atoms within the discharge is also diffusion controlled, with inside surface areas of the source acting as deposition sites for neutral atoms, depleting the Fe neutral population. The atomic population seems particularly concentrated within the discharge region surrounding the front portion of the cathode. Visual inspection of a sputtered cathode reveals greatest sputter erosion of the surface in that same area, particularly on the sharp edges of the cathode tip. The geometry of this cathode region leads to a larger net sputter yield by offering advantageous bombarding ion angles (15) and a smaller surface area for sputtered particle redeposition. Bentz (27) found that over 90% of the analyte ion signal evolves from atoms sputtered from the front one-third of a coaxial cathode similar to the one used in this research. From these studies it appears that the cathode may be reduced in size considerably without decreasing overall atomic or ionic intensities. CathodeAnode Distance. The effect on atomic densities of varying the distance between the cathode and the adjacent anode plate is shown in Figure 3, with data taken at four different discharge pressures. Near the cathode surface the atom populations are rather comparable in each case at a given pressure, with a subsequent reduction in absorbance as the probe is moved away from the cathode. It can be seen however, that the sharpest roll-off in atom density occurs just near the anode plate for each of the two separation distances. This seems to result from deposition and not depletion by beam extraction through the anode orifice, as determined by closing the flange valve and observing no significant population difference. From an analytical standpoint, the 5 mm separation creates a higher atom density than does the 10 mm separation around the anode orifice under comparable conditions. Absorbance is seen to be inversely proportional to source pressure. The lower pressures require higher discharge voltage, producing more energetic Art to bombard the cathode surface. This in turn produces a greater sputter yield and higher atomic absorbance. Thus, within the constraints of discharge stability, lower pressures are favorable for atomic absorbance measurements. The maximum atomic population next to the anode orifice is produced by moving the cathode quite near the anode, but ion signals show a different response. As indicated in Figure 4, the best =Fe+ signal is obtained at cathode distances of 6 1 0 mm from the anode. The decrease in ion signal observed beyond 10 mm is more in keeping with the observed drop in
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Characteristic voitagehurrent plot showing the effect of pressure on the glow discharge.
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Figure 5. Iron atomic absorbance as a function of glow discharge current at selectsd pressures.
Flgure 7.
sputtered atom density as the cathode is pulled back from the anode. h very i3harp decrease in intensity results at distances closer than 6 mrm, resulting from factors which affect ionization and extraction, but which are not significant in producing the precursor atomic population. As the cathode approaches the anode, it, may influence the characteristics of the exiting atomic/ionic beam by changing the energy distribution of the ions or by affecting the ionization process itself as the cathode dark space/negative glow interface proceeds toward the exit orifice. Studies of Discharge Parameters. At the pressures used in these experiments, the effective sampling volume from which ions are extracted is restricted to a small region near the sampling orifice. By taking atomic absorption measurements of the sputtered neutral atoms within this volume, and simultaneously monitoring the extracted ion signals of these sputtered particles, we can examine the relationship between the two populations. In this study, the iron neutral absorbance (taken at the 248.3-nm iron resonance line) and ssFe+ ion intensity, both resulting from sputtering a stainless steel 304 coaxial cathode under various discharge current and Ar source pressure conditions, have been measured. The effect of discharge current on absorbance at various constant pressure conditions is shown in Figure 5. Absorbance increases approximately linearly with increasing current at a given source pressure and increases in the same manner with decreasing pressure a t a given current. The former effect is consistent with the direct relation between the sputtering rate and the primary non current. The greater the rate of incoming sputter ions, the greater is the resultant steady-state sputtered atom population in front of the exit orifice. The latter effect of increased absorbance with decreasing pressure reflects the rise in discharge volltage, as is shown in Figure 6, which gives the
current-voltage characteristics of the discharge. The relative magnitude of this effect depends on the discharge current. At low current conditions, the pressure change creates a relatively small voltage increase, but a significant rise in absorbance. At high currents, the voltage increase is large but the relative increase in atomic density is less striking. For example, at 5 mA the voltage rises by only 30% in going from 1.4 torr to 0.8 torr, but an 80% increase in absorbance is seen. At 35 mA, the same pressure change causes a 270% increase in voltage with a 28% increase in absorbance. Simultaneous measurement of 5eFe+with the Fe atomic absorbance shows a significant difference as illustrated in Figure 7 (and to be compared with Figure 5). Although the overall qualitative effect is similar, in that factors which lead to higher atom density (higher current and lower pressure) also produce higher ion currents, a comparison of Figures 5 and 7 shows a more striking dependence of the ion signal on discharge pressure (and thus voltage). In fact, Figure 7 more closely resembles Figure 6, where voltage is plotted on the ordinate axis. Clearly, changing the pressure affects more than just the voltage of the discharge. Other factors such as mean free path may play significant roles. It is known, however, that electron impact processes leading to the formation of Fe+ are dominant at higher current (28). Electron impact ionization may also be significant a t lower source pressures, because of the relatively larger fraction of high energy electron collisions. Such ionization mechanisms are contributing factors to the larger effect which discharge parameters appear to have on ionic populations as compared to their same effect on atomic populations. Although higher discharge currents and lower Ar pressures enhance both atomic and ionic sensitivities, currents no greater than 30 to 40 mA and pressures no lower than 0.8 torr are used
Effect of discharge current and pressure on 56Fe+ion
intensity.
Anal. Chem. 1983, 55,
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in this study in order to avoid (i) eventual discharge instability brought about by accelerated deposition of cathode material on the Macor insulator about the cathode, (ii) ion exit hole clogging problems with sputtered material, (iii) accelerated deposition of cathode material on cell windows causing drifting of the atomic absorption signal, and (iv) excessive cathode heating problems. Since water or air cooling capabilities were not incorporated in this source design, particular attention had to be paid to this last problem in order to avoid significant thermal contributions to sputter atomization. Simultaneous atomic absorbance and mass spectrometric sampling of a glow discharge allow insight into the effects of discharge parameters and source design on atomic and ionic constituents of the plasma. For additional information on discharge processes, it might be useful to add yet another type of measurement, that of atomic emission. In this manner, one could compare excited state atom populations with the ground state atom and the ionic populations. Registry No. Stainless steel, 12597-68-1; iron, 7439-89-6; copper, 7440-50-8.
LITERATURE CITED Gough, D. S. Anal. Chem. 1978, 4 8 , 1926. McDonald, D. C. Anal. Chem. 1979, 4 9 , 1338. Bruhn, C. 0.; Harrlson, W. W. Anal. Chem. 1978, 50, 16. Kirkbright, G. F. Analyst (London) 1971, 96, 609. Harrison, W. W.; Prakash, N. J. Anal. Chim. Acta 1970, 4 9 , 151. (6) McNally, J. R.; Harrison, G. R.; Rowe, E. J. Opt. SOC.Am. 1947, 3 7 ,
(1) (2) (3) (4) (5)
93.
(7) Grlm, W. Spectrochim. Acta, Part 8 1988, 2 3 8 , 443. (8) Boumans, P. W. J. M. Anal. Chem. 1972, 4 4 , 1219. (9) Coburn, J. W.; Harrison, W. W. Appl. Spectrosc. Rev. 1981, 77, 95.
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(IO) Bruhn, C. G.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1978, 50, 373. ( 1 1 ) Coburn, J. W. Rev. Sei. lnstrum. 1970, 4 7 , 1219. (12) Colby, B. N.; Evans, C . A,, Jr. Anal. Chem. 1974, 46. 1236. (13) Harrison, W. W.; Bentz, B. L. "Trace Element Analytical Chemlstry in Medicine and,Biology"; Waiter De Gruyter and Co.: New York, 1980. (14) Nasser, E. Fundamentals of Gaseous Ionizatlon and Plasma Electronics": Wiley: New York, 1971. (15) Carter, G.; Colllgon, J. S. "Ion Bombardment of Solids"; American Elsevier: New York, 1968; Chapter 7. (16) Chapman, 8. "Glow Discharge Processes"; Wiley: New York, 1980; p 96. (17) Westwood, W. D. Prog. Surf. Sci. 1976, 7 , 71. (18) Coburn, J. W.; Kay, E. Appl. Spectrosc. Rev. 1975, 70, 201. (19) Coburn, J. W.; Eckstein, E. W.; Kay, E. J . Vac. Sci. Technol. 1975, 12, 151. (20) Cobine, J. D. "Gaseous Conductors, Theory and Englneerlng Appllcations"; Dover: New York, 1941, (21) Stlrllng, A. J.; Westwood, W. D. J . Phys. D : Appl. Phys. 1971, 4 , 246. (22) Stlrllng, A. J.; Westwood, W. D. J . Appl. Phys. 1970, 4 1 , 742. (23) Elbern, A. J . Vac. Scl. Technol. 1979, 76, 1564. (24) Savickas, P. J.; Marquis, R. W.: Loving, T. J.; Harrlson, W. W. Paper presented at 28th Paper presented at 28th Annual Conference on Mass Spectrometry and Allied Topics, New York, May 1980; Abstract TPMOC6. (25) Wilson, R. G.; Brewer, G. R. "Ion Beams: Wlth Applications to Ion Implantation"; Wiley: New York, 1973; p 203. (26) Septier, A. "Focusing of Charged Particles"; Academlc Press: New York, 1967; p 289. (27) Bentz, B. L. Ph.D. Thesis, Universlty of Vlrglnla, Charlottesville, VA,
1980. (28) Smyth, K. C.; Bentz, B. L.; Bruhn, C. G.; Harrison, W. W. J . Am. Chem. SOC. 1979, 101, 797.
RECEIVED for review October 18, 1982. Accepted April 18, 1983. The financial assistance of the National Institutes of Health and the Department of Energy is gratefully acknowledged.
Dual-Pin Cathode Geometry for Glow Discharge Mass Spectrometry T. J. Loving and W. W. Harrison" Department of Chemistv, University of Virginia, Chariottesville, Virginia 2290 1
Two small plns are used as dual cathodes in a glow dlscharge for elemental analysls. Slmuitaneous atomlc absorption and mass spectrometric analyses allow comparlsons of atomlc and ionic populatlons In the discharge. Atomlc absorbance proflies are produced for Iron atoms showlng the greatest population near the electrode tlps. The effects of dlscharge current and voltage are demonstrated, and the opilmum dlstances for electrode separation are determlned. Water vapor Is shown lo create serlous population reductlons In iron atoms and Ions. Pressed graphlte electrodes may be used as analytlcal cathodes.
In recent years, the glow discharge has been reported in a range of emission, absorption, and ionization applications (1-6) in analytical chemistry. The primary component of interest in the discharge centers about a conducting cathode which either serves as the sample itself or acts as a matrix which supports the sample. These glow discharge cathodes have taken many different forms, including tubes, disks, hollow cathodes, rods, and wires, depending upon sample requirementa and source designs. The work to be described here had its origin in a study (7) in which two sets of electrodes were
used within a single ion source, the first pair run at high current, low voltage, primarily to sputter atoms from an analytical cathode, and the second set operated at high voltage, low current, and directed across the ion exit orifice to enhance ionization of the neutral atoms. These secondary electrodes, made from graphite or tungsten/rhenium pins, were effective in their intended purpose of ionizing sputtered atoms from the primary cathode, but examination of the mass spectra also showed a significant contribution of ions from the pin electrodes. It was on this basis that we designed a small-chamber ion source to accommodate two pin electrodes. Such a configuration resembles that commonly used in spark-sourcemass spectrometry with the attendant advantages of handling small samples, tipped-out electrodes, or even the ability to run nonconducting samples by pressing a graphite/sample mixture into conducting pins with an appropriate die and hydraulic press. Reported here are the first results obtained with a dual-pin glow discharge ion source. The source has also been designed with optical windows to allow atomic absorption studies of the sputtered atoms.
EXPERIMENTAL SECTION The atomization-ionization source is shown in Figure 1. The vacuum electrical feedthrough assemblies [C] into which the
0003-270O/83/0355-1526$0~.50/0 0 1983 American Chemical Society
