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 populationswith 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
ANALYTICAL CHEMISTRY, VOL. 55. NO. 9. AUGUST 1983
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T O MASS SPECTROMETER v
Atomization-ionization source incaporatingpin eleckodes. See text for defails.
Flgure 1.
Figure 3. Iron atomic absorbance profile from two pin electrodes.
'-1
PRESSURE (TORR) 2 0.0
.I
PIN CATHODE Flgure 2. Cross section of pin cathode holder assembly.
cathodes [A] are inserted enter the chamber through upper and lower ports [Dl at an angle of -IOo with respect to the vertical anode exit hole plate [B]. The cathode holder-electrical feedthrough assemblies slide or rotate over an O-ring inserted at the top of each port, thereby providing free movement of the cathodes along a horizontal plane within the chamber without breaking vacuum. Sputtering gas entrance and source pumping are provided by two porta [E] within a plate which is bolted onto the source chamber and replaces the coaxial cathode holder assembly. Quartz windows [F] allow atomic absorption measurements. A cross section of a pin cathode holder assembly is shown in Figure 2. The 1.0 mm diameter by 7.0 mm long cathode samples me inserted into an electrical pin connedor which is crimped onto a stainless steel rod along which the necessary discharge current is carried. The stainless steel rod and pin connector are insulated from the discharge by quartz tubing. The vacuum is formed around this quartz tubing by a double set of O-rings tightened down within the plastic cap which fits over the O-ring inserted into the chamber port. This design allows for vertical movement of the cathodes without breaking vacuum. The cap is made with Delrin (acetal) due to its exceptional strength at above normal temperature, low moisture adsorption, insulating qualities, and relative ease to machine. The vacuum between the quartz tubing and stainless steel rod is contained by an additional set of O-rings within a second Delrin vacuum electrical feedthrough assembly. Successful vacuum stability well as freedom of cathode motion are both provided in this design. The two cathodes are connected electrically and receive power from an HP 6525A 4-kV power supply, thus forming two individual glow discharges operating at approximately the same current and voltage, with the source housing acting as the anode. The cathodes are typically centered about 4 mm in front of the ion exit orifice with a typical interelectrode separation distance of 1.0-2.0 mm. Bulk solid metal samples (capper, stainless steel, NBS steels, and brasses) and powdered samples mixed with carbon graphite and pressed into cathodes have been studied. Approximately a 3 mm length of each cathode is sputtered, thereby providing a total exposed surface area of about 0.2 cm2. Cathode surface current densities approach 9 mA/cm2at relatively high discharge
DISCHARGE CURRENT
ImA)
Flgure 4. Iron atomic absmbance vs. discharge current at selected pressures of argon: cathode-anode separation. 5 mm. current eonditiom. Argon is used as the discharge gas at pressures of 0.8-1.4 torr. The atomic absorption apparatus and methodology were described previously (8). RESULTS AND DISCUSSION Atomic Absorption Profiles. To gain a better understanding of sputtered atom density and atom diffusion patterns in the glow discharge source, atomic absorption profiles were taken, 88 shown in Figure 3. These profdes (line-of-sight) represent the relative Fe neutral atom population zones which result from sputtering stainless steel 304 pin cathodes at 1 torr Ar and 2 mA constant current. The pin electrodes are positioned at a location within the chamber which provides optimum Fe+ intensity for a given source pressure. Changing the pressure causes the shape of the glow discharge plasma to shift and thus alters the ion extraction pattern. Figure 3 shows that the 2 mA current effectively produces a high atom density in the region between the pin electrodes and the ion exit orifice. A fraction of the a t o m ionized in this region, most likely by electron impact (9) or Penning (10) modes, is extracted as a representative composition of the cathode material. The proximity of the two cathodes to each other causes an overlap or coalescence of their negative glow regions-as in a hollow cathode discharge-which would be expected to enhance the electron ionization step (11). Effects of Discharge Current and Source Pressure. By measuring the Fe atomic absorbance in front of the ion exit orifice simultaneously with the measurement of Fe+ extracted from that same region, it is possible to study the effects of discharge parameters on the atomic and ionic species. Figure 4 shows that the neutral atom population increases rather linearly with discharge current. Decreasing pressure
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DISCHARGE CURRENT
(mA1
Figure 5. "Fe+ ion intensity vs. discharge current at selected pres-
sures of argon.
also causes an increase in atomic absorbance, probably from two effects. At lower pressure, the discharge voltage is higher, thus producing a higher cathode fall and more energetic argon sputter ions. The lower pressure can also reduce the collisional redeposition near the cathode surface. Figure 5 shows that the Fe+ species react differently to changes in current and pressure. The ion intensity increases with discharge current, though at a somewhat more rapid rate than did the neutral atom population. The increase in ion intensity with source pressure may result from the greater pressure differential between the source and the differentially pumped quadrupole chamber which produces a more efficient beam transport. It is also possible that the higher relative pressure is accompanied by an increase in argon metastable density with a corresponding enhancement of the number of sputtered neutrals ionized by the Penning effect. Since only sputtered species may be Penning ionized, Fe+ ion formation is favored over Ar+. Sputter Yield. The pin cathode geometry generates more analyte ions (with respect to background gas ions) than does ow coaxial cathode mode (8)under similar discharge parameters. This may be the result of a higher sputter yield for this geometry. From weight loss measurements after 15 min of operating each cathode geometry under comparable current density, the measured sputter yield is more than twice as high for the pin electrodes. This increase in sputter yield is possibly a result of the different geometry considerations and the coalescence of the two pin cathode glows. Heavy sputter erosion is seen at the tips of the pins, particularly around the front edges. Electrode Separations. As shown in Figure 6, the anode-cathode separation which results in optimum a n a e l ion signals is in the region of 3.0 to 5.0 mm. This distance is essentially half the optimum cathode-anode separation characteristic of the coaxial cathode-anode geometry (8). The decrease in ion intensity at shorter cathode-anode distances is also much less severe in this case, indicating that there is less physical obstruction of the orifice region from the small pin electrodes. The optimum cathode- cathode separation lies in the range up to 2.0 mm, with ion currents decreasing as the distance increases beyond this. Analytical Considerations. A factor which currently inhibits rapid atomic absorption or mass spectrometric analyses by glow discharge sputtering is the relatively long time required to eliminate the effects of residual impurities present in the source chamber. Water molecules adsorbed onto the cathode and discharge chamber surface during the period in which the source is vented for sample exchange represent the primary source of contamination (12). During initial discharge operation, these water molecules are continuously released, and through various ionization and com-
d
2
4
,
CATHOOE-ANOUE SEPARATION
--(mi
Flgure 6. Effect of varying the distance between the anode (ion exit) and the duai-cathodepins on the ion intensity of iron: 2 mA, 10QQmV, 1 torr.
plexation processes within the discharge, a number of contaminant ion species such as H+, H2+> Q+,OH', I-l,Qt, H,O+, and H30+.nH2Q with n = 0 to 5 are formed (13,14). Even though the total partial pressure of water vapor i s commonly
