off scale ( O S ) a t spiking level. The 9.999-mV scale of the data recording unit was exceeded and area calculations were not usable. However, the data did indicate that residue levels were of a magnitude that warranted a sample rerun at higher signal attenuation to bring the response within the 9.999-mV limits. A similar O S . reading was obtained with the S 394 response for captan (20 ppm) a t half tolerance.
CONCLUSIONS A 5-detector GLC system has been described that has potential as the basis of a rapid screening procedure for chemical residues in biological samples. To be fully exploited, various specific detectors with large response ranges are required and their responses should be recorded for computer processing and interpretation. An extensive computer-based reference library of different chemical detector response characteristics is needed. The EC 63Ni detector with its limited dynamic range and non-specificity was satisfactory for milk extracts with minimal cleanup, but not for carrots.
ACKNOWLEDGMENT The authors are grateful for the assistance given by workshop services in modifying equipment and by drafting and photographic services in preparing diagrams. LITERATURE CITED (1) D. M. Daks, H. Hartmann and K. P. Dimick, Anal. Chem., 36, 1560 (1964). (2) J. R. Wessel, J. Ass. Offic. Anal. Chem., 51, 666 (1968). (3) H. Brandenberger. Pharm. Acta Helv., 45, 394 (1970). (4) M. C. Bowman and M.Beroza, Anal. Chem., 40, 1448 (1968). (5) 6.Versino and G. Rossi, Chromatographia,4, 331 (1971). (6) D. M. Coulson, J. Gas Chromatogr.,3 , 134 (1965). (7) W. E. Dale and C. C. Hughes, J. Gas Chromatogr.,6 , 603 (1968). (8) M. C. Bowman and M. Beroza. J. Ass. Offlc. Anal. Chem., 53, 499 (1970). (9) A. Savitzky and M. J. E. Golay, Anal. Chem., 36, 1627 (1964). (10) H. A. McLeod and P. J. Wales, J. Agr. FoodChern., 20, 624 (1972). (11) R. C. Hail, J. Chromafogr. Sci., 12, 152 (1974).
RECEIVEDfor review July 19, 1974. Accepted November 26, 1974.
Mass-Spectrographic Determination of Hydrogen Thermally Evolved from Titanium G. L. Powell, F. W. Postma,' C. Cook, H. Tucker, and A. L. Williamson Union Carbide Corporation, Nuclear Division, Oak Ridge Y- 12 Plant, Oak Ridge, Tenn. 37830
A technique for determining trace amounts of hydrogen in metals using mass-spectrographic detection of hydrogen thermally evolved from a metal sample and calibrated with an expansion of hydrogen gas has been used to determine the quantity of hydrogen evolved at 930 OC from National Bureau of Standards Type 352 hydrogen-in-titanium standards. These standards are rated at 32 pg H/g Ti and were found to deliver 30 f 6 pg H/g Ti in these experiments. At 930 OC, the hydrogen pressure within the vacuum system and the concentration of hydrogen in the sample approach the equilibrium relationships throughout the analysis. These near-equilibrium conditions limited the hydrogen evolution rate from the sample resulting in very long extraction times, difficulty in determining when extraction was complete, and prevented all the hydrogen from being evolved from the sample. These difficulties were sufficiently great that the hydrogen-In-titanium standards co'uld not be used as control samples for thls method of analysis. The proportlonallty constant between the hydrogen concentration In the tltanlurn sample at 930 "C and the square root of the hydrogen pressure at equlllbrlum was directly measured using thls hydrogen analysis technique and yielded good agreement with published values. A method for preparing control samples from uranium or nickel that deliver 1 to 20 pg H/g sample and that do not experience near-equlllbrlum hydrogenmetal lnteractlons during analysls and related dlfflcultles is described.
In the course of developing a hydrogen analysis method based on the thermal evolution of hydrogen from a metal sample in vacuum, and the mass-spectrographic detection of this evolved hydrogen ( I ) , experiments were carried out Present address, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tenn.
to reconcile the ideal gas law calibration of the instrument to hydrogen-in-titanium standards supplied by the National Bureau of Standards (Standard Sample 352) ( 2 ) .These 352 samples are rated to contain 32 bg H/g Ti. The unique metal-hydrogen relationships for a particular metal or alloy determine the hydrogen evolution characteristics a t elevated temperatures in vacuum. This has been demonstrated in the case of solid uranium alloys (3) and solid tungsten-nickel-ion alloys ( 4 ) . The efficiency with which the sample surface converts contaminants to hydrogen during analysis generates a unique lower limit of detection for hydrogen in a particular alloy in a particular condition of surface contamination. At sufficiently high temperatures, evolution from a metal sample is ultimately limited by the diffusion of hydrogen from within the sample. Titanium behaves differently from uranium alloys and tungstennickel-iron alloys, in that the hydrogen pressure over the titanium sample during analysis a t 930 "C approaches the equilibrium hydrogen pressure for microgram per gram concentrations of hydrogen in titanium. This reduces the hydrogen evolution rate from the titanium sample and increases both the time required for an analysis and the uncertainty of the end point of the analysis. The quantity of hydrogen not extracted from the sample, Le., the hydrogen dissolved in the sample in equilibrium with the hydrogen pressure in the extraction chamber, is significant. This report describes the experiments that were necessary to establish a procedure for the routine determination of hydrogen in titanium and the development of working standards of nickel and uranium that are more compatible with this method of analysis.
EXPERIMENTAL Instrumentation The instrument used for these experiments has been described by Condon et al. (I). The instrument is an ultra-high vacuum system consisting of an extraction chamber ANALYTiCAL CHEMISTRY, VOL. 47, NO. 4 , A P R I L 1975
679
E ’
I-l
1
0
50
100
I
‘,io0
Time (seconds)
Flgure 2. Typical hydrogen analysis of a Type 352 hydrogen-in-titaniu m sample Sample weight, 0.0581 gram; total hydrogen evolved, 1.69 pg; hydrogen content, 29 wg H/g TI; mass-spectrometerresponse amplified by 1OX at t = 98 sec Time (seconds)
Figure 1. Mass-spectrometer response to calibration expansions of 2.35 p g H Hydrogen pressure in calibration volume before expansion was 6.4 Torr (850 Pa) (- - -) No samples in the quartz furnace at 930 OC.(-) 0.0618-gram titanium sample in the quartz furnace at 930 OC
containing a quartz furnace maintained a t 930 “C, a sample-feed mechanism to drop individual samples into the furnace, and a calibration volume that can be externally filled with H2 in order to inject microgram quantities of H2 into the extraction chamber. The extraction chamber is evacuated through an orifice connecting the chamber to a mass spectrometer and its associated vacuum system. The H2 evolved from a sample in the quartz furnace generates a mass spectrometer response, i, proportional to the hydrogen pressure (&) over the sample.
PH2= Ai The mass spectrometer response is also proportional to the rate of Hz removal from the extraction chamber which, in turn, is proportional to the hydrogen evolution rate from the sample, provided that this hydrogen evolution rate is slow compared to the pumping rate, k = S/V,, of the extraction chamber. Here S is the pumping speed of the extraction chamber orifice and V , is the extraction chamber volume. An analog computer calculated the number of micrograms of hydrogen, n, that had been evolved by the sample from the equation
where N is a calibration factor. Both i and n are displayed on a two-pen recorder in real time during the analysis. The system is calibrated by rapidly expanding hydrogen from the calibration volume, V,, into the extraction chamber. The calibration volume consists of two Varian all-metal %-inch valves mounted with their seats together. A solid steel cylinder is in the volume between the valve seats to further reduce the free volume of the system. This calibration volume ( V , = 3.40 f 0.01 cm,’) was determined by evacuating the volume and titrating the evacuated volume with ethanol. The extraction chamber volume was calculated from geometric considerations to be V , = 1500 f 100 cm’. The calibration volume is filled with H2 from a reservoir. The H1 pressure in the reservoir is measured with f 0.1 Torr ( f 1 3 Pa) accuracy by a Wallace and Tiernan Type FA-160 pressure gauge. The quantity of H2 trapped within V , is calculated using the ideal gas law. The time required to expand the calibration volume charge into the extraction chamber is less than 0.3 second. The mass-spectrometer response to this expansion (Figure 1, broken line) is a step-function followed by an exponential decay having a rate constant, k = 0.185 sec-I, that is the same as the pumping rate used in Equation 2. The resulting hydrogen pressure, PH?,in the extraction chamber is readily calculated from Boyles law. Using the maximum in the mass-spectrometer response curve, the quantity A = 4 X Torr/volt ( A = 0.5 pascals/volt) in Equation 1 is determined. For such an expansion, the analog computer is adjusted to display n as a step-function with the height of the step related to the quantity of hydrogen injected by a factor of 1.0 pg H/volt. 680
ANALYTICAL C H E M I S T R Y , VOL. 4 7 , NO. 4 , APRIL 1975
One change in the instrument from that described by Condon et al. ( I ) was a Varian all-metal Yd-inch valve (sample isolation valve) inserted in the extraction volume between the orifice and the furnace, calibration system, and sample-feed mechanism. When this valve is closed, the sample being analyzed evolves hydrogen into a static volume, V , = 980 f 5 cm3. An ideal gas law expansion of argon into the static volume while the furnace was at operating temperature was used to determine V,. The sample isolation valve was initially installed to facilitate maintenance of the quartz furnace. When open, this valve does not affect the conductance of gas from the extraction chamber. Materials. Samples used for these measurements were Type 352 titanium samples as received from the h’ational Bureau of Standards weighing approximately 0.24 gram per sample. Some samples were sawed into smaller samples, approximately 0.06 gram per sample, using a water-cooled diamond cut-off wheel followed by acetone degreasing. Nickel (A-nickel) and uranium (Reactor Grade) samples were prepared by annealing 1.2-cm-diameter by 30-cm-long rods of the respective metals in quartz tubes under some fixed hydrogen pressure ranging from 1 X Torr (1 X 10-4Pa) to 700 Torr (9.33 X lo5 Pa). At the end of the two-hour anneal, the quartz tube containing the metal rod was removed from the tube furnace, immersed in water, and the quartz tube broken in a time period of less than 15 seconds. The hydrogen pressure over the sample during the anneal was determined in the low-pressure regime using an ionization gauge. Granville-Phillips capacitance manometers, calibrated against Wallace and Tiernan Bourden-Type pressure gauges, were used to cover the pressure range for 1 Torr (133 Pa) to ’700 Torr (9.33 X lo5Pa). These waterquenched rods were machined into 0.64-cm-diameter by 0.25-cmthick disks such that the specimens were representative of the center portions of the rods. The specimens were acetone degreased before analysis.
RESULTS AND DISCUSSION The initial studies on Type 352 hydrogen-in-titanium standard samples consisted of the analysis of seven 0.25gram samples (1 mm thick) in succession with the spent titanium samples accumulating in the 930 “ C quartz furnace. The hydrogen evolution appeared to stop after approximately 500 seconds a t which time the value of n was taken and the hydrogen content of the samples determined. Those results, in the order that the samples were analyzed, were 27, 21, 22, 19, 18, 18, and 15 fig H/g Ti. Attempts to calibrate the instrument during this experiment indicated an apparent drop in sensitivity that was approximately proportional to the number of specimens accumulated in the quartz furnace. The mass-spectrometer response for calibration expansions with and without a titanium sample in the quartz furnace are compared in Figure 1. This comparison shows that the hot titanium samples enhance the H2 pumping rate for the extraction chamber, i . e . , absorb hydrogen a t high hydrogen pressures and slowly re-evolve hydrogen at very low pressures. Figure 2 shows the results of a typical analysis for a 0.06-gram Type 352 sample, analyzed in the absence of
other titanium samples in the hot zone. After 1300 seconds (21 minutes) the mass-spectrometer response, Le., hydrogen evolution rate, is very small but still measurable. Eighteen samples analyzed individually without allowing the accumulation of spent titanium samples in the 930 O C quartz furnace averaged 30 f 6 pg H/g Ti. The large uncertainty in this value is due to the uncertainty in determining the time at which the mass-spectrometer response returned to zero. The error limits thus obtained for titanium are a t least a factor of two greater than those observed for uranium alloys ( 3 ) or tungsten alloys ( 4 ) analyzed in a similar manner. If the hydrogen pressure over the titanium sample was maintained sufficiently low during the analysis that the activity of hydrogen at the sample surface was at least two orders of magnitude less than the average activity of hydrogen within the bulk sample, the hydrogen evolution rate would be controlled by diffusion of hydrogen through the titanium. This would result in the mass-spectrometer response decaying exponentially, once the sample is heated to the quartz furnace temperature (3, 4 ) . The observed mass-spectrometer decay (Figure 2) is much slower than could be fit to a simple exponential decay and this, along with the observed absorption of hydrogen from the gas Dhase (Figure 1) dictated an interaction between the hv.., drogen pressure, P H ~over , the titanium sample and the average hydrogen concentration, [HI, in the titanium. A fulllogarithmic plot of mass-spectrometer response us. hydrogen concentration in the titanium is shown in Figure 3. Both quantities displayed in Figure 3 were calculated from the analysis described in Figure 2. During most of the analysis, the relationship
(3) was observed. A value of B = 0.029 g atom H/(g atom T i X [B = 2.5 X g atom H/(g atom T i X A)] was calculated from Figure 3. McQuillan ( 5 ) has shown that, under equilibrium conditions a t 930 "C, Equation 3 (Sieverts law) is valid with B = 0.0157 g atom H/(g atom T i X G) [B = 1.35 X g atom H/(g atom T i X During the 930 "C analysis, the hydrogen concentration a t the sample surface decreases because of desorption to approach the equilibrium value but cannot decrease below g atom H/(g the equilibrium value ([HI = 0.0157 atom Ti). The rate that hydrogen is evolved from the sample is controlled by diffusion of hydrogen atoms dissolved in the titanium. This diffusion process is driven by the hydrogen concentration gradient between the sample surface and the center of the sample. Under these experimental conditions, the rate hydrogen is evolved is also the rate that hydrogen is removed from the extraction chamber. The surface concentration and the diffusion gradient both decrease in such a manner that the average hydrogen concentration in the sample and the surface hydrogen concentration decrease a t the same rate resulting in the one-half power hydrogen pressure dependence shown in Figure 3. The average hydrogen content remained a factor of 1.8 greater than the equilibrium value. The factor 1.8 is the ratio of the value of B for Equation 3 calculated from Figure 3 to the equilibrium value reported by McQuillan ( 5 ) . In the later stages of the analysis, the average hydrogen concentration approached the surface (equilibrium) hydrogen concentration resulting in the deviation to higher pressures (higher mass-spectrometer response) than predicted by the one-half power curve shown in Figure 3. The diffusion gradient within the sample and, thus the hydrogen evolution from the sample, is being decreased during the
a)
-
0.3 -
-
1 .o
3.0
10
30
Residual Hydrogen C a n ~ n . (pg H/g
Ti)
Figure 3. Mass-spectrometer response vs. residual hydrogen content in the titanium sample calculated from the analysis described in Figure 2 (-
- -1 [H]proportionallo v ' ~
analysis by the decrease in the hydrogen concentration a t the center of the sample and an increase in the hydrogen concentration at the sample surface relative to that at the center of the sample. In order to further investigate this near equilibrium process during a hydrogen analysis, the equilibrium value of R from Equation 3 was determined directly on the hydrogen analyzer. A Type 352 titanium sample was dropped into the quartz furnace (at 930 f 5 "C) while the sample-isolation valve was closed. A time period of a t least 50 seconds was allowed for equilibrium to be achieved between the hydrogen in the hot titanium sample and the 980 cm isolated volume (VJ. The isolation valve was then opened .in less than 0.3 second, producing a Hz pressure pulse at the mass spectrometer due to gaseous HP released from V,. The pulse was followed by a slowly decaying Hz pressure due to the evolution of hydrogen from the titanium metal. The analog computer response to this pulse was a step-function having a height equivalent to the number of micrograms of hydrogen in the gas phase in V , the instant the isolation valve was opened. This step was followed by a response due to the number of micrograms of hydrogen removed from the titanium sample. After a quantity of hydrogen had been removed from the sample, the isolation valve was closed, the analogue computer rezeroed, equilibrium re-established, and the isolation valve reopened. This procedure was repeated until the hydrogen pressure-pulse height became constant indicating that the static volume base pressure had been reached. Typical data taken in this manner are shown in Figure 4. Starting from the last expansion from V,, the amount of hydrogen remaining in the titanium sample and the hydrogen pressure over the sample prior to each opening of the isolation valve could be calculated. The results of three separate experiments of this type are shown in Figure 5. The first experiment used a 0.0618-gram titanium sample. In the second experiment, a 0.243-gram titanium sample was used. For the third experiment, a 0.251-gram titanium sample was degassed overnight at 930 "C, a liquid nitrogen cold trap was added to V , to suppress the hydrogen base pressure, and V , was redetermined ( V , = 1090 f 10 cm3). Hydrogen (2.40 wg) was then added to the system via the calibration volume. The first two experiments yielded a
a))].
6
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4 , APRIL 1975
681
t
0.000
l
i
i
i!I!l.A 'i iH,/j 800 800
1000 1000
1200 1200 T i T e !seconds)
,o.
l-----l ,; 20.0
1
1400 1400
0.0 0.0
10.0
20.0
30.0
. I
'( Pressure
(G r l
Flgure 6. Hydrogen analysis results for uranium metal waterquenched from a H2 atmosphere at 850 O C . Hydrogen extraction temperdture, 930 "C
Figure 4. The final three of eight hydrogen analyzer responses to a titanium sample (0.0618 g) analyzed by periodically sampling the hydrogen in a static volume Sample dropped into quartz furnace (930 "C) at t = 0 sec. ( A ) Isolation valve opened, ( s ) Isolation valve closed, (0Analog computer erased
E
I V G
Flgure 7. Hydrogen analysis results for nickel metal water-quenched from a Ha atmosphere at 850 O C Hydrogen extraction temperature, 930 "C
(.
?Prerrure (-q%r)
x IO \ \o x 102)
Flgure 5. Equilibrium hydrogen solubility coefficient calculated from Figure 4 and other data ( 0 )A 0.0618-gram Type 352 sample. (a)A 0.243-gram Type 352 sample, (0)A 0.251-gram Type 352 sample degassed overnight at 930 by the addition of 2.4 pg H
O C
followed
value of B (Equation 3) of 0.0163 f 0.005 g atom H/ (g atom Ti X )[1.41 X g atom H/(g atom Ti X and the third experiment yielded 0.0174 f 0.0005 g [1.50 X 10-3 g atom H/(g atom H/(g atom Ti X These values agree reasonably well with atom Ti X B = 0.0157 reported by g atom H/(g atom Ti X 6) McQuillan ( 5 ) . The implications of these near-equilibrium conditions during a hydrogen analysis are significant. In the instrument described here, titanium samples cannot be allowed to accumulate in the quartz furnace because they perturb and delay the hydrogen evolution from samples of other materials (uranium alloys, tungsten alloys, steel, etc.) being analyzed. This contraindicates the use of these standards as in situ control samples. If samples are not allowed to accumulate in the extraction furnace, the furnace must be frequently cycled to air which tends to increase the vacuum system H2 base pressure. If the hydrogen activity in the analyzer due to the vacuum system base pressure (or contamination of an inert gas carrier stream used in some hydrogen analyzer designs) were equivalent to 1 X 10-3 Torr H Z (0.13 Pa), an analysis of titanium samples a t 930 "C would fail to extract the 11 pg H/g Ti in equilibrium with the hydrogen activity of such a system. The resulting hydrogenin-titanium analyses would be in error by an incremental amount. T o reduce this incremental error to 1 pg H/g Ti,
a)] a) a)].
682
ANALYTICAL C H E M I S T R Y , VOL. 47, NO. 4 , APRIL 1975
the effective hydrogen base pressure must be reduced to 1 X Torr (1.3 X Pa). In order to extract (at 930 O C ) 31 pg H/g Ti from a titanium sample containing 32 Kg H/g T i using an argon carrier gas a t 1 atmosphere pressure, the effective H2 contamination level in the argon supply must be no greater than 1 part in lo8 by volume. Another way to circumvent these near-equilibrium conditions is to carry out the analyses a t much higher temperature since the solubility coefficient B (Equation 3) has a temperature coefficient of exp (13.6 kcal/RT) (5). That is, the hydrogen solubility coefficient decreases markedly with increasing temperature. A t the higher temperatures, titanium sublimation may be significant and a small quantity of titanium absorbed on the analyzer walls (where the temperature may be 500-700 "C) can absorb significant quantities of hydrogen. Therefore, in analyzing for hydrogen in titanium metal or in Group IIIB, IVB, and perhaps VB metals, there is good probability that the results will be low because of an incremental error related to the equilibrium between the metal and residual hydrogen in the system. The difficulties associated with routine control samples composed of hydrogen-charged Ti can be circumvented by the preparation of control samples that have hydrogenmetal phase relationships that are similar to the material being analyzed. Figures 6 and 7 show the results of hydrogen analyses (at an extraction temperature of 930 "C) of, respectively, uranium and nickel samples. These samples were machined from rods that were water-quenched from a well controlled hydrogen atmosphere a t 850 O C . The hydrogen analyses for both metals displayed a positive incremental error equivalent to 0.5 f 0.2 pg H/g metal from the composition predicted by Equation 3 which can be directly assigned to the sample surface contamination (3, 4). From
the slope of the curve in Figure 6, a hydrogen solubility g atom H/ (g coefficient a t 850 OC of (1.20 f 0.05) X [1.03 X g atom H/(g atom U X atom U X was calculated compared with 1.3 X g atom H/ (g atom U X 6) [1.14 X g atom H/(g atom U X measured by Mallett and Trzeciak (6). A similar treatment of the data in Figure 7 yielded a hydrogen s o h bility coefficient a t 850 "C of (1.43 f 0.06) X g atom H/(g atom Ni X G) [1.24 X g atom H/ (g atom T i X for nickel compared with 1.60 X g atom H/ (g atom Ni X &) [1.38 X g atom H/ (g atom Ni X measured by Sieverts (7). Note that the hydrogen solubility coeffients for nickel and uranium are three orders of magnitude smaller than that for titanium. The hydrogen analysis chamber H2 base pressure for nickel or uranium analyses can be six orders of magnitude greater than that for hydrogen analysis of titanium before comparable near-equilibrium interactions occur between the evolving hydrogen and the sample. Furthermore, the hydrogen solubility coefficients for uranium and nickel decrease with de-
m)
a)]
m]
a)]
a)]
creasing temperature (6, 7); thus, sublimed metal at temperatures below the furnace temperature has less affinity for hydrogen than that at furnace temperature and, therefore, does not pose a significant hydrogen absorption problem. LITERATURE C I T E D (1)J. 6.Condon, R. A. Strehlow, and G. L. Powell, Anal. Chem., 43, 1448 (1971). (2)J. T. Sterling, F. J. Palumbo, and L. L. Wyman, J. Res. Nat. Bur. Stand., Sect. A, 66, 483 (1962). (3)G. L. Powell and J. B. Condon, Anal. Chem., 45, 2349 (1973). (4) G. L. Powell, Anal. Chem., 44, 2357 (1972). (5)A. D. McQuillan,Proc. RoyalSoc. Ser. A, 204, 309 (1950). (6) M. W. Malled and M. J. Trzeciak, Amer. SOC.Metals, Trans. Quart., 50, 981 (1958). (7)A. Sieverts, 2. Metallk., 21, 37 (1929).
RECEIVED for review August 28,1974. Accepted December 4, 1974. Work performed at the Oak Ridge Y-12 Plant under Contract W-7405-eng-26 with the U.S. Atomic Energy Commission.
Surface Sputter Effects in a Hollow Cathode Discharge E. H. Daughtrey, Jr., D. L. Donohue, P. J. Slevin, and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, Va. 22903
A scanning electron microscope is used to study sputter effects in a hollow cathode discharge. Copper, stainless steel, and graphite were sputtered as cathode materials. The effects of fill gas, net sputter time, and acid cleaning were noted. Both mlcro- and macro-changes were observed. Conical hillocks formed rapidly after sputter initiation. An approximation toward a spherical cavity emerges with long term sputtering. These phenomena appear to be significant to analytical hollow cathode emission studies.
Cathode surface sputtering in the hollow cathode discharge provides the basis for two trace element analysis techniques. Hollow cathode emission (HCE) has been used in analysis for many years ( I , 2), providing excellent elemental sensitivity for solids (3, 4 ) as well as for certain elements deposited from solution as a film in the cathode cavity (5-7). The recently developed (8) hollow cathode ion source (HCIS) has been shown to be a sensitive, stable ion source for analytical use in solids mass spectrometry. It can also provide valuable information about the species produced in the sputtering and ionization processes of the hollow cathode discharge. However, large changes in sensitivity and reproducibility, especially with the emission source, were observed periodically and appeared to correlate with changes in appearance of the cathode surface, some visible to the unaided eye, some requiring microscopic examination. A number of factors affected these surface phenomena. Cathode material and its recent surface history, including preparation, cleaning, and net sputter time, were of particular significance. Basic studies of sputtering in the hollow cathode discharge have received little attention, generally because of the lack of control over sputtering parameters, making in-
terpretation of results difficult. A study of sputtering rates in the planar glow discharge cites the increased role of backsputtering or redeposition as a factor in material transport as the discharge pressure range increases (9).The incident ion energy, and angle of incidence cannot be as carefully controlled in the hollow cathode configuration as in ion bombardment experiments (IO, I l ) , which generally operate a t pressure low enough to allow escape of the sputtered atoms. The energy and angle of escape of the sputtered atoms will also be influenced by the hollow cathode configuration. The cylindrical cup hollow cathode geometry, as opposed to the simple flat disk target used in ion bombardment experiments, further complicates sputter considerations. Because of these factors, rigorous agreement with sputtering theories and ion bombardment experiments cannot be expected. Because of interest in this laboratory in both hollow cathode emission and ionization, a study of hollow cathode sputter effects was conducted, using a scanning elecron microscope as the major examination tool. The main objective was to determine the micro- and macro-effects which the hollow cathode discharge produced on a cathode surface. We hoped to see some relationship of these surface phenomena with observed analytical sensitivities and experimental difficulties. EXPERIMENTAL The cathodes were sputtered using a Glomax demountable hollow cathode tube (Barnes Engineering Company), modified for high current operation, coupled to an existing vacuum system (7). The tube was powered by a Kepco BHK power supply which was capable of supplying 200 mA controlled current at up to 1000 V. The Cambridge Stereoscan scanning electron microscope was used to provide high resolution examination of each cathode surface. The samples were prepared in two different ways. In the first ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4, APRIL 1975
683
