I NOTES Electrode Temperature Considerations in Spark Source Mass Spectrometry W. W. Harrison and D. L. Donohue Department of Chemistry, University of Virginia, Chariottesviile, Va. 22903
Spark source mass spectrometry (SSMS) has been shown to be a valuable trace element survey technique for environmental quality studies ( I ) . Mercury is a n element of particular interest in such applications, b u t its well known volatility characteristics provide special problems involving sample preparation. For SSMS, as will be shown in this study, further difficulties may result from the vacuum spark discharge even after satisfactory sample preparation. S S M S would not normally be a method of choice for mercury determination. Flameless atomic absorption techniques are simpler and more sensitive. However, for elemental survey analysis by SSMS, it would be valuable to obtain reasonably reliable mercury analysis data, free of source discharge volatility problems. For this reason, merkury was included in a group of several elements used to study various factors affecting the analysis of solutions by SSMS. Compared to the other elements, mercury showed unique reproducibility problems which were very dependent upon sparking conditions. This study reports the relative magnitude of the bulk electrode temperature variations and their effect on resultant mercury data. EXPERIMENTAL Apparatus. The temperature probe used for all measurements
was a glass insulated ceramic bead thermistor (Fenwal type GB32P38), 51 mm long by 1.5 mm in diameter, with a nominal resistance of 2 Kohms at 25 "C and powered by a variable dc voltage up to 1.5 V. The inverse relationship of resistance and temperature was monitored by a panel meter or a recorder, the latter providing a means of displaying rapidly changing temperature effects. Two different electrode-thermistor configurations were used to determine relative electrode temperatures, as shown in Figure 1. The arrangement in Figure l a takes advantage of the conductive heating of the tantalum shield by the electrode. The thermistor fits into an enveloping mounting bracket and effectively indicates shield temperature, which is an indirect measure by conduction of electrode temperature. Through the use of appropriate shield masking, heating of the shield and thermistor probe by spark discharge radiation or particle bombardment was shown to be less than 10% of the total temperature rise. In Figure l b is shown a more direct method, involving drilling out a Ys-in. graphite electrode to a Ysz-in. tip thickness. The thermistor is then inserted to a snug fit of the probe element in the electrode tip. The leads from the thermistor are taken out a specially designed source port and protection circuit to the measurement device. The thermistor is mounted on the RF ground electrode and measurements are made with the accelerator voltage off. The SSMS data were taken with an AEI MS-702. The instrumental parameters and modifications have been previously described ( 2 ) . The flameless atomic absorption data were obtained using a Hitachi-Perkin-Elmer Model 139 spectrometer and Sar(1) R. Brown and P. G . T . Vossen, Anal. Chem , 42, 1820 (1970). (2) C. W. Magee. D. L. Donohue, and W . W . Harrison, Anal. Chern., 44 2413 (1972).
gent Model SRG Recorder. A Lucite absorption cell (1-in. diameter, lyd-in. long) with quartz end windows was secured in the optical train in line with a Jarrell-Ash mercury hollow cathode tube. The mercury doped matrices were placed in a heating chamber made from a small, two-piece vacuum trap (similar to Kontes K-926300) and brought to the desired temperature by immersing the lower half of the trap in a heated silicone oil bath. Compressed air was metered into the heating chamber, sweeping the evolved mercury through connecting Tygon tubing to the flowthrough absorption cell. Reagents. Spec-pure or reagent grade chemicals were used to prepare all standards. Ultra Superior Purity graphite (Ultra Carbon Corporation) and powdered silver (Gallard-Schlesinger Corporation) were used as matrix materials. Procedure. The mercury was added to the matrix using freeze drying techniques (significant mercury losses were experienced by infrared drying). Electrodes were formed using an AEI die, loaded into the source, and shot without pre-sparking. Photoplate detection was used for most determinations because of the capability of simultaneous detection of all elements beginning at spark initiation. Relatively high concentrations of mercury (loo-600 ppmw) were used to allow short exposures. RESULTS AND DISCUSSION Relative temperature changes were considered sufficient to correlate with observed irregularities of mercury retention in the sparking electrodes. With the thermistor mounted in the shield bracket, the measured temperature was much lower than that of the sparking electrode. In the alternate internal mode, the thermistor was much closer to the sparking surface, with considerably higher temperatures recorded, but still effectively measuring only bulk electrode temperatures. However, the relative magnitudes of the induced temperature changes could be monitored and were of interest as described. Choice of M a t r i x . Initial studies which led to the temperature measurements involved a comparison of graphite us. several metal powder matrices, particularly silver. We had expected the amalgamation capabilities of the metals to retain mercury better t h a n would the graphite. As expected, graphite electrodes were rapidly depleted of mercury under sparking conditions while compacted silver retained the mercury to a much greater degree. However, graphite has many desirable features as a sample matrix, notably its availability in a very pure state and a fine particle size which allows the preparation of homogeneous electrodes. Therefore, further graphite studies were of interest. Workable conditions were eventually realized by using milder spark parameters, 100 pulses per second-50 psec pulse length us. the previous 300-100. It seemed clear that the difference in behavior resulted from a temperature effect in the electrodes. Effect of Spark P a r a m e t e r s . T h e spark pulse length and repetition rate are often varied to control ion flux rate in SSMS. Electrode temperature would be expected to in-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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crease with the net real on-time of the spark ( 3 ) . This may be extended to extremely rigorous conditions wherein graphite electrodes glow red all along their length. Figure 2 shows relative temperature changes for a “family” of response curves for different spark conditions. The nominal instrument values for pulse length a n d repetition rate are only approximate guides to net spark on-time. The pulse repetition rate on our instrument has a greater effect on electrode temperature (and ion flux) than does pulse length so t h a t the calculated on-time and temperature do no always correlate. Clearly, however, the lower conditions would have much less effect on volatile constituents. This substantiates our SSMS d a t a wherein reproducible mercury results could be obtained only a t low spark conditions. The only analytical problem with using the lower conditions is the low ion flux which results in longer exposure times. A series of satisfactory test runs can be made on a single set of electrodes a t low conditions, but a n increase in (31 R. E. Honig. Twelfth Annual Conference on Mass Spectrometry and Allied Topics. Montreal. 1964. Paper No. 38.
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Figure 4. Change in temperature produced by changing only the spark gap width Spark conditions 300; 100, internal thermistor mount
spark conditions during the run causes a sudden surge of mercury expulsion, followed by rapid loss of the remaining mercury (Figure 3). This sudden peaking of the mercury is explained in Figure 2 by the large temperature shift produced a t different spark conditions. We have also observed a loss of cadmium from graphite electrodes a t high spark conditions. In particular, a n increase in spark voltage has a large effect on elemental volatilization. This is normally not subject to as much run-to-run variation, however, as are the other parameters discussed. Effect of Gap Width. At this point, a n automatic spark gap control ( 4 ) became available to set and control any pre-selected gap width. Up to this time, the gap had been monitored on an oscilloscope and maintained manually. Significant effects were seen for mercury. At a narrow gap (50 p m ) , little if any loss of mercury was observed even u p to 300 pps/100 psec, conditions which had created serious loss previously (Figure 3 ) . At a medium gap (100 pm), which was approximately that which was maintained for the earlier part of the study, the effects seen in Figure 3 were reproduced. For a wide gap (150 pm), sample loss occurred even more rapidly than for t h e medium gap. The temperature changes accompanying these gap variations are shown in Figure 4. These significant tempera(4) C. W. Mageeand W. W. Harrison, Ana/. Chem.. 45.220 (1973).
ANALYTICAL CHEMISTRY. VOL. 45, NO. 9, AUGUST 1973
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Figure 5. Atomic absorption profiles of mercury released upon heating from doped graphite samples used for SSMS electrodes. See text for details
Figure 6. Atomic absorption profiles of mercury released upon heating from doped silver powder used for SSMS electrodes. See text for details
ture changes would explain the mercury response just described and also suggest t h a t gap control could be a critical parameter in mercury analysis. Effect of Temperature on Mercury Loss. There is no simple way to very gradually and reproducibly increase electrode temperature and monitor exactly when mercury loss occurs. Therefore, a routine flameless atomic absorption ( 5 ) apparatus was constructed for mercury determination to obtain a relative measure of mercury volatilization as a function of temperature. Graphite may act as a reducing agent in the spark discharge. To determine the response of several mercuric and mercurous salts with a more neutral matrix, A1203 was first used. The mercuric salt showed only a single absorption peak centered about 230-240 "C. However, the mercurous salt appears to break up to produce two different mercury-yielding species, possibly involving a disproportionation reaction to mercury and mercuric chloride. One peak a t 235 "C coincides with the single response from the mercuric salt. A second absorption peak a t about 100 "C may represent mercury released from mercurous chloride. Other mercuric salts were also studied and each produced a single peak response curve, similar to the chloride salt. A mercury doped graphite sample, prepared from a mercuric chloride solution, was next used to prepare three sets of electrodes for a joint AA-SSMS analysis. One pair was not sparked and was set aside as the control. The other two pair were sparked for 20 minutes a t 100/100 conditions for one pair and 300/100 for the other. All three electrode pairs were then crushed in a Wig-L-Bug and evaluated by flameless AA, with the results shown in Figure 5 . The control set exhibited a dual peak response very
similar to mercurous salts, even though the mercury was applied as the mercuric form, indicating a n interaction with the graphite which was not observed with the A1203. The 100/100 pair was reduced in mercury concentration, b u t two peaks were still seen. The 300/100 pair showed no significant response until a temperature was reached beyond the point where the second mercury peak is ordinarily seen. This experiment also points out that the sample loss is throughout the electrode and not merely a t the sparking tip. The entire electrodes were powdered and used in the absorption studies. If only a localized volatilization occurred, little or no difference would be seen in the net absorption d a t a for the entire electrodes. In Figure 6, results are shown for a n experiment similar to t h a t of Figure 5, except a silver matrix was substituted for the previous graphite. The control set and the 100/100 set match very closely in response and no significant sample loss is observed. For the 300/100 set, significant mercury loss is apparent, although not to the extent shown with the graphite matrix. In addition, only a single absorption peak is obtained. The silver matrix evidently does not cause (or allow) reduction of the mercuric chloride in a manner similar to t h a t shown by graphite. The net result is t h a t while silver may act to hold mercury in a prepared electrode somewhat better than graphite, loss can still easily occur. The sparking conditions necessary to cause volatilization problems may vary between instruments and electrode preparation and should be determined for each laboratory.
(5) W.R. Hatch and W . L. Ott, Ana/. Chern., 40, 2085 (1968)
Received for review December 8, 1972. Accepted March 2, 1973. This study was supported by research grants from LEAA and EPA.
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