Anal. Chem. 1991, 63, 2982-2984
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Glow Discharge Mass Spectrometry Using Pulsed Dual Cathodes Sir: Glow discharge mass spectrometry (GDMS) has become an increasingly useful tool for the trace elemental analysis of solid materials (I). Most GDMS systems rely on a steady-state, direct current (dc) voltage to initiate the discharge. Recently, an alternative method was introduced whereby the high voltage was pulsed (2), creating time-dependent anomalies in the observed ion signal. Pulsing the glow discharge improves the ion signal intensity, influences ionization mechanisms, and allows the advantageouscollection of time-resolved mass spectra (3). This communication describes the use of two pulsed discharges in the same discharge housing with the aim of improving quantitative analysis. Supplementing a primary glow discharge with an auxiliary discharge has been reported as a way to boost spectral line intensity from hollow cathode lamps ( 4 , 5 ) and analytical emission sources (6, 7). In these applications, electrons from the positive column of a lowvoltage, high-ment discharge were used to enhance excitation of sputtered atoms from the primary cathode. In mass spectrometry, a dual discharge ionization source using steady-state dc discharges has been reported (8) in which a second discharge was positioned adjacent to the exit orifice; the additional discharge increased the atomic ion signal and altered interfering signals from polyatomic species. Another multiple discharge arrangement for mass spectrometry involved two identical cathodes 80° either side of a line normal to the exit orifice (9), enhancing the observed ion signal. Described here is a dual-discharge configuration quite different in design and purpose from previous studies. In this work, two isolated cathodes are placed parallel to each other and pulsed alternately to achieve complementaryeffects. The thrust of this new configuration is to permit virtually simultaneous comparison of an analytical sample and a reference standard. It is also possible to use the second discharge to study fundamental glow discharge proceases (e.g., employing two different cathode materials and studying atomic interaction of sputtered species). EXPERIMENTAL SECTION The quadrupole mass spectrometer system used in this study is described elsewhere (IO),except for some minor upgrades. Figure 1A illustrates the dual-cathode insertion probe used in this work. Each cathode is electrically isolated and can be positioned independently in the probe sample holders. Separatingthe two cathodes is a grounded stainless steel shield. The metal shield and the surrounding vacuum housing act as the circuit anode, allowing both cathodes to experience the same discharge environment. In order to maintain precise synchronization when alternately pulsing the two discharges, a single power supply (Kepco, Model OPS3500)is used and the high voltage directed to the desired cathode using a pulsesteeringbox (PSB).The duty cycle of each discharge is controlled independently (between 25 and 50% at present) while a constant overall pulse frequency is maintained. The FET switches currently used in the PSB are limited to a maximum applied voltage of -1250 V. Figure 1B illustrates the typical pulse sequence used when signals from two samples are compared. Reagent grade argon was used in all experiments with a typical operating pressure around 1 Torr. Analytical samples were prepared from NIST Standard Reference Materials (SRM) of copper and low-alloy steel. Samples were cut into pins 2 mm in diameter with 5 mm exposed to the discharge. Experiments designed to examine sample interaction were performed with high-purity nickel and copper wire (Alfa, 1 mm in diameter).
RESULTS Solid elemental analysis by mass spectrometry (e.g., SSMS 0003-2700/91/0363-2982$02.50/0
and GDMS) suffers in relation to solution-based techniques (e.g., ICPMS) in the relative ability to make rapid and frequent ion intensity comparisons between sample and standard. Of course, solution-based techniques have their own set of problems, not the least of which may be the dissolution process itself, particularly in the case of refractory materials. There are many good reasons to run solids in their natural state. For the direct mass spectrometric analysis of solids, however, one is normally reduced to the use of a single internal standard element (e.g., the matrix) against which all other elements are then referenced, often by use of relative sensitivity factor (RSF)values to normalize interelement response differences. The dual pulsed discharge (DPD) developed in this study allows the real-time correlation of the ion signals from a standard reference sample with the ion signals observed from an unknown sample. In this way, two major advantages may be gained over conventional GDMS analysis: (1)The single internal standard method, using RSF values, is replaced by the more conventional (and reliable) element-by-element comparison. (2) The time scale for comparison becomes almost simultaneous, reducing significantly those errors arising from changes in the discharge equilibrium or shifts in instrumental parameters. The obvious problem that would be of concern in this dual-discharge approach to quantitation is the potential for the mass spectrum of one sample to be contaminated with components from the second cathode. Cross contamination between samples was evaluated by alternately pulsing two cathodes made from different materials, with the signal intensity of ions from the complementary cathode measured. High-purity samples were used so that one cathode would not have traces of the complementary cathode in its matrix. Hence detection of elements from the complementary cathode is indicative of contamination in the discharge chamber. Cross contamination experiments were conducted with one cathode made from copper wire and the other cathode from nickel wire. The samples were alternately pulsed at 50 Hz, each with a 25% duty cycle, and a maximum applied voltage of -1100 V. Without shielding between the cathodes, the typical carryover of nickel observed in the copper cathode pulse was 7-10% of the nickel signal observed when the nickel cathode was pulsed. This contamination is the result of sputtered nickel atoms diffusing across the discharge space and depositing onto the copper cathode's surface. In order to protect the copper sample from nickel contamination, a stainless steel shield was positioned between the cathodes such that all direct line-of-sight interactions were blocked. When the above experiment was repeated with the shield in place, the amount of contamination of the copper pulse with nickel was reduced to less than 0.3% of the nickel signal intensity during the nickel pulse. Similar results were obtained when observing copper contamination on the nickel discharge. Likewise, the use of iron and copper cathodes produced comparable elemental cross talk. Pressure changes from 0.5 to 1.5 Torr did not appear to alter significantly the average degree of cross contamination. These data indicate, under the present source design, 1000 ppm of an alloying element in one cathode could introduce up to 3 ppm contamination in the signal of the other cathode. A n improved shield design is expected to lower this effect still more. Before application of the DPD to the analysis of unknown samples, its performance was first evaluated using identical cathodes to determine the effect of sample positioning. Ideally, two identical cathodes should yield ion peaks with the same intensities, assuming all other experimental factors 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991 samDle
probe housing
A
\I
d
I
/
i
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Table I. Comparison of Signal Intensities for Two Identical Sample Electrodes Using the Dual Pulsed Discharge“ element
sample 1
% RSD
sample 2
% RSD
iron-56 nickel-58 cobalt-59 copper-63
11340 2686 1230 698
5.2 3.0 3.4 5.3
11312 2669 1231 688
3.3 2.4 2.5 2.2
“Values listed are peak height counts.’ Both samples are SRM 1262.
Table 11. Comparison of SRM 1262 and SRM 1263
data collection gates
B
ratio”
59C0
60Ni
63Cu
wZr
93Nb
IwMo
obsd calcd % error
0.153 0.160 4.5
0.537 0.533 0.75
0.188 0.192 2.1
0.282 0.250 13
0.171 0.163 4.9
0.393 0.429 9.1
“All ratios based on (SRM 1263/SRM 1262). i
.
:
DISCHARGE #I2
Figure 1. Dual discharge instrument schematic: (A) position of dual pin insertion probe relative to shield; (B) discharge pulse sequence and timing of data collection gates. tumbuckles (3) for adjusting probe position
Cajon high-vac fitting
bellows assembly
insertion Drobe
I
discharge housing
Figure 2. Bellows assemble for positioning the insertion probe and dual cathodes inside the discharge housing.
are controlled. Two SRM 1262 low-alloy steel pins were mounted on an insertion probe. The pins were fixed by matching insulatorsto control the area sputtered and resultant current density. Each cathode was placed with its center face 4 mm from the grounded shield, allowing the dark space to form around each sample pin while minimizing the separation between the samples. The insertion probe assembly was inserted through a high-vacuum Cajon fitting into the discharge chamber (see Figure 2). Between the Cajon fitting and the discharge housing is an adjustable bellows assembly that controls the alignment of the probe inside the source housing relative to the exit orifice and shield. The probe’s position was adjusted to place the cathodes symmetrically on either side of the stainless steel shield and an equal distance from the exit aperture. To verify the cathodes’ position, the 56Fe+pulsed ion signal from each pin was displayed on an oscilloscope and the position of the probe adjusted so that the ion intensity of individual discharge pulses were of the same magnitude. The bellow assembly permits “fine-tuning” of the electrode position to balance the signal contribution from each electrode.
Comparative mass spectra were accrued using data acquisition windows placed over the plateau region of each pulse. After a single mass spectral sweep was completed in synchronization with the pulse discharge of one pin, a complementary spectrum was immediately acquired in synchronization with the second pin discharge, thus alternately collecting mass spectra from the two identical samples. Table I is a listing of the observed peak heights for each sample pin using a minor isotope of the base matrix (iron) and isotopes of three alloying elements (nickel, cobalt, and copper). The values shown are mean peak heights from four separate runs for each pin, along with the resulting % RSD. These data indicate that it is possible for the DPD to obtain mass spectra that accurately represent the composition of the cathodes and that direct comparison of the two samples is a viable procedure. The next step in appraising the utility of the DPD for the analysis of unknown samples was to compare the mass spectra of two different low-alloy steels. In this experiment, SRM 1262 and SRM 1263 sample pins were prepared in the same manner as the previous experiment. As before, the position of the probe inside the discharge chamber was adjusted to yield 56Fe+signals of the same intensity for both cathodes. The peak heights in the mass spectrum for elements in the 1263 sample were divided by the peak heights for the same elements from the 1262 sample and then compared to the anticipated 1263/1262 ratio (based on the composition of these materials as reported by NIST). Table I1 shows the results obtained in these initial experiments, with errors calculated for the “unknown” averaging about 6% relative to the “standard”. These errors are comparable to those reported in other GDMS analyses of bulk low-alloy samples using sensitivity factors developed from the analysis of several standards (11,12),yet required less than 15 min of operator time. We believe that these results can be improved as we understand better the factors that will most affect two discharges operating in such close proximity to each other. For example, selecting the optimal pressure may be important in the creation of adjacent glow discharges that are not interactive. The ability to compare sample pins “simultaneously” eliminates the need for relative sensitivity factors by allowing a direct comparison of the unknown sample and standard. While the use of RSF values can be quite satisfactory, there exists always the concern that, over the extended time period, when such factors are sometimes used, instrumental or procedural changes may alter relative ion sensitivities without
Anal. Chem. 1991, 63, 2984-2985
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knowledge of the analyst. Currently, the sensitivity of the glow discharge system on which the DPD is mounted is limited to the low ppm range. It should also be noted that the location of the two electrodes somewhat off the central ion beam axis, along with the presence of the shield, reduces the observed ion signal intensity. We plan to use the DPD to study diffusion of sputtered species into the effective ionization volume of the ion source. Studies are currently underway to improve the sensitivity of the technique and to expand its applicability to samples with other matrices. Registry No. Co, 7440-48-4;Cu, 7440-50-8; Fe, 7439-89-6;Mo, 7439-98-7; Ni, 7440-02-0; Nb, 7440-03-1; Zr, 7440-67-7; steel, 12597-69-2.
LITERATURE CITED (1) King, F. L.; Harrison, W. W. Mass Spectrom. Rev. 1990, 9 , 285. (2) Kllngler, J. A.; Savickas, P. J.; Harrison, W. W. J. Am. SOC. Mass Spectrom. 1900, 7 , 138. (3) Klingler, J. A.; Barshick, C. M.; Hanlson, W. W. Anal. Chem., in press. (4) Sullhran, J. V.; Walsh, A. Spectrochim. Acta 1085, 27, 721. (5) Lowe, R. M. Spectrochim. Acta 6 1971, 26, 201. (6) Lowe, R. M. Spectrochim. Acta 6 1978, 37,257.
(7) Sullhran, J. V.; Gough, D. S. Analyst 1078, 703, 887. (8) Harrison, W. W.; Bentz, B. C. Anal. Chem. 1079, 57, 1853. (9) Loving, T. J.; Harrlson, W. W. Anal. Chem. 1089, 55, 1526. (10) Bruhn, C. G.; Bentz, B. C.; Harrlson, W. W. Anal. Chem. 1978, 5 0 , 373. (11) Jakubowski, N.; Stuewer, D.; Vleth. W. Anal. Chem. 1987, 59, 1825. (12) Sanderson, N. E.; Hall, E.; Clark, J.; Charalambous, P.; Hall, D. Mikrochim. Acta 1987, 7 , 275. Corresponding author. Shell Development Co., Westhollow Research Center, Houston, TX.
' Current address:
J. A. Klingler' W. W. Harrison* University of Florida Department of Chemistry Gainesville. Florida 32611-2046
RECETVED for review June 10,1991. Accepted October 4,1991. This work was made possible by support from the Department of Energy, Basic Energy Sciences, for which we are most grateful.
Anomalous Surface Area Change at an Ultramicroelectrode during the Reduction of Molybdenum Oxide Powder in the Absence of a Solution Phase Sir: We previously reported the reduction of Moo3 to H,Mo03, with x = 0.4, in a biamperometry cell (1). In that study, powdered Moos was simply pressed between two glassy-carbonelectrodes of conventional size (5-mm diameter). Along with other recent reports (2-5), that study demonstrated the feasibility of using common electroanalytical methods such as cyclic voltammetry and chronoamperometry on the study of solids in the absence of a solution phase or of an analyte that originates in a gas phase. The work was limited by three aspects of the cell design. The counter process a t the glassy-carbon electrode required small amounts of water as an anodic depolarizer to initiate the reduction of MOO> The current passed led to a large iR distortion of voltammograms. Perhaps the most severe problem was that with a biamperometry cell cyclic voltammetric peaks symmetrical about 0 V are inherent since a reduction at one electrode is indistinguishable from an oxidation a t the other. The present study employs a cell which consists of a modified glass-carbon electrode as the quasireference/counter and an ultramicroelectrode as the indicator. The sample powder is pressed between them. The above shortcomings of a biamperometric cell with electrodes of conventional size are alleviated; however, with a chemical system where the electrolysis product is much more conductive than the sample matrix and where mass transport is slow, the apparent area of the indicator electrode can greatly exceed that of the ultramicroelectrode surface after generation of some product. EXPERIMENTAL SECTION AU chemicals were reagent grade and were used without further purification. Moo3, PdC12,and NazIrC&were purchased from Aldrich Chemical Co., Morton Thiokol, and Strem Chemicals, respectively. Electrochemicalmeasurementson the solid-state samples were performed with a Bioanalytical Systems, Inc. (BAS) CV 37 voltammograph. Measurements were carried out in the two0003-2700/91/0363-2984$02.50/0
electrode mode. Samples of fiiely-groundMo03were sandwiched between the working electrode (BAS 10-pm-diameter glassycarbon ultramicroelectrode)and the auxiliary/reference electrode (BAS 3 mm diameter ghay carbon) which were vertically aligned in a glass tube. Both electrodes were sealed in Teflon rings to form pistons which fit the inside diameter of the tube. A mass of 100 g of mercury was used to force the two electrodes together, thereby providing reproducibility in the sample size and in the contact between the electrodes and samples. The reaction chamber was purged with an argon stream that was saturated with water. This procedure also removed oxygen from the sample. Unless stated otherwise, the electrode used as the auxiliary (GC-Pd/Ir) was modified with an iridium oxide film (0.75-pm thickness) containing palladium (6) by cycling it 50 times between +1.2 and -0.3 V vs SCE at 0.01 V/s in a solution containing 1.0 mM PdC12, 2.0 mM NazIrC&,0.2 M KzS04,and 0.1 M HC1. Electron spectroscopy experiments showed that the palladium does not occur in the outer surface of the film, so the formation of super stoichiometricmolybdenum bronzes containingpalladium is not likely. The ratio of Ir(II1) to Ir(1V) in the oxide film was established by applying0.6 V vs SCE in 0.1 M HCl, which is about the formal potential for the Ir(II1,IV)couple. The electrode was removed from the solution and equilibrated in air for 24 h in a controlled humidity chamber (relative humidity of 64 k 3% and temperature of 20 k 0.5 "C) prior to use in the solid-state cell. Air oxidation of the Ir(II1) was negligible under these conditions.
RESULTS AND DISCUSSION Figure 1illustrates a cyclic voltammogram of Moos at an ultramicroelectrode. The reduction of Moo3 to hydrogen molybdenum bronze (HMB) is initiated a t -0.5 V vs the quasireference. The anodic process at 0.2 V is the reoxidation of HMB. This is substantiated by the fact that the peak current increases if the negative-going scan is delayed for various times at -1.0 V before reversing the direction. This mechanism is consistent with that in our report on potential scan biamperometry of MooBpowders (1). A cathodic peak is not developed because on the time scale of the experiment Moo3 is not depleted in the vicinity of the working electrode. 0 1991 American Chemical Society
