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Anal. Chem. 1909, 6 1 , 1048-1052
Ultramicroelectrode Array Behavior of One-Dimensional Organic Conductor Electrodes Michael S. Freund and Anna Brajter-Toth* Department of Chemistry, University of Florida, Gainesville, Florida 32611
Tetrathlafulvalene-tetracyanoqulnodlmethane (TTF-TCNQ) electrodes were prepared by two methods that produced electrodes wlth dlfferenl structures. This was conflrmed by scanning electron mlcroscopy, whlch also revealed the ultramlcroelectrode arraylike structure of TTF-TCNQ electrodes. Electrochemlcal behavior at both types of electrodes followed the predictions of the theory for ultramkroelectrode arrays. At the two types of surfaces that were prepared, apparent electrochemical rate constants of ferricyanide and ascorbate were different. The resultlng changes In the apparent rate constants of redox couples such as ferricyanide and ascorbate as a function of surface structure suggest that at TTF-TCNa control over reactivlly can be achleved through structural manlpulatlon.
One-dimensional donor-acceptor organic conductors, TTF-TCNQ and NMP-TCNQ, have been attracting considerable attention as electrode materials (1,2). The interest follows reports of catalytic oxidation of flavoenzymes at electrodes constructed with these conductors (3, 4). The mechanism of catalysis remains unknown (1, 5 ) . Interest in metallike organic conductors is a part of the developing importance of new electrode materials in electroanalysis (5-10). Focus on new surfaces is triggered by the desire to control selectivity, sensitivity, and the long range stability of electrodes. With new electrode materials, the range of applications of electroanalysis can also be extended. Development of biosensors is an example of effective effort in this area ( 1 , 2). We have recently shown that, in addition to flavoenzymes, oxidation reactions of small biological molecules are catalyzed a t TTF-TCNQ electrodes ( 5 ) . At electrodes made of this material, improved stability toward passivation was also observed. Both results are of considerable interest in the analysis of biological compounds. We report here measurements that were undertaken to clarify the nature of electrochemical reactions at TTF-TCNQ electrodes. Rational design of new analytically useful surfaces depends on clear understanding of the parameters that control electrode activity. The conclusions that are presented address the issues of reactivity at TTF-TCNQ electrodes and identify a set of parameters that control activity and lead to the apparent catalytic response of redox couples ferricyanide and ascorbate. EXPERIMENTAL SECTION Methods and Materials. All solutions were prepared in 0.5 M phosphate buffer, pH 7 unless otherwise specified, and were deoxygenated by bubbling with water-saturated nitrogen. All measurements were made at 25 f 1 "C. Potassium ferricyanide was obtained from Mallinckrodt; ascorbic acid was obtained from Chem Service. Tetrathiafulvalene (TTF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and poly(viny1 chloride) (PVC) were obtained from Sigma. TTF-TCNQ salt was prepared by refluxing TTF and TCNQ separately in equimolar quantities in acetonitrile (11). After a
* Author to whom correspondence should be addressed. 0003-2700/89/0361-1048801 S O / O
half hour of refluxing, the hot solutions were mixed, resulting in the immediate formation of a black precipitate. The combined solutions were protected from light and allowed to cool. The precipitate was filtered and washed 5 times with cold acetonitrile and 5 times with cold diethyl ether to remove any traces of neutral materials. The precipitate was then dried for 24 h under vacuum, while care was taken to protect it from light. The resulting salt was stored in a freezer until it was used. All electrochemical measurements were carried out with a Bioanalytical Systems electrochemical analyzer, BAS-100. Solution resistance was compensated with BAS-100. Electrodes. The TTF-TCNQ polymer paste electrodes were constructed by using the procedure described by McKenna et al. (5). First, the polymer, poly(viny1 chloride), was thoroughly dissolved in tetrahydrofuran after which TTF-TCNQ complex was added to make a PVC/TTF-TCNQ mixture of 1:13 (w/w). Care was taken to ensure that all PVC was dissolved before the salt was added, and care was taken to maintain a uniform suspension by stirring constantly until the paste had thickened. The polymer paste electrodes were allowed to dry for 12-24 h before use. The resistance of the electrode and solution was measured with the Bioanalytical Systems electrochemical analyzer, BAS-100 and never exceeded 100 Q. The areas of several polymer paste TTF-TCNQ electrodes were determined by chronocoulometry using 0.264mM K,Fe(CN), in 1M KC1 and a diffusion coefficient of Do= 7.63 X lo4 cm2/s. The potential was stepped from 0.400 to 0.100 V vs SCE at varying pulse widths. An area of 0.259 tt 0.001 cm2was determined at pulse widths between 400 and 1000 ms by extrapolating to time zero. Pretreatment of TTF-TCNQ electrodes consisted of lightly polishing with a Kimwipe and poising the electrode at 0.225 V vs SCE in a blank solution for 5 min. After this treatment typical background currents of ca. 0.3 pA were obtained at 0.200 V vs SCE. Although extensive polishing led to negative shifts in the anodic peak potential of ascorbic acid, the most reproducible peak potentials were achieved by the pretreatment described above. High-pressure electrodes were prepared by using a pressure of ca. 3000 psi (11). The TTF-TCNQ powder was pressed into disks of ca. 2 mm thickness. The electrode area determined by chronocoulometry (pulse width of 250 ms) was 0.195 tt 0.001 cm2. The electrical contact was made by attaching a piece of copper wire to the pellet with nickel paint (GC Electronics, Rockford, IL).Nickel paint wm also used to secure the back of the electrode in the Teflon holder. The resistance of the pellet electrodes was ca. 50 Q . Other electrodes used in this study included a saturated calomel (SCE) reference electrode and a 1 cm2 platinum foil auxiliary electrode. Scanning electron microscopy was carried out with a Joel JSM 35CF microscope. Simulations. Reversible cyclic voltammograms of ferricyanide were simulated using the current function values for a reversible charge transfer (12),the geometric electrode areas determined by chronocoulometry as described above, and Eo' values determined experimentally. Quasi-reversible cyclic voltammograms were simulated by digitizing current function values at A values closest to those determined from the experimental results. A was determined from experimental results by first calculating $ from peak separations (aEJ using the method of Nicholson (13). The 4 values were in turn used to calculate A (12) as 0.7 and 0.8 at pressed pellet and polymer electrodes, respectively. The voltammograms were simulated by using current function values for h = 1.0 and a = 0.5. The temperature used in the simulation was 25 "C. 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 61, NO. 9. MAY 1. 1989
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Fbure 1. Scanning electron micrograph of (A. E. C) 1:13 ( w i w ) PVC:TTF-TCNQ polymer paste electrcde and (D) high-pressure pellet TTF-TCNQ electrode. Key: (A) untreated; (E) polished: (C) polished and electrochemically pretreated polymer paste electrcde.
RESULTS AND DISCUSSION Electrode Characterization. Analysis of reactivity at ITF-TCNQ electrodes described in this paper required assessment of the structure of the electrode surface. Electro-
Table 1. Electrochemical Response of Fe(CN))/'Ascorbate at TTF/TCNQ Electrodes
chemical characterization of the electrodes revealed that the ratio of PVC to TTF-TCNQ in the polymer paste and the method of electrode treatment had an effect on the oxidation peak potentials of small biological molecules (5). The least positive oxidation potentials were observed a t electrodes with a PVC to 'ITF-TCNQ ratio of 1:13 (w/w) (5). In this study it was also found that by polishing k13 electrodes, oxidation peak potentials of ascorbic acid were further reduced. Comparison of the results of McKenna e t al. (5)at polymer paste electrodes, with those of Jaeger e t al. ( 1 2 ) a t high-pressure pellets, confirmed that the method of electrode preparation played an important role in the electrochemical kinetics of probe molecules. Figure 1A-C shows the scanning electron micrograph (SEM) pictures of electrode surfaces that were studied electrochemically. The micrographs show that the method of preparation determines the structures of the electrode. The micrograph of the 1:13 (w/w) PVCsalt polymer paste elect r d e (Figure 1A) also clearly shows that in the polymer paste TTF-TCNQ forms an arraylike structure. In the array 'ITF-TCNQ crystals (ca. 40-80 r m ) are separated by an average distance of ca. 40 rm. The polymer (PVC) presumably allows the crystals to adhere. Mechanical polishing causes a more even distribution of the salt (Figure lB), effectively reducing the average distance between 'ITF-TCNQ crystals. At this polished electrode, the oxidation peak of ascorbate shifts to less positive potentials
unpolished 86 0.350 polished 69 0.100 pellet 69 O.Oo0 L1 1 m M Fe(CN),*I'-. 5 mV 8-l. pH 7.0, 0.5 M phosphate buffer. b 1 mM asmrbate, other mnditians the game a8 a.
electrode
AEE,PmV
and
E-? V
(Table I) and the background current increases (Figure 2), when compared to the behavior at unpolished electrode. The effect of electrochemical treatment on the structure of electrodes that were first polished is shown in Figure 1C. Poising the electrode at 0.225 V causes roughening of the surface. Background current recorded on these electrodes decreases compared to the background current at electrodes which were pretreated only by polishing (Figure 2). Some slowing down of ascorbate kinetics is observed which is apparent in the positive shift of the oxidation peak. As shown in Figure lD, high-pressure pellet electrodes have a more uniform surface and in the micrographs the crystal structure of TTF-TCNQ is not easily discernible. Ascorbate oxidation occurs a t the least positive potentials (Table I), i.e., the kinetics become fastest on these electrodes (see the following discussion) and the background current increases substantially (Figure 2). Microelectrode A r r a y Properties of TTF-TCNQ Electrodes. The SEM data clearly show the arraylike structure of the polymer paste TTF-TCNQ electrodes. These electrodes consist of very small ?TF-TCNQ crystals, which
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
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-=f
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text). 1 I
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Flgure 2. Background cyclic voltammograms at lTF-TCNQ electrodes recorded in 0.5 M, pH 7.0 phosphate buffer at a scan rate of 5 mV s-I, electrode area 0.259 cm2 (A, E. C) and 0.195 cm2 (D). Key: (A) untreated; (E) polished; (C) polished and electrochemically pretreated 1:13 polymer paste electrode; (D) high-pressure pellet electrode.
represent the electroactive surface, separated by an average distance of ca. 40 pm. The dimensions are typical of ultramicroelectrode arrays (14). At electrode arrays the background current is proportional to the active electrode area. In the array, this is the conducting surface a t which the potential is controlled. As shown by the SEM, a t TTF-TCNQ electrodes the active area will be determined by the method of electrode preparation. At high-pressure pellet electrodes the active area is highest and consequently, as expected, so is the background current (Figure 2). At polymer paste electrodes the background current is much lower. At electrodes with 1:13 (w/w) polymer to salt ratio, a current of ca. 25 nA (at 0.225 V for 0.10 cm2 geometric area) has been reported ( 5 ) . Untreated polymer paste electrodes have the lowest active area and consequently the lowest background (Figure 2A). Polishing causes a more even distribution of the salt, increasing the active area and the background current. SEM results indicate that the response a t TTF-TCNQ electrodes should follow the predictions of ultramicroelectrode array theory (15). By use of this theory, the shape of cyclic voltammograms and the relationship between peak current and scan rate can be predicted. Both depend on the distance between individual electrodes in the array that form the active fraction of the total geometric area. They also depend on this distance relative to the thickness of the diffusion layer from which electroactive material is supplied. The thickness of the diffusion layer is determined by the time scale of the experiment and depends, in cyclic voltammetry, on scan rate (15). The theory also predicts that the shape of cyclic voltammograms and the measured current depend on the ratio of the active area (at which potential is applied) to the total geometric area of the array (15). This determines an average diameter of each electrode in the array when the distance between individual electrodes is fixed. In case of TTF-TCNQ electrodes the distance between individual electrodes in the array is an important parameter because of the method of
preparation of the electrode surface and because the TTFTCNQ salt that forms the ultramicroelectrode array is a one-dimensional organic conductor (16).On the basis of the SEM data it is clear that a t polymer paste electrodes the distance between individual electrodes will be larger than a t high-pressure pellets. The fraction of active area (i.e. the ratio of active to inactive area) will be larger at high-pressure pellet electrodes. The shape of cyclic voltammograms and the potential at which the reaction of probe molecules will occur at arrays (as a t other electrodes) are also determined by the apparent standard heterogeneous rate constant of electron transfer of the probe molecule (15). As a result, at the same array electrode, the shape of cyclic voltammograms may be different for different probe molecules under otherwise identical conditions. Quantitative verification of the ultramicroelectrode array structure can be obtained by varying the time scale (e.g. pulse width in chronocoulometry) of the electrochemical experiment. In a slow experiment faradaic currents which will be measured will be proportional to the totalgeometric area (17,18) without evidence for the microelectrode structure. At short time scales actual electroactive area (area of individual microelectrodes) will determine the faradaic current. For a microelectrode array, electroactive area is smaller than the total geometric area. To determine the electrode area by chronocoulometry, the pulse widths were varied from 200 to lo00 ms. As shown in Figure 3 at 1:13 polymer paste electrodes, the measured area starts to decrease at pulse widths lower than 400 ms. This result supports the microelectrode structure of these surfaces. The areas measured at times greater than 400 ms reflect the geometric area. The small increase in electrode area at times greater than 400 ms may be a result of the porosity of polymer paste electrodes. Electrochemical Behavior of Fe(CN),3-/4- and Ascorbate at TTF-TCNQ Array Electrodes. As described above, the values of parameters, which include distance between electrodes in the array, fraction of active area, and the apparent standard heterogeneous rate constant will determine the shape and potential of cyclic voltammograms. The shape and potential are also functions of scan rate. For example, as the distance between individual electrodes in the array increases, it can be predicted (15) that the shape of cyclic voltammograms will approach that of a steady-state voltammogram. This will be observed for the array where the fraction of active area is fairly low, as the scan rate increases. At
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
3
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Figure 4. Experimental (solid lines) and calculated (points) cyclic in 1.O M KCI at high-pressure voltammograms of 5 mM Fe(CN):-'& pellet (A, C) and 1:13 polymer paste (6, 0)TTF-TCNQ electrodes. Scan rates were 100 mV s-I (C, D) and 5000 mV s-I (A, B). The parameters used In the calculations are described in the Experimental Section.
TTF-TCNQ electrodes this behavior can be expected at polymer paste electrodes (where the distance between electrodes in the array is highest and the fraction of the active area is fairly low) as the scan rate increases. As shown in Figure 4 (parts B and D), behavior of Fe(CN)63-/4-at TTF-TCNQ polymer paste electrode follows these predictions. With the increase in scan rate the shape of cyclic voltammograms approaches that of a steady-state voltammogram. The apparent heterogeneous rate constant of electron transfer of ferricyanide is not high under the solution conditions of this experiment (14),which as expected results in an increase in peak-to-peak separation with the increase in scan rate. At high-pressure pellet electrodes the distance between the ultramicroelectrodes in the array decreases and so does, as a result, the fraction of inactive area. At these electrodes the predicted (15)and observed (Figure 4A and C) shapes of cyclic voltammograms will be the same over a wider range of scan rates as those at electrodes of standard size where the active and geometric areas are the same. The theory predicts that when the shape of cyclic voltammograms is the same as that at electrodes of equivalent total geometric area, the apparent standard heterogeneous rate constant will provide the only indication that the array structure is indeed present. The apparent standard heterogeneous rate constant will be a function of the fraction of active area and will decrease as the active area decreases. In practice this means that peak-to-peak separation and the oxidation peak potentials will be larger and more positive, respectively, at array electrodes than at normal electrodes where the total area is active. A t TTF-TCNQ pellet electrodes, peak-to-peak separation for Fe(CN)63-/4-is ca. 69 mV at slow scan rates (Table I). Under the same conditions, at active graphite electrodes this value is ca. 57 mV (19). A t untreated polymer paste electrodes under conditions where normal shape of cyclic voltammograms is observed (at the same low scan rates), peak-to-peak separation is ca. 86 mV (Table I) which is consistent with the increase in the fraction of inactive area. Additional verification of the microelectrode array structure is illustrated in Figure 4 where the cyclic
E(vOLT)
Figure 5. Cyclic voltammograms of ascorbic acid at TTF-TCNQ polymer paste (1:13) electrode: scan rate, 5 mV s-' (A) and 500 mV s-' (B); 5 mM solution in pH 7.0, 0.5 M phosphate buffer.
voltammetric response at polymer paste and pellet electrodes is shown at fast and slow scan rates. As shown in Figure 4, the polymer paste electrode yields significantly smaller currents at fast scan rates in comparison to the pellet electrode. This is consistent with the chronocoulometricresults at short pulse widths which reflected smaller effective area due to the microelectrode structure. Simulations, represented by points in Figure 4A,C,D, show that the effective area is equal to the geometric area at slow scan rates at the untreated polymer paste electrode and over the full range of scan rates at the pellet electrode. The response of ascorbate, a small biological molecule, was also investigated at TTF-TCNQ electrodes to determine if the behavior followed that predicted for array electrodes. Ascorbate was studied at pH 7.0. The shape of cyclic voltammograms of ascorbate at polymer paste electrodes as a function of scan rate followed that of ferricyanide. With the increase in scan rate the shapes approached those of steadystate voltammograms. This is shown in Figure 5. At pellet electrodes, the reduced distance between active electrodes in the array led predictably to a negative shift in anodic peak potential (Table I) which corresponds to an increase in the apparent heterogeneous rate constant of ascorbate (20). At these electrodes the oxidation peak of ascorbate shifted to ca. 0.0 V vs SCE compared to that of 0.350 V which was obtained at untreated polymer paste electrodes (Table I). The shift corresponds to a ca. 900-fold increase in the apparent heterogeneous rate constant (5). As was described above, the apparent heterogeneous rate constant of Fe(CNIs3-/*-also increases at pellet electrodes (Table I). CONCLUSIONS The electrochemical response at TTF-TCNQ electrodes is consistent with their arraylike structure. The method of electrode preparation allows control of background currents. The decrease in the fraction of active area at polymer paste electrodes favors lower backgrounds. The fraction of the active area also controls the apparent rate constant which increases
1052
Anal. Chem. 1989, 6 1 ,
with the increase in active area. The improved response of Some at mF-TCNQ to that at nonactivated graphite has been previously (5) reported. The results reported here for molecules such as ferricyanide and ascorbate support the conclusion that the apparent catalysis may have its source in structural features of the electrodes.
LITERATURE CITED Albery, W. J.; Bartlett, P. N.; Craston. D. H. J. Nectroanal. Chem. 1985, 194, 223. McKenna. K.; Brajter-Toth, A. Anal. Chem. 1987, 5 9 , 954. Kulys, J. J.; Samalius, A. S.;Sirmickas. G. J. S. FEBS Len. 1980, 114, 7 . Cenas, N.;Kulys, J. 8ioelectrochem. Bioenerg. 1980, 8 , 103. McKenna. K.: Bovette. S.E.; Braiter-Toth, A. Anal. Chim. Acta 1988, 206,75. Shaw, B. R.; Creasy, K. Anal. Chem. 1988, 6 0 , 1241. Kulesza, P. J.; Brajter, K.; Dabek-Zlotorzynska, E. Anal. Chem. 1987, 5 9 , 2776. Saraceno, R. A.; Pack, J. G.; Ewing, A. G. J. Nectroanal. Chem. 1986, 197, 265
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(9) Coury, L. A., Jr.; Birch, M. E.; Heineman, W. R. Anal. Chem. 1988, 6 0 , 553. (10) Gehron, M. J.; Brajter-Toth, A. Anal. Chem. 1986, 58, 1488. (11) Jaeger, C. D.; Bard, A. J. J. Am. Chem. Soc. 1979, 701, 1690. (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; pp 218, 225, and 230. (13) Nicholson, R. Chem. 1965, 3 7 , 1351, (14) Sleszynskl, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56, 130. (151 . . Amatore. C.: Saveant.. J. M.:. Tessier. D. J . Electroanal. Chem. 1983. 147, 39. (16) Bryce, M. R.; Murphy, L. C. Nature 1984, 309, 119. (17) Reller, H.; Kirova-Eisner, E. J. Electroanal. Chem. 1984, 161, 247. (18) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2825. (19) Poon, M.; McCreery, R. L. Anal. Chem. 1988, 58, 2745. (20) Deakin, M. R.; Kovach. P. M.; Stutts. K. J.; Wlghtman, R. M. Anal. Chem. 1086, 58, 1474.
s.
RECEIVED for review May 3, 1988. Resubmitted November 15, 1988. Accepted February 3, 1989. This work was supported, in part, by grants from the National Institutes of Health (GM 35341-01A2) and the Interdisciplinary Center for Biotechnology Research at the University of Florida.
CORRESPONDENCE High-Resolution Fourier Transform Spectrometer To Identify the Rotational Structure of the B2&+-X2Zg+ Transition of N,+(O,O) in a Helium Inductively Coupled Plasma Sir: The use of Fourier transform spectrometers in the near-, middle-, and far-infrared spectral region is common in many analytical labs. Since a wealth of atomic information lies in the visible and ultraviolet regions, there has been a great deal of effort by various groups worldwide to build Fourier transform spectrometers that operate over this region of the spectrum. In 1975 Horlick and Yuen (1) reviewed spectrochemical analysis with a Fourier transform spectrometer (FTS). More recent reviews by Faires (2) and Thorne (3) discuss theory, instrumentation, and applications of a FTS in atomic emission spectroscopy (AES). Stubley and Horlick (4-6), Marra and Horlick (7), Faires (8, 9), Faires, Palmer, Engleman, and Niemczyk (IO),and Faires, Palmer, and Brault (11) carried out pioneering work in Fourier transform inductively coupled plasma atomic emission spectroscopy. Various aspects of the LAFTS have been previously reported (12, 13).
Los Alamos Fourier Transform Spectrometer (LAFTS). The LAFTS, housed in a 3360-ft2building, is the highest resolution visible and ultraviolet FTS in the world. Table I lists some of its characteristics. The instrument is currently operating with single pass optics allowing for a maximum resolution of 0.0026 cm-l (0.023 pm at 300 nm). The interferometer is housed within a 14 f t by 7 f t vacuum tank (30 mTorr achieved) and utilizes two entrance ports for experiments. The FTS sits on a 4 f t by 10 f t NRC vacuum compatible optical table isolating the instrument from vibration. Figure 1 shows the folded design of the instrument. The LAFTS utilizes a double-sided interferogram that improves the signal-to-noise ratio and helps eliminate phase problems. The analog-to-digital (A/D) converter is composed of seven 16-bit A/D converters, scale amplifiers, and an 18-bit digital-to-analog (D/A) converter for calibration. Parsons and
Table I. Los Alamos Fourier Transform Spectrometer Specifications Spectral Ranges region
UV-vis
range
beam splitters
detectors
200 nm-1.1 pm q u a r t z / a l u m i n u m P M T : s i PIN diode
N e a r - I R 0.8-5.5 G r n Far-IR 4-20 pm
CaF-GaP KC1-Ge
InSb HgCdTe
Resolution and Operating Parameters optical p a t h 2.5 m (single pass) 5.0 m (double pass) difference 0.026 cm-' (single pass) resolution 0.0013 cm-' (double pass) sample rate 5000 Hz n o t digitally filtered 4oooO Hz w i t h digital filtering reduced t o 5000 Hz throughput >4 000000 points number of points m a x i m u m transform size 2** (4 194304)
Palmer (13) give detailed characteristics and research plans for the LAFTS. For inductively coupled plasma atomic emission spectroscopy (ICP-AES) the advantages of the FTS are intensity precision, wavenumber accuracy and precision, and high resolution. Provided electronic noise is minimized, the limiting noise in the E"I'-ICP system will be the plasma. Several papers (14-18) have discussed sources of noise in argon ICP's. Helium Inductively Coupled Plasma (He-ICP). Argon is the most common plasma gas used in inductively coupled plasmas for elemental analysis. Abdallah and Mermet (19) compared temperatures of helium and argon in ICP and MIP systems. Abdallah et al. followed with a paper (20) utilizing a 50-MHz He-ICP at atmospheric pressure for analytical studies. Seliskar et al. (21-24) presented work on a reduced
0003-2700/89/0361-1052$01.50/00 1989 American Chemical Society
