Anal. Chem. 1983, 55, 1586-1591
1586
Phenylenediamine-Containing Chemically Modified Carbon Paste Electrodes as Catalytic Voltammetric Sensors K. Ravlchandran and Richard P. Baldwin” Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
Chemlcally modlfled carbon paste electrodes were fabricated by Incorporating approprlate quantities of N,N,N’,N’-tetramethyl-p-phenylenedlamina (TMPD) dlrectly Into the paste mixture, and thelr performance was characterized wlth cycllc voltammetry. Analogous to solution-phase TMPD, electrodes constructed in this manner exhlblted two pH-dependent 0x1datlons at +O.lO V and +0.4 V vs. SCE, respectively. The currents observed for these waves were directly proportional to the amount of TMPD added to the paste, and new electrode surfaces were generated In mlnutes with 5-10% reproducibility. The TMPD electrodes were able to catalyze the electrooxldatlon of ascorbic acld at the first wave and nicotinamlde adenlne dlnucleotlde vla the second. Electrode response to ascorblc acld was linear over the range from 5 X lo-’ M to 5 X lo-* M, and the catalysis was such that mixtures of ascorbic acid and dopamlne in ratlos from 1:20 lo 5:l could be resolved.
The construction, characterization, and application of chemically modified electrode (CME) systems possessing specifically functionalized surfaces represent an important new area of electrochemical research which, in principle, should open up several unique analytical possibilities to the electrochemist. However, to date, only a few instances demonstrating the use of CMEs capable of enhanced analytical performance have actually been reported. The most noteworthy of these analytical applications has utilized the ability of the modifier molecule to react chemically with solute species as a means of selectively preconcentrating either metal ions (1-4)or organic compounds possessing a reactive functional group ( 5 , 6 )prior to their conventional voltammetric determination. CMEs containing a covalently immobilized enzyme layer have been shown to exhibit amperometric and potentiometric response similar to that of conventional enzyme electrodes formed by physical entrapment of the enzyme near the electrode surface (7). Most recently, electrochemical pretreatment of graphite electrodes by cycling between extreme anodic and cathodic potentials has been effective in allowing the in vivo discrimination of dopamine and ascorbic acid (8, 9). Numerous examples of electrocatalytic CME systems have been reported (IO). In most of these systems, the catalytic CMEs have been designed to mimic on a heterogeneous level previously observed solution-phase redox systems. Usually, the surface-bound modifier selected is a reversibly electrolyzable molecule capable of mediating an oxidation or reduction process which under normal conditions would exhibit high overpotential because of slow heterogeneous electron transfer at the bare electrode surface. The most thoroughly studied example of such electrocatalytic CME activity is probably the oxidation of the reduced form of nicotinamide adenine dinucleotide (NADH) at o-quinone containing electrodes at potentials several tenths of a volt less than that required a t a corresponding unmodified electrode surface (11-15). In theory, the attachment of analogous 0003-2700/83/0355-1586$01.50/0
“electrocatalysts” should present the analyst with the possibility of increasing the range and selectivity of conventional electrochemical systems by selectively lowering the overpotential observed for specific analytes. In the work reported here, we have examined the analytical utility of electrocatalytic CMEs employing phenylenediamine-based systems recently reported by Kitani and Miller (16, 17) for the solution-phase catalysis of the oxidation of NADH and ascorbic acid. The electrodes employed here were chemically modified carbon paste electrodes fabricated by incorporating the phenylenediamine species directly into ordinary carbon paste (15). Both the catalytic properties of these ClMEs toward NADH and ascorbic acid and their overall analytical utility (reproducibility, sensitivity, convenience,etc.) will be described. EXPERIMENTAL SECTION Reagents. N,N,N’,N’-Tetramethyl-p-phenylenediamine (TMPD),p-phenylenediamine(PD), and dopamine were obtained from Aldrich Chemical Co. and were used as received without further purification. Ascorbic acid and NADH were purchased from Sigma Chemical Go. All solutions were prepared fresh for each experiment with deionized water and were deoxygenated before use by degassing with nitrogen. All experiments were performed in buffer solutions prepared by titrating either 0.15 M Na2HP04or 0.15 M citric acid to the required pH. Electrodes. Pure carbon paste electrodes were prepared by thoroughly handmixing 3 mL of Nujol oil (McCarthy Scientific Co., Fullerton, CA) and 5 g of graphite powder (Spectropuregrade, Fluka) in a mortar and pestle. Chemically modified carbon paste was prepared in a similar fashion except that the graphite powder was first mixed with the desired weight of the modifier by forming an ether slurry and placing the mixture in an ultrasonicator for 15 min. Subsequently, the ether was allowed to evaporate; and the graphite/modifier mixture was added to the Nujol in a mortar and pestle. Both pastes were packed into a home-built electrode assembly consisting of two concentric lengths of glass tubing arranged in a pistonlike configuration so that the paste could easily be extruded and a fresh surface prepared. The geometrical surface area of the electrodes was approximately 0.20 cm2. Slight variations in current were observed for different batches of modified carbon paste which contained the same nominal quantity of modifier. Thus, the preparation of new calibration curves is suggested as a precaution each time a new batch of paste is employed. The calibrationcurves cited below each corresponded to the average of at least three different voltammograms for three different electrode surfaces for each concentration reported. All ascorbic acid and dopamine current levels were corrected by subtraction of the background current due to the oxidation of the TMPD modifier. Surface coverages of the modifier were estimated by determining the effective electrodearea for the CMEs and the quantity of modifier electrolyzed during a cyclic voltammetric scan. The electrode area was determined by potential-step chronoamperometry according to the method suggested by Adams (28)using a pure carbon paste electrode and potassium ferrocyanide as the solute. For the electrode assemblies employed in this work, the effective electrodearea was found to be 0.23 cm2(k0.02 cm2). The quantity of modifier electrolyzed was determined by integration of the cyclic voltammogram observed for the CME in blank buffer solution. Integration was accomplished by the “cut-and-weigh” approach. 0 1983 American Chemical Society
AL-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
110 P A
,I-‘
A’
I
--+ 0.5
1587
+0.3
+ 0.1
-0.1
POTENTIAL V v s SCE
Flgure 1. Cyclic voltammograms at carbon paste electrode of solutlon M PD and 1.0 X containing (A) 2.0 X lom4IW PD and (6)2.0 X 10-3 M NADH.
Apparatus. Cyclic voltammograms were obtained with a Bioanalytical Systems (West Lafayette, 1N)l Model CV-1B cyclic voltammetry unit and a Hewlett-Packarcl Model 7015B X-Y recorder. Saturated calomel reference and platinum wire counterelectrodes were also employed. All potentials were reported with respect to the saturated calomel reference. A scan rate of 20 mV/s was employed unless otherwise indicated. The potential-step chronoamperonletricexperiments described utilized a Wavetek Model 187 funlction generator to supply the required voltage pulse. RESULT8 AND DISCUEISION Solution-Phase Catalysis. Typical cyclic voltammograms (CVs) obtained for the oxidation of PD and TMPD at ordinary carbon paste electrodes are shown in Figures 1A and 2A, respectively. In all respects, the observed anodic behavior corresponds closely to that which has previously been reported in the numerous electrochemical investigations that have been performed on these systems (19). Under the conditions employed here, PD undergoes a single, apparently reversible two-electron oxidation to the quinone diimine
At pH 7.0, the peak potential for the oxidation occurs at +0.18 V vs. SCE; and the hydrolysis and coupling reactions which have been reported for the oxidation product do not occur appreciably under the conditions of these experiments. TMPD, on the other hand, shows two well-defined one-electron oxidations. The fiirst, with a peak potential of +0.10 V vs. SCE, is reversible anid can be cycled indefinitely while the second occurs at +0.40 V and is rendered irreversible by a following chemical reaction probably related to the hydrolysis of the electrochemical product. Kitani and Miller (16,17)have recently demonstrated that, in solution, PD is able to catalyze the oxidation of NADH by a mechanism presumably involving hydride transfer from the NADH to the oxidized iminium form of the catalyst. TMPD, however, was shown to catalyze the oxidation of ascorbic acid via its first wave and that of NADH only a t the second. In all cases, the catalyses are evidenced by a dramatic increase in the anodic current level observed for the particular PD or TMPD oxidation wave involved in the process and a corresponding decrease in the cathodic current, observed for these waves on the reverse CV scan.
I
+ 0.5
I
I
+ 0.3
I
I
+ 0.1
I
I
- 0.1
POTENTIAL V v s SCE
Flgure 2. Cyclic voltammograms at carbon paste electrode of solution M TMPD and M TMPD and (6)2.0 X containing (A) 2.0 X 1.0 X M ascorbic acid. In this work, both of these solution-phase electrocatalytic systems were examined a t carbon paste electrodes. By themselves, NADH and ascorbic acid were oxidized irreversibly in broad waves having peak potentials of +0.45 V and +0.26 V vs. SCE, respectively. The CV behavior typically observed for solutions containing these solutes and a 1.0 X M concentration of the phenylenediamine catalysts as well is shown in Figures 1B and 2B. Both of these reaction sequences follow the well-known “catalytic EC” mechanism P-,Q+ne (2) kcAT
Q+A-P+B (3) where P represents the reduced form of PD or TMPD, Q is the oxidized form, and A corresponds to the NADH or ascorbic acid substrate. (Note that the terminology used here is the reverse of that conventionally used by electrochemists to describe this mechanism. From the point of view of the primary electrode process, the term “catalyst”would ordinarily be used to refer to species A. However, as the reaction of ultimate interest here is the second, chemical redox process, the PD and TMPD are viewed as catalyzing the NADH and ascorbic acid oxidations.) In these cases, the catalytic rate constant k C A T can be determined according to the methods of Nicholson and Shain (20). Experimentally, this is accomplished by selecting a known, excess concentrationof substrate and plotting the ratio of the peak currents in the presence and absence of catalyst against the inverse o€ the potential scan rate. The kCAT value obtained in this manner for the catalytic oxidation of NADH by PD was 2.3 X lo3 (f0.24) L mol-l s-l. This value was found to be independent of both scan rate and NADH concentration and compares favorably with the value of 2.1 X lo3 L mol-1 s-l reported by Kitani and Miller (16). The kCAT value measured for the catalytic oxidation of ascorbic acid by PD was 3.1 X lo3 (f0.31) L moll1 s-l, and that by TMPD was 0.89 X lo3 (&0.04)L moll1 s-l. Phenylenediamine-Containing CMEs. In principle, it should be possible to carry out these same catalytic processes at phenylenediamine-containingCMEs. The CMEs selected for use here were chemically modified carbon paste electrodes fabricated simply by adding the desired quantity of PD or TMPD to the graphite powder prior to mixing up the carbon paste. This type of CME was chosen because of its very easy mode of constructionand the capability which it afforded both to vary conveniently the surface concentration of the modifier/electrocatalyst and to generate fresh electrode surfaces
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983 /------
\I 1
+o 5
I
I C03
I
I to1
I -01
POTENTIAL V v s S C E
Figure 3. Cycllc voltammogram of carbon paste CME containlng 0 . 2 5 % TMPD: (A) initial scan: (B) scan obtained after 15 min of continuous cycling between -0.10 V and $0.28 V vs. SCE. with quantitative reproducibility (15). Although these considerations are of extreme importance for CMEs which are to be employed in any practical quantitative applications, they are not readily obtained with CMEs formed via the usual covalent or adsorption attachment approaches. On the first anodic scan for a PD-containing carbon paste CME, a large oxidation wave occurred at a potential region close to that at which the solution-phase oxidation of PD had been seen to take place. However, as the potential was increased beyond the peak potential value, the electrode behavior became erratic. Subsequently, no reduction wave was seen on the reverse scan; and repeated scans in the positive direction failed to exhibit the PD oxidation observed on the initial anodic scan. Apparently, a rapid solubilization of the PD occurred upon oxidation and resulted in fouling of the electrode surface. As a result of the PD electrode’s instability, its electrocatalysis of NADH oxidation was not investigated further. TMPD, however, did form a stable and reproducible carbon paste CME which, upon CV scan, exhibited the same two waves as solution-phase TMPD (Figure 3). This CME could be scanned repetitively over the first oxidation for periods of 15-20 min without a noticeable decrease in the current level obtained. However, scanning the potential to the second TMPD wave rapidly rendered the electrode unsuitable for further use. Additionally, the TMPD-CME could be deactivated either by allowing it to stand for several hours in contact with an aqueous solution or by holding its potential continuously at the first TMPD oxidation wave for several minutes. In both cases, observation of the characteristic deep blue color of the oxidized form of TMPD in the solution surrounding the electrode indicated that the modifier was being stripped from the electrode surface and leached into solution. However, the initial CME activity could always be immediately restored by simply removing the outer layer of paste and briefly smoothing the newly exposed portion. Mixed paste electrodes renewed in this manner exhibited stable and reproducible electrochemicalbehavior over periods of weeks; in fact, no apparent deterioration of performance has yet been observed for any of the pastes prepared during the course of this work. New electrode surfaces could be routinely regenerated from the same batch of modified carbon paste with a reproducibility of 5-10% in the peak current measured for the initial TMPD oxidation wave. It is significant to note that this degree of reproducibility is not appreciably poorer than that expected for the renewal of ordinary carbon paste electrode surfaces. In most respects, the redox processes observed for the TMPD-CME appeared completely analogous to those of the
0
I 3
I 4
I 5
I 6
I 7
0
I
L
8
9
PH
Figure 4. Peak potential vs. pH for a 0.25% TMPD CME: (A) anodic wave: (8) cathodic wave. Solid lines represent values obtained for TMPD solutions at ordlnary carbon paste electrode: circles correspond to anodlc (0)and cathodic ( 0 )values at TMPD-CME. solution-phase redox couple. In both cases, the peak potentials
E, for the anodic and cathodic TMPD waves were affected identically by changes in the potential scan rate. For values up to 50 mV/s, peak potentials were independent of scan rate while, at higher values, both the surface-bound and solution-phase processes gradually shifted toward more extreme potentials. Furthermore, the pH variation of the peak potentials was identical for both systems. In both cases, the E,-pH plot (Figure 4) showed a shift of 59 mV per pH unit below pH 6 and a leveling off at higher pH values. The pH dependence suggests that the electronctive sites on the carbon paste CME behave as true surface active groups influenced by specific solution conditions and not shielded within the electrode interior. However, the carbon paste CMEs do not behave exactly as expected for “ideal” monolayer CMEs correspondingto the attachment of a single, uniform layer of the modifier onto a solid electrode surface. The experimental criteria ordinarily used to characterize CME behavior are the potential separation of the anodic and cathodic peaks observed for the modifier and the scan rate dependence of the observed peak currents. Because of the absence of diffusion mass transfer effects in monolayer CMEs, cyclic oxidation and reduction of the attached modifier characteristically yields symmetrical peak-shaped waves for which both the anodic and cathodic peaks occur at the same potential. Further, the associated anodic and cathodic peak currents, .,i and ipo,for such electrodes vary directly with the potential scan rate employed as opposed to the square root dependence ordinarily observed for the diffusion-controlled electrolysis of solute species. As Figures 2 and 3 clearly show for the carbon paste CMEs employed in this work, the shapes and peak separations of the anodic and cathodic waves observed for the first TMPD electron transfer closely resemble those observed for the CV of TMPD solutions at ordinary carbon paste electrodes; this behavior persists even at relatively slow scan rates. Moreover, for the TMPD-CME, there is a linear relation between i, and scan rate only for scan rates of less than roughly 100 mV/s. A plot of log ,i vs. log u is linear up to a scan rate of 300 mV/s possessing a slope of 0.77. Similar behavior, intermediate between that expected for diffusionless monolayer CMEs and purely diffusion-controlled solute systems, has previously been observed for CMEs containing thick films of an adsorbed
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
l
/
1589
Table I. CME Characteristics as a Function of Modifier Concentration % surface TMPD E , (V coverage, VS, %CE) i, M A nmol/cm2 concn 1.00 0.75 0.50 0.25
tO.090 t0.090 t0.095 +0.090
29.0 i: 2.4 17.3 f 2.4 11.0 f 1.0 5.0 i: 0.4
9.72 f 6.09 f 3.98 f 2.01 f
0.10 0.38 0.40 0.04
/---
L
c- -
I ’ I 0.1
0.2
0.3
t-%
Figure 5. ivs. t-”* after ia potentlal step from 0.00 V to 4-0.30V vs. SCE: (A) 2.0 X M TMPD solutjon at carbon paste electrode: (B) 0.25% TMPD-CME; (C) electrolyte only at carbon paste electrode.
electroactive polymer for which a model involving charge diffusion via an “electron-hopping”mechanism through the polymer film to the actual electrode surface is thought to be applicable (21,22). Subsequently, potential-step chronoamperometry using a voltage pulse across the initial TMPD oxidation (Le., from 0.00 V to +0.30 V) wai3 also employed for CME characterization purposes. The resulting currents obtained for the CME, for an unmodified carbon paste electrode in a 1 X M TMPD solution, and for the unmodified electrode in a blank solution are shown plotted in Cottrell form in Figure 5. The condition of semiinfinits linear diffusion should be confirmed by the existence of a linear relation between current and t-’I2 as evidenced by the TMPD solute (curve A) even at relatively long times, but by the TMPD-CME (curve B)only at times up to approximately 201 s. Further, the TMPD-CME current does not decay rapidly down to the background level (curve C) but remains at a relatively high and fairly constant value. Apparently, some diffusional processeri are operative in the functioning of the carbon paste TMPD-CMEs. One possibility was that some leaching of TMPD into the solution surrounding the electrode surface was occurring and that the subsequent diffusion of this liberated TMPD back to the surface was responsible for the scan rate and time dependences observed for the CME currents. Similar behavior was reported by Kuwana for carbon paste electrodes dloped with ferrocene and scanned to a potential sufficiently anodic to generate ferricinium ion (23). This hypothesis was tested by intentionally stirring the electrolysis solution between successive CV scans. When this was done, it was foiund that the current amplitude observed for the second scan was significantly diminished compared Q that observed for the fist. In no case, however, was the current completely eliminated by the stirring. Thus, it appeared that some TMPD was lost into the surrounding solution and that this ‘free” TMPD accounted for a portion of the current observed with this CME. Further investigations concerning the mode of action of the carbon paste CME and particularly of the role of modifier diffusion both within and out of the carbon paste matrix are continuing and will be reported elsewhere. Finally, CMEs containing different concentrations of TMPD in the carbon ]paste mixture were constructed and compared. As shown in Table I for CMEs fabricated from paste mixtures doped with 0.25% to 1.0% TMPD (by weight), the resulting current levels and surface coverages were pro-
I
+ 0.5
I
I
I
+0.3
POTENTIAL V
v8
I
+ 0.1
I
I -0.1
-
SCL
Figure 6. Cyclic voltammograms at 0.25% TMPD-CME: (A) 1.0 X M NADH. M ascorbic acid; (B) 1.0 X
portional to TMPD concentration. Concentrations greater than 1.0% were not used because of the unmanageable current levels which resulted. Attempts at using TMPD concentrations of 0.10% or less were never successful in producing CMEs which exhibited readily identifiable TMPD waves. Effective surface coverages shown in Table I were computed as described in the Experimental Section and were found to be independent of the scan rate employed. Compared to surface coverages reported for most types of CMEs (IO),the values found here for carbon paste CMEs represent a relatively high effective concentration of surface-active groups intermediate between that normally observed for monolayer CMEs and for polymer-coated CMEs. CME Catalysis. Cyclic voltammograms obtained for the TMPD-CME in solutions containing either 1.0 X M NADH or 1.0 X M ascorbic acid are shown in Figure 6. It is evident that the catalytic behavior of the CME is similar to that of the solution-phase TMPD toward these two species-with evidence of the catalysis consisting of a dramatic increase in the current level associated with the first TMPD oxidation wave when ascorbic acid is present and a corresponding increase in the second wave for NADH. In addition, depending on the ascorbic acid concentration present, the correspondingreduction wave is either completely suppressed or markedly decreased on the reverse scan. The pH dependence of the catalytic processes was investigated and was found to correspond exactly to that previously observed for the TMPD oxidations themselves. Two aspects which related directly to the analytical utility of the CME catalysis were subsequently investigated: its concentration dependence and the selectivity enhancement
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
that it imparted to related electroanalysis. For both purposes, ascorbic acid was employed as a test analyte in conjunction with the TMPD-CME. Ascorbic acid calibration curves obtained for CMEs containing a range of TMPD concentrations and corrected for the background currents observed when no analyte is present all exhibited a region of roughly linear response to the ascorbic acid concentration. The correlation coefficients ranged from 0.997 for the 0.25% electrode to 0.987 for the 1% CME. In general, this linear response range was found to occur at higher ascorbic acid concentrations for CMEs fabricated with greater TMPD amounts. At lower ascorbic acid concentrations, electrode response was limited by the background current present because of the oxidation of the TMPD modifier itself while, at higher ascorbic acid concentrations, the principal source of interference consisted of background current from the onset of the direct, uncatalyzed oxidation of the ascorbic acid which occurs at the CME at +0.26 V vs. SCE Gust as at a conventionalcarbon paste electrode). Quantitation of lower ascorbic acid concentration was limited by the former effect and thus was facilitated by the use of CMEs containing the lower TMPD paste concentrations. Accordingly,maximum sensitivity was achieved with the electrode constructed from the 0.25% TMPD paste. A detection limit of 5 X M was obtained for ascorbic acid by using CV at this CME; this detection limit compares favorably with that normally observed for CV at conventional unmodified electrodes. Furthermore, this electrode possessed a linear response range from 0.05 mM to 5 mM. The reproducibility of peak current measurements made with different electrode surfaces over this range was 5-10%. Subsequently, assays of commercially available vitamin C tablets containing nominally 250 mg of ascorbic acid were performed with a 0.50% TMPD paste electrode. The results, obtained by using a calibration curve approach, indicated an ascorbic acid content of 243 mg. This reported value represented the average of four individual determinations using four different electrode surfaces and possessed a standard deviation of 22 mg. In general, CME results were comparable in accuracy and reproducibility to those obtained with plain carbon paste electrodes. When using the CME, however, the current was monitored at the TMPD oxidation wave at +0.10 V vs. SCE instead of at the conventional ascorbic acid wave at +0.26 V. The most interesting aspect of the CMEs performance was this potential shift as it offered the opportunity to determine this analyte in the presence of other species which, at ordinary electrodes, are oxidized together in the same potential range. An example would be ascorbic acid and dopamine which exhibit E,’s of +0.26 V and +0.28 V vs. SCE, respectively, at a plain carbon paste electrode. In fact, the resolution of this pair of analytes represents a situation of some practical significance as the discrimination of these species in brain tissue has been the object of considerable previous activity by researchers interested in the chemistry of the neurotransmission process (24). Voltammograms for a solution containing an equimolar mixture of ascorbic acid and dopamine, run at both a plain carbon paste electrode and a 0.25% TMPD-CME, are shown in Figure 7, curves A and B. Two separate anodic waves were clearly resolved only with the CME, the ascorbic acid appearing at the first TMPD oxidation and the dopamine at its customary reversible potential of +0.28 V. Since the CV scan was reversed at 0.35 V, the second TMPD wave did not appear. The overall facility of the CME for determining mixtures of the two analytes is demonstrated in Table I1 where the peak current obtained for a 1.0 X M concentration of ascorbic acid is shown to be constant in the presence of a wide range
I
I
+ 0.5
I
I
+03
I +01
I
I -0.1
POTENTIAL V v s SCE
Figure 7. Cyclic voltammograms of solution containing 1.O X M ascorbic acid and 1.0 X IO3 M dopamine: (A) obtained at unmodified carbon paste electrode; (B) obtained at. 0.25% TMPD-CME.
Table 11. Ascorbic Acid Current in the Presence
of Dopamine
[ascorbic acid], M 1.0 x 10-3 1.0x 10-3 1.0 x 10-3 1.0 x 10-3 1.0 x 10-3 a
[dopamine], M 5.2 x 2.1 x 1.1 x 5.2 x 2.0 x
i,,a P A 10-3 10-3 10-3 10-4
10-4
36.0 33.0 30.4 32.3 29.7 mean 32.3
Observed at t 0.10 V vs. SCE.
of dopamine concentrations. The calibration curve observed for varying concentrations of ascorbic acid in the presence of a constant 1.0 X M amount of dopamine was linear in the range from 0.05 to 5.0 mM (correlation coefficient = 0,991). In the presence of dopamine, however, the calibration curve did possess a greater slope compared to that observed earlier for ascorbic acid by itself. The more difficult but, in this instance, more realistic case in which the analyte of interest occurs at the second wave and the excess interferent at the first is simulated by the reverse situation involving the determination of trace amounts of dopamine in the presence of excess ascorbic acid. The increased level of analytical difficulty arises from the fact that, even though the two waves still occur at easily distinguishable potentials, the second dopamine wave is superimposed on the tailing but still quite significant portion of the ascorbic acid oxidation. Even in this case, however, a distinct dopamine wave could be observed in the presence of a &fold excess of ascorbic acid. This greatly exceeds the selectivity obtainable with ordinary carbon paste electrodes. It is interesting to compare the analytical performance of the carbon paste CMEs described in this work to that obtained by Cheng (9) for graphite/epoxy electrodes electrochemically pretreated by initially cycling the electrode between +7 and -5 V prior to the analysis. In general, our CMEs were not able to achieve as great a degree of discrimination between the ascorbic acid and dopamine oxidations as was reported for the electrochemically pretreated electrodes. One of the reasons for this is that ascorbic acid oxidation is not as effectively catalyzed by incorporated TMPD, thereby allowing its oxidation to be separated from that of the uncatalyzed dopamine by only 0.18 V. This compared with a peak separation of 0.24 V found by Cheng. Preliminary results indicate that the use of differential pulse voltammetry with its narrow, peak-shaped signals will permit much greater excesses of interferent to be
Anal. Chem. 1903, 55, 1591-1595
employed in the carbon paste approach. In terms of experimental convenience, flexibility, and quantitative reproducibility, however, the carbon paste CMEls clearly possess attractive properties. This electrode modification procedure is one of the most general approaches available. In principle, virtually any water-insoluble species can be added to the paste mixture; and, although the modifying species may eventually be lost into solution, the initial quantity added can be directly controlled. Thus, extension of the carbon paste CME scheme to numerous other electrocatalytic systems is easily envisioned. Furthermore, regenerakion of new electrode surfaces can be performed in a matter of minutes and with a 5-10% reproducibility that is virtutally the same as that observed for ordinary carbon paste ellectrodes. The development of CMEs possessing these properties is especially important if CMEs are to be employed for routine quantitative measurements. Registry No. TMF'D, 100-22-1;PD, 106-50-3;N M H , 512-68-4; ascorbic acid, 50-81-7; dopamine, 51-61-6.
LITERATURE CITED (1) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 1 1 , 393-402. (2) Oyama, N.; Anson, F. C. J . Am. Chem. SOC.1979, 101,3450-3456. (3) Rublnsteln, I.; Bard, A. J. J . Am. Chem. SOC. 1980, 102, 6641-6642. (4) Cox, J. A,; Majda, M. Anal, Chlm. Acta 1980, 118, 271-276. (5) Slria, J. W.;Baldwin, 13. P. Anal. Lett. 1980, 13, 577-588. (6) Price, J. F.; Baldwin, R. P. Anal. Chem. 1980, 52, 1940-1944.
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54, 1980-1984. a n o n , F. G.;Fombariet, C.; Buda, M. J.; Pujol, J. Anal. Chem. 1981, 53, 1386-1369. Faiat, L; Cheng, H.-Y. Anal. Chem. 1982, 5 4 , 2108-2111. Murray, R. W. Acc. Chem. Res. 1960, 13, 135-144. Tse, D. C.; Kuwana, T. Anal. Chem. 1978, 50, 1315-1318. Degrand, C.; Miller, L. L. J . Am. Chem. Soc. 1980, 102, 5728-5732. Fukui. M.; Kitani, A.; Degrand, C.; Miller, L. L. J . Am. Chem. SOC. 1982. 104. 28-34. (14) Jaegfeldt, H.; Torstensson, A.; Gotton, L.; Johansson, G. Anal. Chem. 1981, 53, 1979-1982. (15) Ravlchandran. K.; Baldwin, R. P. J . flecfroanal. Chem. 1981, 126,
293-300. (16) Kitani, A.; Miller, L. L. J . Am. Chem. SOC. 1981, 103, 3595-3597. (17) Kitani, A.; So, Y.; Mlller, L. L. J . Am. Chem. SOC. 1981, 103, 7636-7641. (16) Adams, R. W. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969;pp 56-58. (19) Adams, R. W. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969;pp 356-361. (20) Nicholson, H. S.;Shah, I. Anal. Chem. 1984, 36, 706-723. (21) Daum, P.; Lenhard. J. R.; Rollson, D.; Murray, R. W.J . Am. Chem. SOC. 1980, 102. 4649-4653. (22) Roullier, L.; Waldner, E.; Laviron, E. J . flecfroanal. Chem. 1982, 139,
199-202. (23) Kuwana, T.; French W.G. Anal. Chem. 1964, 3 6 , 241-242. (24) Adams, R. N. Anal. Chem. 1976, 48, 1126A-1136A.
RECEIVED for review March 7,1983. Accepted May 10, 1983. This work was presented in part at the 1983 Pittsburgh Conference on Analytical Chemistry and Applied SpectrosCOPY.
Differential Pulse Polarography and Differential Pulse Anodic Stripping Voltammetry for Determination of Trace bevels of Thallium Llnda
K. Hoeflich,' Robert J. Gale," and Mary L. Good2
Depatiment of Chemistty, Louisiana State University, Baton Rouge, Loulsiana 70803
Slmuttaneous determlnfitlonof TI+ and (CH3)ZTI' Ionic specles by differential pulse polarography (DPP) and differential pulse anodic strlpplng voltammetry (DPASV) is described for various buffered matrices. Detection llmits for TI+ and (CH3)ZTI' were 130 ppb and 250 ppb, respectively, by conventlonal DPP. Corresponding values are 3.2 ppb and 3.4 ppb with DPASV. Pb( I I), Zn( I I ), and Cd( I I ) presence interfered wlth these thallium species identifled by DPP, although EDTA addition prevented Interference of the TI+ peak by these metals. When EDTA is used to remove interferences in DPASV, the thallium species peak ehlfts to a more nogatlve potentlai and the peak current Is decreased such that the portion due to (CH,),TI+ is unaffected, while that due to TI+ Is modified. The electrochemlcal response of TI' ion is unaffected by pH while that for (CH3),TI+ Ion is decreased with Increasing pH. (CH3)ZTI' ion reduces in an lrreverslble $)-electronwave with kinetic parameters a := 0.368 f 0.091 and k , = (4.05 f 0.55) X IO-' cm 8-l for E , = -0.800 V SCE, and a diffuslon coefficient D o = 1.06 X lo-' cm s-l.
The past 20 years have been characterized by an awareness of environmental polluition from a variety of sources. One of the major contributors is energy production, where problems 'Present address: E. I. du Pont de Nemours and Co., Inc., Seaford, DE 19973. Present address: UOP,Inc., Corporate Research Center, Des Plaines, IL 60016. 0003-2700/83/03551591$01.50/0
of concern include the environmental impact of stack gas effluents and residual waste products. Special attention has been given to the quantitation and speciation of heavy metals, because of their emission with coal fly ash and subsequent deposition into natural waters, Certain inorganic compounds are biomethylated to organometallic forms by microbial action in these waters, which complicates their determinations (1-5). Thus, procedures must be designed to determine total heavy metals content and to differentiate the chemical forms present. One of the heavy metals of interest is thallium because of its toxicity. This metal undergoes a series of bioenvironmental transformations and, although it is not present at concentrations as high as certain other heavy metals (e.g., Cu, Cd, Pb), its inorganic compounds are considerably more toxic (6-11). Concentrations of thallium and thallium compounds are likely to be at the parts-per-billion level (12-191, so that any methods chosen should be capable of both quantitative and qualitative determinations at these levels. In addition, for full utility the analytical methods preferably would be adaptable to field use. Electroanalytical methods are obvious choices for speciation studies, and while organothallium compounds have been identified electrochemically, the results are not especially applicable to procedures for routine analyses of low level natural samples (20-29). The present study was an investigation of the electrochemical analysis procedures appropriate to environmental needs. Conventional voltammetric methods, namely, cyclic voltammetry (CV), differential pulse polarography (DPP), and differential pulse anodic stripping voltammetry (DPASV), were used to characterize 0 1983 American Chemical Society
