Anal. Chem. 1092, 64, 2706-2710
2708
Stripping Voltammetry with Preconcentration through Chemical Reactions Coupled to Charge Transfer in an Ionomer-Coated Electrode: Application to the Determination of a Nitrosoamine Waldemar Gorskit and James A. COX* Department of Chemistry, Miami University, Oxford, Ohio 45056
A cathodic drlpplng voltammetrlc method for the determinatbn ol an aromatic nitrosoamine, ~n~roro-N,Kd~thyianliine (NDEA), was devebpad urlng a glassy carban lndlcator coated with a catlon-exchangemembrane fllm. The preconcentratbn atop was shown to involve reductionol the analyte by an ECE mechanism, the product of which couples with NDEA 8ub"ttylntroducedtotlwfnnl. T h k c ~ t b n p r o d u c t k reduced at a lower potential than NDEA. The resuiting dMerentiai pulse drlpplng current I8 directly proportional to the roiutlon concentratlon over the range 5-810 nM NDEA. The detection ilmit by this method I8 3 nM with a 10mln preconcentration of the condensation product followed by a 5mln prwxldatbnat 800 mV. A comparabledrippingmethod baud on the dlrect reductlon of NDEA has a detection llmit of 0.1 pM.
INTRODUCTION Analytical applications of voltammetry a t modified electrodes have been slow to appear although several investigations have shown promising results.' Among the more interesting possibilities is to use an ionomer-coated electrode or a surface that has been modified by incorporating a chelating agent as the preconcentrator for a stripping voltammetry experimentaZ4 The advantage over classifical stripping analysis is that the electrode process does not need to be reversible since the preconcentration is by a chemicalexchange process. For example, Cr(V1)was preconcentrated from acid solution by ion exchange a t a poly(4-vinylpyridine) film on glassy carbon and determined on the basis of the current produced when it was reduced during a negativegoing linear potential scan.3 The working range with a 60-s preconcentration was 10410-8 M. Inclusion of fluoride in the supporting electrolyte and application of 0.9 V vs SCE during the preconcentration prevented the accumulation of Cr(II1) oxides which would otherwise passivate the electrode surface. Adsorptive stripping voltammetry also shares the attribute of not requiring electrochemical reversibility.7~8Wang et al.8 determined proteins such as trypsin and chymotrypsin using preconcentration by adsorption at open circuit followed by
* Author to whom correspondence should be addressed.
+ On leave from the Department of Chemistry, Warsaw University, Pastaura 1, Warsaw 02093, Poland. (1) Cox, J. A.; Jaworski, R. K.; Kulesza, P. J. Electroanalysis 1991,3, 869-877. (2) Oyama, N.; Anson, F. C. Anal. Chem. 1980,52, 1192-1198. (3) Cox, J. A.; Kulesza, P. J. Anul. Chim. Acta 1983, 154, 71-78. (4) Eepenscheid, M. W.; Martin, C. R. Electroanalysis 1989, I, 93-95. (5) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, A l l , 393-402. (6) Guadalupe, A. R.; Abruila, H. D. Anal. Chem. 1986,57, 142-149. (7) PaleEek, E.; Postbieglovl, I. J . Electroanal. Chem. Interfacial Electrochem. 1986,214, 359-371. (8) Wang, J.; Villa, V.; Tupia, T. Bioelectrochem. Bioenerg. 1988,19, 39-47.
0003-2700/92/0364-2706$03.00/0
linear scan voltammetry to yield an analytical signal due to reduction of the analyte. The present study is part of a program to devise improved methods for the determination of trace levels of nitrosoamine8 in water. By using an acidified sample in conjunction with an electrode coated with a cation-exchange film, an ionexchange stripping voltammetric method can be envisioned. Indeed, others have reported the preferential uptake of aromatic cations over alkali metal ions into Nafion films, presumably because of strong nonspecific interaction between the organic compound and the backbone of the perfluorosulfonated membrane.9 Based on previous reports,1OJ1 reduction of the immobilized NDEA was predicted. As detailed herein, we found that a superior approach was to preconcentrate the NDEA as the product of a condensation reaction which can be promoted within the film by an electrochemical pretreatment step. This product is reduced at a less-negative potential than is NDEA; moreover, the sensitivity of the method based on a chemical preconcentration step is greater than that for the method which utilizes direct preconcentration and subsequent reduction of NDEA. The use of a chemical conversion of an organic analyte in the preconcentration step constitutes, to our knowledge, a new variation of stripping voltammetry. In perhaps the mostrelated study, Fogg and Lewis derivatized amines to assist in the preconcentration step of an adsorptive stripping voltammetry procedure.lZ
EXPERIMENTAL SECTION Chemicals. 4-Nitroso-N,jV-diethylaniline (Sigma) was used without further purification. Ndion (equivalent weight 1100 g) was purchased from Aldrich as a 5 % (wt) solution in lower aliphatic alcohols and 10% water. Methanol (Fisher),used for dilution of the Ndion solution to 2.5 % , was of spectral purity. Stocksolutions of 4-nitroeo-N,jV-diethylaniline(NDEA),c" = 6 X lo-" M (wherex = 2,4, or 6),were prepared in 0.05 M HCl (Fisher). Distilled water, purified with a Sybron/Barnstead NANOpure cartridgesystem, was used in a l l of the experiments. As NDEA is highly toxic, all solutions of the compound were prepared in a fume hood. Experimental Techniques and Instrumentation. Cyclic and differential pulse voltammetric measurements were made with an EG&G PAR Potentiostat/Galvanostat Model 273A connected with a MC-GPIB interface board (National Instrumenta) installed on an IBM PS/2 Model 55 SX computer and controlled by EG&G PAR Model 270 electrochemical software. Voltammograms were recorded on a Hewlett-Packard 7470A digital plotter. (9) Nagy, G.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B., 111; Szentirmay, M. N.; Martin, C. R. J. Electroanul. Chem. Interfacial Electrochem. 1986, 188, 85-94. (10) Leedy, D. W.; Adams, R. N. J. Electroanal. Chem. Interfacial Electrochem. 1967,14, 119-122. (11) Kemula, W.; Krygowski, T. M. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1979; Vol. XIII, Chapter XIII-3 and references therein. (12) Fogg, A. G.; Lewis, J. M. Analyst 1986,111, 1443-1444. 0 1992 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 84, NO. 22, NOVEMBER 15, 1992 2’10’1
Differentialpulse voltammograms(DPVs)were recorded using a negative-going dc scan at a rate 4 mV s-l. The step time was 0.5 a. The pulse amplitude and the current sampling time were varied in the ranges of 20-200 mV and 5-50 ma, respectively. A conventional three-electrode cell, with a Pt wire counter electrode,a Ag/AgCl/3M NaCl reference electrode (Bioanalytical Systems, Inc., BAS), and a 3-mm-diameter glassy carbon (GC) disk (BAS) working electrode, was used. Procedures. The glassy carbon surface was prepared as follows. First, it was polished with an Alpha A polishing cloth (Mark V Lab) using successivelysmaller particles (1.0-, 0.3-, and 0.05-pm diameter) of alumina suspended in 17.6 MQwater. The surface was sonicated for 2 min in a closed beaker of distilled water after each polishing step. A final sonicationwas performed in methanol for 15 min and in water for 5 min. The surface was then air-dried. The dry GC electrode was coated by adding 10 pL of the 2.5 % Ndion solution and evaporating the solvent. The area covered by the film was ca. 6 mm in diameter. Two methods of drying the Nafion films were used. The fiist involved evaporating the solvent for 15 min under ambient conditions of ca. 20% relative humidity (“dry-cured”film). In the second method, which was based on a recent report,’3 the GC electrodewith the added Nafion solution was placed immediately into a closed desiccator which was partially filled with water; it was dried for 48 h under the resulting 85 % relative humidity conditions (‘wet-cured” films). Freshly-coated electrodes were preconditioned by immersing in 2 M HC1 for 45 min and subsequently cycling the potential between +0.800 and -0.300 V in 0.05 M HC1 (background electrolyte) until a steady-state voltammogram was achieved (usually 30 min). This pretreatment procedure was repeated prior to performing a sequence of experimentswith the objective of characterizing the electrode performance or of elucidating the mechanism. When applying the electrode to NDEA determinations, sequential standard additions were used, so treatment between experiments was not employed. All procedures for the determination of NDEA involved a 15min preconcentration step during which the solution studied was stirred for the first 10 min. The preconcentration into the Nafion film was performed either at open circuit (OC mode) or with the electrode polarized for 10 min at -300 mV and then for 5min at +800 mV (electrolyticmode). After the preconcentration step, a differential pulse polarogram was obtained by scanning from +800 to -300 mV. The typical range of NDEA concentrations studied was from 5.0 X 10-4to 8.7 X lo4 M,however, in some experiments it was extended up to 1.5 X 10“ M NDEA. Background currents were obtained by following the procedures described above except that the solutions contained only 0.05 M HCI. These currents were subsequently subtracted from those measured in the presence of NDEA. All experiments were done at room temperature (20 f 1 “C) in deaerated solutions under an argon blanket.
RESULTS AND DISCUSSION A cyclic voltammogram of NDEA recorded a t a bare GC electrode in acidic aqueous solution is shown in Figure la. The shape is essentially the same as that reported by Leedy and Adams‘o for the cyclic voltammetry of 4-nitroso-NJVdimethylaniline (NDMA) on a carbon-paste electrode. They demonstrated that the electrolysis mechanism for NDMA was an ECE-type. Specifically, they proposed that NDMA is reduced in a two-electron step (E) to [p-(N,N-dimethylamino)phenyl]hydroxylamine which dehydrates (C) to form N,N-dimethyl-p-phenylenediimine, Under the same cathodic peak, the imine is further reduced in a two-electron step (E) to the corresponding diamine. The diamine undergoes a quasireversible oxidation a t a potential about 400 mV less negative than the peak for the reduction of NDMA. In fact, the electrochemical reductions of aromatic nitroso compounds generallyoccur by ECE mechanisms.11 Since the replacement ~~
~~
~
(13) Striebel, K. A.; Scherer, G. G.; Haas, 0. J. Electroanal. Chem. Interface Electrochem. 1991, 304, 289-296.
;mi I
I
I
I
I
I
I
I
00
QO
E / ~ V Flgure 1. Cyclic voltammograms for 1.1 X lo4 M NDEA on (a) bare GC electrode at a scan rate of 50 mV s-l and (b) for 1.5 X lo4 M N M A on dry-cured GClNaflon electrode after preconcentratlon wlth the OC mode at a scan rate of 4 mV s-l. Backgroundelectrolyte, 0.05 M HCI. Numbers at curves indicate order of the trlals.
of the methyl with an ethyl group should not markedly change the voltammetric behavior, this mechanism is assigned to the reduction of NDEA in Figure la. When the bare GC electrode, after several potential cycles in NDEA solution, is removed, rinsed with water, and transferred to 0.05 M HC1, the pair of peake centered at about 500 mV is still observed, but the currents are diminished. Apparently, adsorption is involved in the electrode process. In a solution of 0.05 M HC1, NDEA, for which the PKAis 4.48,14 exists primarily as a cation which can be sorbed by a Nafion layer on a GC electrode. Because of strong, nonspecific interactions between organic compounds and the membrane backbone? cationic NDEA can preconcentrate even in the presence of a large excess of inorganic cations. A cyclic voltammogram of NDEA bound in a Nafion layer on a GC electrode is shown in Figure lb. An important characteristic is that the current of cathodic peak A (Figure lb) is diminished after the first cycle by a factor of more than 20. Apparently, most of the nitroso compound present in the Nafion layer is irreversibly reduced during the first potential excursion to values more negative than 200 mV. As a result, peaks A and B, which are coupled, both practically vanish with cycling. The only voltammetric peak which is sustained a t steady state is cathodic peak C (Figure lb). Peak C was not characterized in the study of the electrochemistry of NDMA in homogeneous solution,”’ but the related chemistry can be surmised from other studies on the oxidation of compounds related to the diamine. For example, the oxidative dimerization of aniline to an azo compound was demonstrated by surface-enhanced Raman spectroscopy (SERS).I5 Using SERS in conjunction with cyclic voltammetry, Shi et al.le demonstrated that nitroso intermediates in the reduction of p-nitrobenzoic acid condense with an aromatic hydroxylamine to form an azoxy compound. The latter reaction, as well as coupling between aromatic nitroso compounds and aromatic amines to form azo compounds, is well-known.17 Therefore, it is suggested that peak B results in the formation of an azo compound, the four-electron reduction of which leads to peak C.18 The proposed mech(14) Gornostaev, L. M.; Skvortaov, N. K.; Belyaev, Yu. E.; Ionin, B. I. Zh. Org. Khim. 1974,10,2484-2486. (15) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1989, 93, 3753-3760. (16) Shi, C.; Zhang, W.; Birke, R. L.; Gosser, D. K., Jr.; Lombardi, J. R.J.Phys. Chem. 1991,95,6276-6285. (17) Boyer, J. H. In The Chemistry of the Nitro and Nitroso Groups; Feuer, H., Ed.; Interscience: New York, 1969; Chapter 5.
2708
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992
anism for the cyclic voltammetry of NDEA sorbed into the Nafion film at a glassy carbon electrode is summarized as follows. At about 30 mV during the fiist negative-going scan, NDEA is reduced (peak A, Figure lb) by the process
-
+ 2e- + 2H' PHA PHA H,O + PDI
NDEA
PDI
-
-
+ 2e- + 2H'
PDA
(1)
-.
4e- + 4H'
+ EAAB
(3)
(4)
where EAAB is 4,4'-(N,h'-diethylamino)azobenzene. During the second negative-going cycle, EAAB is reduced at about 400 mV (peak C, Figure lb) stepwise to PDA EAAB
+ 2e- + 2H'
-
EAHAB + 2e- + 2H'
EAHAB
-
2 PDA
(5)
(6)
where EAHAB is 4,4'-(NJV-diethylamino)hydrazobenzene. Only a single, narrow peak is developed, which suggests that reaction 6 is facile relative to reaction 5. The above mechanism is supported by the results of stripping voltammetry experiments performed in the concentration range, 5.0 nMto0.51 pM NDEA. Here, differential pulse voltammetry was used in conjunction with preconcentration into the Nafion layer in order to obtain faradaic currents above the background level. A series of experiments was performed at a Ndion-coated GC electrode (dry-cured) on a solution of 0.05 M HCl to which NDEA was added by sequential spikes. Prior to each trial, the indicator electrode was contacted to the sample solution for 15 min in the OC mode. After each pulse-stripping measurement step, the sample was spiked to the next concentration of NDEA, and the OC preconcentration was initiated. Based upon our recent study of the transport of an aromatic cation through a cation-exchange membrane (Nafion),lgpreconcentration of protonated NDEA by an ionexchange mechanism was expected even though the ionic strength of the sample was high relative to the NDEA concentration. The stripping voltammogram which results is shown in Figure 2. The first appearance of a faradaic peak was with a NDEA concentration of 30 nM. The stripping potential corresponded to peak C rather than to peak A of Figure lb. The threshold concentration for the appearance of peak A, which is due to the direct reduction of NDEA (reaction 11, is in the 0.1-0.3 pM range (Figure 2). These observations are consistent with the hypothesis that peak C is due to the reduction of an azo compound formed by condensation of the parent nitrosoamine and a diamine which is present in the film from the ECE reduction of NDEA in the previous trial. NDEA + PDA
-
EAAB
1.0 Q
4 -
(2)
where PHA is [(N,N-diethy1amino)phenyllhydroxylamine, PDI is NJV-diethyl-p-phenylenediimine,and PDA is N,Ndiethyl-p-phenylenediamine.During the reverse scan, PDA is oxidized at about 570 mV (peak B, Figure lb) 2 PDA
I
(7)
Such reactions are well-knom.17 By reaction 7, the PDA that accumulates in the f i b during a sequence of experiments, (18) Thomas, F. G.;Bob, K. G. In The Chemistry of Hydrazo, Azo and Azoxy Groups; Patai, S., Ed.; John Wiley: London, 1975; Chapter 12. (19) Cox, J. A.; Poopisut, N. Anal. Chem. 1992,64, 423-426.
0.0 800
400
E /mV
0.0
-400
Flgure 2. Background corrected DPVs on a dry-cured QCINaflon electrode atter preconcentrationwith the OC mode in a solutlon spiked (1) 5 X (2) 3 X lo4; with increasing amounts of NDEA. kA: (3) 1.1 X (4) 3.1 X and (5) 5.1 X M. Background electrolyte, 0.05 M HCI. Pulse amplitude, 50 mV; pulse time, 50 ms.
which does not include a rather lengthy (ca. 45 min) acidwash step between trials, chemically converts the sorbed NDEA to an azo compound. The azo compound is more easily reduced than the parent NDEA. The mechanistic study provides a guideline for describing conditions for the determination of trace levels of NDEA. The peak at 440 mV (Figure 2) permits detection at lower concentration than does that for the direct reduction of NDEA. This experimental observation is probably indicative of the relative mobilities of these compounds in Ndion. In order to use the reduction of EAAB for analytical purposes, the preconcentration step must include conversion of NDEA to EAAB. With a modified electrode that was not previously used or which was acid-washed between experiments, open circuit preconcentration will give a low result for the first trial since PDA, which is needed in reaction 7, is not present in the film during the preconcentration step. Two or three trials on a given sample are generally sufficient to load the film with the necessary amount of PDA by reactions 1-3. The electrolytic preconcentration mode described in the Experimental Section does not require a pretreatment of a new or acid-washed film. During the first 10 min of preconcentration at -300 mV, NDEA is converted to PDA by reactions 1-3. The electrode is then polarized at 800 mV for 5 min. The azo compound is formed by reaction 4. Additional NDEA which is sorbed into the film during the period at 800 mV and while the stripping potential scan is applied will not be detected at peak C when a new or acid-washed film is used; however, this quantity will be small relative to that accumulated at -300 mV. That the sample is stirred only during the 10-min preconcentration at -300 mV accounts for this difference in the amount of NDEA sorbed. Prior to optimizing the preconcentration and stripping steps, the influence of the drying step on the performance of the Nafion-coated GC electrode was examined. This investigation was inspired by a recent report of improved ionexchange behavior for hydrophobic cations when a Nafion film is wet-cured.13 A comparison of the stripping voltammetry of NDEA at wet- and dry-cured Ndion is shown in Figure 3. At the wet-cured film, the analytical signal is more than twice the value observed with the dry-cured film. Also significant is the fact that the peak for the reduction of NDEA is attenuated at the wet-cured film; it is not observed above the background until the NDEA concentration exceeds 1pM. Although it is not surprising that the peak for the direct reduction of NDEA is small, the data in Figure 3 suggest that the coupling reaction of NDEA is more efficient in the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22. NOVEMBER 15, lQQ2 2709 r
I
I 800
400
00
J -400
E /mV Background corrected DPVs on (1) dry-cured and (2) wetcured QC/Naflon electrode recorded after loading In NDEA solutlon under conditions corresponding to curve 5 In Flgure 2. Experlmental condltlons as In Figure 2. Flgure 3.
I
J-+----
0
0
E /mV
Background corrected DPVs on a wetcured QC/Nafion electrodeafter preconcentratbn wlth the electroiyUc mode In a solution spiked wlth Increasing amounts of NDEA. &: (1) 5 X 10-a;(2) 3 Flgure 5.
50
100 Time /mtn
150
Figure 4. Dependence of the peak current on the preconcentration tlme In a solutlon of 1 X M NDEA. Other experimentalconditions are those In Figure 5.
wet-cured film. In all subsequent experiments the wet-cured Ndion was used. In order to establish the optimum analytical procedure, the following experimental parameters were varied: preconcentration potential, pulse amplitude, and current sampling time. First, using stripping conditions of 4 mV 8-1 dc scan rate, 50-mV pulse amplitude, and 50-ms current-sampling time, the OC and electrolytic preconcentration modes were compared. In all cases,the film contained PDA from previous experiments. That is, the acid-washing steps described in the Experimental Section was not used in this part of the study. The peak current at 440 mV (Figure 3) comprised the analytical signal. In the 5-100 nM DNEA range, the sensitivities of the electrolytic and OC modes were the same, but above 100 nM the former was more sensitive. The electrolytic mode was employed in subsequent work. Using pulse amplitudes of 20, 50, and 100 mV, the peak currents were 18.5 f 0.2,63 f 1, and 162 f 3 pA (n = 51, respectively, with a 1.5 X 10-4 M NDEA solution and a 10-ms currentsamplingtime. This observation is consistentwith the general behavior of differential pulse volta”etry.20 With amplitudes above 100 mV, further increases in sensitivity were not significant. Also expected was the observation that shorter sampling times yielded higher sensitivities. Below 10 ms, the greater background current offsets the increase in sensitivity. The Subsequentexperimentswere performed with a 100-mV pulse amplitude and a 10-mecurrent-samplingtime. The sensitivity of the method is a function of preconcentration time. As shown in Figure 4, the analyticalpeak current increases with preconcentration times at -300 mV up to 1h. At that point, when 0.1 pM NDEA is the sample, the film is apparently saturated. A preconcentration time of 10 min at -300 mV was selected because, as shown below, a useful sensitivity was obtained without deleterious effects such as fouling, which may occur with high loadings of the Nafion layer. Stripping voltammograms obtained under the optimized conditions are shown in Figure 5. A linear least squares fit of peak current vs concentration over the range 5.0-810 nM NDEA (eight points) under the Figure 5 conditions yielded the following: slope, 19.3 f 0.1 nA/nM; intercept, 0.02 f 0.02 (20) Hasebe, K.;Osteryoung, J. Anal. Chem. 1976,47, 2412-2418.
X (3) 7 X (4) 1.1 X (5) 3.1 X (6) 8.1 X (7) 1.81 X and (8)4.77 X lod M. Background electrolyte, 0.05 M %I; pulse amplitude, 100 mV; pulse time, 10 ms.
and correlation coefficient, r, 0.9999. Here, the NDEA concentration was increased by sequential standard addition. The detection limit was 3.0 nM using the criteria of the concentration that yields a background-corrected signal of 3 times the standard deviation of the blank. The wet-cured Nafion-coated GC electrode was stable in the present study. A single electrode was used for 3 weeks without any change in behavior when cleaned in 2 M HC1 and stored in 0.05 M HC1 after a series of experiments. Concentrations above 10 pM NDEA were not used during this period, however. As a test for matrix effects on the method, a calibration curve was obtained by spiking Miami University tap water with various concentrations of NDEA. Each solution contained 0.05 M HC1. In the range 5.0-810 nM, a linear least squares fit of current vs concentration (eight points) yielded the following: slope, 13.6 f 0.1 nA nM-l; intercept, -0.01 f 0.02 PA;and correlation coefficient, r, 0.9998. The detection limit was 8 nM NDEA. The lower slope probably reflects an influence of multicharged inorganic cations on the sorption of protonated NDEA. The water sample had atotal hardness of 344 f 4 mg/L (fivetrials with a EDTA titration with results expressed as CaCO3). If the decrease in sensitivity of 30% is unacceptable, quantifying the concentration by sequential standard addition can be employed. The water sample used in the above study had a sodium ion content of (5.0 f 0.4) 10-4 M based on five trials by flame atomic absorption spectrometry. Because of the high affiiity of Nafion for aromatic cations relative to alkali metals! the sodium ion concentration was not expected to influence the sensitivity. This was verified by performing an experiment that compared the responses of 20 nM NDEA in supporting electrolytes containing 0, 0.05, 0.10, and 0.15 M NaCl in addition to 0.05 M HC1. Five trials under the conditions in Figure 5 were performed on each solution. The respective peak currents were 0.44 f 0.03,0.43 f 0.03,0.46 f 0.03, and 0.49 f 0.03 pA. The currents in the presence of NaCl are not significantly different at the 95% confidence level (t-test) except for the 0.15 M NaCl caae, where the departure from the mean current for the experiments in the absence of NaC1, 0.05 PA, is slightly out of the 95% confidence interval, 0.44 f 0.043 pA. In conclusion, the reported stripping analysis procedure is promising for the determination of nanomolar levels of aromatic nitrosoamines. The method may be well-suited to other classes of aromatic compounds which form cations in acidic solution and are reduced by a mechanism involving pA;
2710
ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992
couded following chemical reactions to form electroactive products. With kalytes for which it is suited, this procedure may be more selective and less subject to matrix effects than comtmable electrochemical techniaues such as differential pulse voltammetry (with or without preconcentration) and adsorptive stripping voltammetry. However, more Practical applications of the described mechanism may result if the chemistry is initiated and quantified in a dual-indicator electrode amperometric cell for high-performance liquid chromatography. That possibility is presently being explored.
ACKNOWLEDGMENT This work was supported by Grant No, R816507-01-0from the U.S.Environmental Protection Agency.
RECEIVEDfor review May 27, 1992. Accepted August 24, 1992,
RegistryNo. NDEA, 120-22-9;carbon,7440-44-0;water, 773218-5.
