Anal. Chem. 1988, 58, 2777-2781
high-sensitivity spectroelectrochemical analysis. Finally, improvements in to monitor may lead to the redox processes in polymer films on electrodes in a spatially resolved fashion, to determine rates and mechanisms of charge transport in the films. Registry No. TAA, 13050-56-1;TAA+,34516-46-6;p-benzoquinone, 106-51-4; p-benzoquinone radical ion(1-), 3225-29-4.
LITERATURE CITED Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1960; Chapter 4. Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1968; Chapter 3. Levich, V. G. fhysicochemical Hydrodynamics ; Advanced Publications Ltd.: London, 1977; Voi. 1 and 2. Newman, J. S.Electrochemical Systems ; Prentice-Hall: Engiewood Cliffs, NJ, 1973. Bockris, J. O'M.; Reddy, A. K. N. Modern flectrochemistry; Plenum: New York, 1973; Vol. 1, Chapter 4. Adams, R . N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969; Chapter 4. Bard, A. J.; Fauikner, L. R. flectrochemical Methods; Wiley: New York, 1980; Chapter 8. Wightman, R . M. Anal. Chem. 1981, 53, 1125A. Shoup, D.; Szabo. A. J. Electroanal. Chem. 1982, 740, 237. Fujinaga, T.; Kihara, S. Grit. Rev. Anal. Chem. 1977, 6 ,223. Eibickl, J. M.; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978. McLarnon, F. R.; Muller, R. H.; Tobias, C. W. J . Electrochem. SOC. 1982, 729,2201. Ward, W.; Le Blanc, 0. Science (Washington, D.C.) 1984, 225, 1471. Weisshaar, D. E.;Tailrnan, D. E. Anal. Chem. 1983, 55, 1146. Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1978, 89, 247. Sieszynaski, N.; Osteryoung, J. Anal. Chem. 1984, 56, 130. Fleischmann, M.; Ghoroghchian, J.; Pons, S.J. fhys. Chem. 1985, 89,5530. Fleischmann, M.; Bandyopadhyay, S.;Pons, S.J . fhys. Chem. 1985, 89,5537. Schuette, S.;McCreery, R. L. Anal. Chem. 1986, 58, 1776-1782. Kuwana, T.; Winograd, N. I n Electronanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 1-74. Heineman, W. R . Anal. Chem. 1978, 50,390A.
2777
(22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; Chapter 14. (23) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. I n Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, P 1. (24) Muller, R. H. Adv. flectrochem. Electrochem. fng. 1973, 9 , 281-368. (25) Srinivasan, V. S. Adv. Electrochem. Electrochem. Eng. 1973, 9 , 369-422. (26) Kondo, Y.; Awakura, Y. J. Electrochem. SOC. 1976, 723. 1184. (27) McLarnon, F. R.; Muller, R. H.; Tobias, C. W. flectrochim. Acta 1978, 27, 101. (28) Matysik, J.; Chrnlel, J.; Cieszyk-Chmiel, A. J . flectroanal. Chem. 1985. 795,39. (29) Fukunaka, Y.; Minegishi, T.; Nishioka, N.; Kondo, Y. J . Electrochem. SOC. 1981, 728,1274. (30) Hsueh, L.; Newman, J. Electrochim. Acta 1971, 76,479. (31) Awakura, Y.; Okada, M.; Kondo, Y. J. flectrochem. SOC.1977, 724, 1050. (32) Pawiiszyn, J.; Weber, M. F.; Dignarn, M. J.; Mandelis, A,; Venter, R. D.; Park, S.Anal. Chem. 1986, 58, 236. (33) Pawliszyn, J.; Weber, M. F.; Dignam, M. J.; Mandelis, A.; Venter, R. D.; Park, S.Anal. Chem. 1986, 58, 239. (34) Pruiksrna, R.; McCreery, R. L. Anal. Chem. 1979, 57,2253. (35) Pruiksrna, R.; McCreery, R. L. Anal. Chem. 1981, 53, 202. (36) Ikeshoji, T.; Sekine, T. Denki Kagaku 1977, 45,575. (37) Aamodt, L. C.; Murphy, J. C. J. Appl. fhys. 1981, 52, 4903. (38) Royce, B. S.H.; Mandelis, A. Appl. Opt. 1984, 23, 2892. (39) Rossi, P.; McCurdy, W.; McCreery, R. L. J. Am. Chem. SOC. 1981, 703, 2524. (40) Rossi, P.; McCreery, R. L. J . flectroanal. Chem. 1983, 157,47. (41) Jan, C.-C.; Lavine. B. K.; McCreery, R. L. Anal. Chem. 1985, 57, 752. (42) Fukunaka, Y.; Denpo, K.; Iwata, M.; Maruoka, K.; Kondo, Y. J . f l e c trochem. SOC. 1983, 730, 2492. (43) Jan, C.-C.; McCreery, R. L.; Gamble, F. T. Anal. Chem. 1985, 57, 1763. (44) Galus, 2 . Fundamentals of Electrochemical Analysis ; Haisted: New York, 1976; p 101. (45) Engstrorn, R. C.; Weber, M.; Wunder, D. J.; Burgess, R.; Winquist, S. Anal. Chem. 1986, 58, 844-848. (46) McLarnon, F. R.; Muller, R. H.; Tobias, C. W. J . Electrochem. SOC. 1975, 722, 59.
RECEIVED for review April 3, 1986. Accepted July 1, 1986. This work was supported by the NSF Division of Chemical Analysis.
Liquid Chromatography with Electrochemical Detection and Coulometric Investigations of Carbamate and Urea Pesticides Q. Gordon von Nehring,' J. West Hightower, and James L. Anderson* Department of Chemistry, University of Georgia, Athens, Georgia 30602
Llquld chromatography wlth electrochernlcal detection has been aopiled for the detennlnatlon of selected urea pesticides, uslng a Kelgraf compostte electrode. Detection llmlts of 62-410 pg/L (1-8 ng) were obtalned In a water matrix wlthout sample pretreatment. A flow lnjectlon coulometrlc system based on a retlculated vltreous carbon worklng electrode was applied for the evaluation of electron transfer stolchlometry (nvalues) for oxldatlon of carbamate and urea pestlcldes. Excellent precision was obtained wlth relatlve standard devlatlons of less than 1% for coulomeirlc n values of nanomole pestlclde quantttles over a wlde range of operatlng condltlons.
Increasingly widespread use of carbamate and urea pesticides has spurred the development of sensitive new detection Present address: Dow Chemical Co., M i d l a n d , MI 48640.
schemes for their determination in water and other media. Liquid chromatography (LC) detection methods are particularly promising to avoid problems due to the thermal lability of these compounds. Conventional ultraviolet absorption detection frequently lacks sensitivity and selectivity for application t~ typical groundwater analysis. This work evaluates the efficacy of liquid chromatography with electrochemical detection (LCEC) for the direct detection of these pesticides in water and demonstrates the value of flow injection coulometry to evaluate the electron transfer stoichiometry of nanomole quantities. Anderson and co-workers ( I , 2 ) have reported the application of Kelgraf composite electrodes to the electrochemical detection of selected carbamates with picogram detection limits. The present work extends the previously reported work with carbamates ( I , 2 )to selected urea pesticides, using the Kelgraf electrode. For comparison, Hill et al. (3) have recently reported a sensitive postcolumn reaction for fluorescence detection of the carbamate Aldicarb. This latter approach has been of limited value for the urea pesticides, leading to interest in the
0003-2700/86/0358-2777$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
LCEC approach as an alternative. Recently Frei e t al. ( 4 ) reported excellent detection limits through the combination of on-line preconcentration coupled with electrochemical detection a t a glassy carbon electrode for selected urea pesticides. Since the sensitivity of response varied significantly between different pesticides, flow injection coulometry was used to assess the number of electrons transferred per molecule and to determine whether LCEC sensitivity was influenced primarily by electron transfer stoichiometry or kinetic limitations in the electron transfer process. Coulometric analysis in flowing streams has been extensively reviewed in the recent literature (5-16). The use of porous electrodes has been treated in detail by Sioda e t al. (16-19), as well as other workers (20-22). One of the most promising porous electrode materials for oxidative reactions is reticulated vitreous carbon (RVC), as reviewed by Wang (22). This material is quite inert and can be fashioned into a variety of shapes. T h e recent literature contains numerous applications of RVC as an electrode for coulometric and other analyses (23-31). Flow coulometric methods have therefore been used almost exclusively for the quantitation of analytes (5-16, 23-31) or purity analysis of analytes of known electron transfer stoichiometry (15). We present here the application of flow injection coulometry for the precise evaluation of the electron transfer stoichiometry of nanomole quantities of samples, an application which has apparently been briefly reported only once before (32),for much higher concentrations (ca. 2 mM) than reported here. Relative standard deviations less than 1% have been achieved in the present work over a range of operating conditions, significantly surpassing the performance of the more conventionally applied approaches of bulk or thin-layer coulometry for the nanomole sample quantities and rather positive potential range investigated here. The system has been used to evaluate the stoichiometry of oxidative electron transfer for selected carbamate, urea, and related pesticides. These results have been correlated to the observed LCEC response of these compounds, largely accounting for the variability of detector sensitivity for different species. EXPERIMENTAL S E C T I O N Chemicals. Reagent grade chemicals were used unless noted otherwise. The methanol (MeOH) was Burdick & Jackson Distilled in Glass grade or Baker Resi-Analyzed, used without further treatment. House-distilled water was purified using a Barnstead Nanopure system equipped with one organic purifying cartridge, two mixed-bed, high-purity ion exchange cartridges, and a submicrometer filter cartridge in series. Analytical standard herbicides and pesticides were obtained from the US. environmental Protection Agency standards repository. Samples of l,l’-bis(hydroxymethy1)ferroene (BHMF) were synthesized by reduction with lithium aluminum hydride of the dimethyl ester of ferrocene-1,l’-bis(carboxy1ic acid). Solvents were vacuum filtered through a 0.45-pm Pall Ultipor NX membrane filter prior to use. An aqueous buffer of Na2HP0, (0.010 M), KHZPO, (0.015 M), and NaOOCCH3 (0.10 M) adjusted to pH 7.00 was mixed I:1 with methanol to prepare the chromatographic eluent and flow injection carrier stream. Cells. The thin-layer LCEC detection cell, with 50-pm-thick gasket and 2-pL cell volume, was as described previously ( I , 2 ) . The reference (Ag/AgC1/2 M KCl) and auxiliary electrodes were located immediately downstream. The flow coulometric cell, shown in Figure 1, was similar in design to cells of Richards and Evans (13),Schieffer (15),and Curran and Tougas (31). The outer body was a heavy-wall (13 mm 0.d. X 8 mm i.d.) glass tube equipped with two side tees capped by 7/25 ground glass joints. These joints were used to introduce the auxiliary (platinum and reference (Ag/AgC1/1 M KCl) electrodes. The reference electrode was isolated from the auxiliary chamber by a porous Vycor (Corning Glass) plug. The inner tube of the cell was a porous Vycor tube, which isolated the working electrode chamber from the reference/auxiliary electrode chamber. The auxiliary electrode
AghgCl
1 F
REFERENCE
’L -
SEPTUM ,,,
J
VITON
dING
1
MONOLITHIC 100M
Figure
1.
PL~~~N~MCONTACT
m
Flow coulometric cell.
(26-gauge platinum wire) was wrapped around this porous Vycor tube to reduce the distance and resistance between the working and auxiliary electrodes to a minimum. The porous Vycor tube contained a single monolith of 100 pore/in. RVC (ERG, Inc., Oakland, CA), cut with a cork borer to yield an electrode 4.8 mm 0.d. X 5 cm in length. The cell was mounted in an aluminum cylinder with a screw cap closure to seal the poly(viny1 chloride) end caps with uniform compression against the cell body with O-ring gaskets. The body of the holder was cut out for insertion of the cell and access to the side arms. The porous Vycor tube was sealed by the close tolerance fit into the end fittings. A platinum wire served as the working electrode connection, introduced via a septum and a tee connector in the inlet line. The outer cell auxiliary chamber was filled with eluent solvent drawn from the solvent reservoir. The solution in the auxiliary chamber required only occasional replacement, which was accomplished via the reference electrode port. Apparatus. The liquid chromatographic system, equipped with a 20-pL sample injection loop, was as described previously (2). An HPLC Technology column (25 cm X 4.6 mm i.d.) was packed with 5-pm Hypersil SAS particles. The coulometric system used the I,C system as a well-regulated flow system. A Valco sample injection valve (50-pL loop volume) equipped with an air actuator was connected at the outlet of the LC via l/ls-in.-o.d. X 0.25-mm-i.d. Teflon tubing. This valve introduced the sample to the cell inlet tee pictured in Figure 1, using Teflon tubing. A BAS cyclic voltammetry potentiostat (Model CV-1B) modified for nulling of background current was connected to the cell. Charge was integrated for 80 s with a locally constructed integrator based on a Philbrick Model 1429 chopper-stabilized operational amplifier, 10-pF precision Mylar capacitor (Plastic Capacitor, Chicago, IL), and switch-selectable precision input resistances. The current-time curves were recorded on a Houston Instruments Omniscribe 5000 strip chart recorder, and the integrated charge output was read by using a Data Precision Model 259 digital multimeter. All times were recorded by stopwatch. The coulometric system routinely used injections of 50-pL samples at M into the flowing solvent. This concentration level gave a good signal-to-noise ratio while minimizing electrode fouling. Between runs, the system was operated a t the desired applied potential with solvent recirculation through the cell in a closed loop. This approach allowed the solvent background to converge to a constant level, while conserving solvent. The solvent was only allowed to flow to waste when samples were being injected. The background current signal was obtained by injecting an aliquot of carrier solution and integrating for an 80-s period just prior to the introduction of the sample. This observed background was then subtracted from the sample signal integral to yield a corrected signal. The system was calibrated using 1,l’-bidhydroxymethy1)ferrocene or ferrocene as a standard, known to undergo a simple one-electron oxidation. R E S U L T S A N D DISCUSSION The responses of the ureas in the LCEC experiment are shown in Table I. The observed detection limits range from ca. 1 to 8 ng injected (ca. 60-410 pg/L), with no sample pretreatment or preconcentration, somewhat higher than those
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table I. LCEC Response of Selected Urea Pesticides
compd BHMF" Metoxuron Monuron
Monolinuron Chlortoluron Fluometuron Metobromuron Diuron Difenoxuron
Linuron Siduron Chlorobromuron Neburon
estimated detection limits M rg/L ng
retention capacity time, min factor (k? 6.0 7.0 8.2 10.0 10.4 11.0 11.0 12.5 14.0 16.5 17.0 18.0 36.0
0.71 1.00 1.34 1.96 2.00 2.10 2.14 2.60 3.00
2.2 X 2.7 x 2.9 X 6.7 X 6.7 x 9.8 x 6.5 X 5.0 X 2.5 X 7.4 x 5.4 x 6.9 X 1.5 X
3.88 3.90 4.10 9.30
Used for calibration-not
a
10-7 10-7 10-7
10-7
10-7
10"
5.4 62 58 140 140 230 160 120 72 180 120 200 410
0.11 1.2 1.1 2.7 2.7 4.6 3.2 2.4 1.4 3.6 2.4 4.0 8.2
urea.
The steady-state limiting current ZL for continuous flow of a sample stream through a porous electrode has been shown (19) to be
ZL = nFuC*R
J U ( 0
20
(1)
where n is the number of equivalents transferred per mole, F is the Faraday constant, u is the volumetric flow rate (cm3/s), and C* is the bulk concentration of the analyte (mol/cm3). The coulometric conversion efficiency, R (fraction of C* which reacts, which should ideally approach unity (100%)) is given by the expression
R = 1 - exp(-jsa'-"v*-'l)
(2)
where j is a proportionality constant and a is a dimensionless flow coefficient (0 5 a 5 l),which both depend on flow rate, s is the specific surface area per unit volume (66 cm2/cm3(22)), a is the cross sectional area of the flow stream (cm2),and L is the electrode length (cm). Parameter a is determined from the relation a = VRvc/L = 0.175 cm2,where VRv, = 0.878 cm3, assuming a pore volume of 97% of total volume for the RVC electrode. For flow injection coulometry under mass transfer control, the charge Q L is given by the expression QL
i
li:
2779
= nFVoC*R
(3)
where Vois the volume injected (cm3). The integration time is not critical, provided that the time is long enough for the sample to pass completely through the detector cell, but not so long that excessive integration of background current introduces errors. The coulometric efficiency,R, can be estimated, if the value of n is known (31),from rearranging eq 3 as follows:
4
R = QL/nFVoC*
40
TIME ( m i d
Typical chromatogram of urea pesticides: (1) 3,4dlchloroanllne, 1 X lo4 M; (2) Diuron, 5 X lod M; (3) Unwon, 5 X lod M; (4) Neburon, 5 X lo-' M. Flgure 2.
reported by Frei et al. (4), taking into consideration their use of preconcentration. A typical chromatogram is shown in Figure 2 for a flow rate of 1.0 mL/min and an applied potential of 1.15 V vs. Ag/AgC1/2 M KC1. These detection limits are comparable to detection limits obtainable by UV and postcolumn reaction fluorescence ( 3 ) ,when considering the quantity of material detected. Sparacino and Hines (33) showed similar detection limits with UV for selected carbamates through the use of preconcentration. The detection limits electrochemicallyobtained vary over as much as a factor of 4 for species of similar capacity factor (k'), and the relative response is not simply related to molecular structure. The flow injection coulometric experiments were therefore undertaken to supplement preliminary cyclic voltammetry data and clarify the responses observed for both the ureas and carbamates. Introduction of a sample aliquot to an electrode held a t a constant potential avoids the long background equilibration time and large background correction associated with the potential step methods. This advantage is significant a t the relatively positive potentials used here, where currents due to oxidation of the solvent, supporting electrolyte, and electrode surface may be larger than that due to the analyte. In addition, the ease of making background measurements immediately before and after sample injections under identical conditions enables rapid and precise background correction and allows continuous assessment of any alterations of electrode response with time. This approach facilitates the measurement of small sample quantities, even a t high oxidative potentials.
(44
As a corollary, the number of electrons transferred is estimated as
n = QL/FVoC*R
(4b)
In all cases, R must either be determined or corrected for (via normalization to a standard of known n value) to assess absolute coulometric response. The flow dependence of coulometric efficiency, R, can be estimated by rearranging and evaluating expression 2 as follows:
P = In
[-ln (1- R ) ] = In (jsa'-*L)
+ ( a - 1) In u
(5)
The value of a is obtained from the slope of a plot of P us. In u. The value of j is estimated from substitution of the values of a and the other parameters into eq 5. Average values of R of 0.914,0.855, 0.850, and 0.795 were obtained for flow rates, u, 0.0100,0.0133,0.0167,and 0.0222 cm3/s, respectively, based on composite results for both ferrocene, used as a reference standard with n = 1.0, and two concentrations of Aminocarb, a carbamate pesticide found to have n = 4.0 (see later discussion). The values of a = 0.485 f 0.093, j = 0.0017 f 0.0006, and R = 0.85 at a flow rate of 1 cm3/min (0.01667 cm3/s) are lower than the values of a = 0.73, j = 0.016, and R = 0.9999 a t a flow rate of 1 cm3/min, estimated for a nominally comparable RVC sample from the data of Blaedel and Wang (26),but more comparable to the values of a = 0.42, j = 0.0089, and 0.90 5 R 5 1-01at the flow rate of 1cm3/min from the data of Curran and Tougas (31). The experiments of Blaedel and Wang were conducted under steady-state conditions a t slower flow rates and greater cross-sectional areas, with the species of interest continuously present in the mobile phase. Curran and Tougas used both the steady-state approach of Blaedel and Wang and the finite sample injection method reported here. Their steady-state
2780
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table 11. Aminocarb Response Corrected to Ferrocene
flow rate, mL/min
701 mV
800 mV
1.33 Ab
1.84 1.77 2.15 2.08 2.32 2.27 2.73 2.68
3.72 3.44 3.82 3.47 4.01 3.79 4.17 3.98
B' 1.00 A
B 0.80
B 0.60 B
response, equiv/mol, at given applied potential 900 mV 1000 mV 1050 mV 1100 mV 3.70 3.70 3.80 3.81 3.76 3.84 3.89 3.97
Based on estimated extrapolation of ferrocene correction.
3.97 3.80 4.12 3.87 4.07 3.85 2.5
X
3.88 3.78 4.08 3.97 4.03 4.01 4.02 4.04
1300 mV
4.06 3.92 4.18 4.06 4.28 4.14 4.29 4.65
M Aminocarb.
5.0
X
4.08" 3.93" 4.Ola 3.95" 4.10a 4.08" 4.02"
1310 mV 4.19" 4.34" 4.17n 4.07 4.32 4.19
M Aminocarb.
Table 111. Coulometric Response of Selected Urea Pesticides response, equiv/mol (70RSD), at given applied potential 1100 mV 1250 mV 1300 mV
compd 08
IO
I2
P O T E N T I A L ( V v s Ag/AgCII
Figure 3. Normalized hydrodynamic voltammogram of Aminocarb.
results (0.90 IR 5 1.01) for a series of compounds including hydroquinone (R = 1.00) bracketed the flow injection result for hydroquinone ( R = 0.96). The coulometric efficiency in the present work approaches 100% only a t the lower flow rates and a t the higher applied potentials. From the above data, it is estimated that a value of R > 0.99 would be achieved for u < 0.15 mL/min. An electrode of smaller pore size and less void volume, with corresponding increases in surface area, would improve the performance of this system considerably, as might an increase of the area of the auxiliary electrode, which was signficantly less than that of the RVC working electrode. Flow rates of 0.6, 0.8, 1.0, and 1.33 mL/min were used, with current integration for 125, 100, 80, and 60 s, respectively, to allow the sample to pass completely through the working electrode. The volume of the RVC electrode plus connecting tubing was less than 1 mL. The potential range from 0.7 to 1.3 V (vs. Ag/ AgC1/1 M KC1) was investigated. The need for extrapolation of the results back to zero flow rate was obviated for the actual measurements of interest, assuming that no slow chemical steps limited the reaction rate, by alternating each unknown sample with a ferrocene sample to obtain a calibration standard at each measurement point and normalizing the pesticide response to the known ferrocene response. In this case, the R terms for the pesticide and for ferrocene essentially cancel, and the normalized n values obtained become independent of flow rate, as long as R is reasonably close to unity and the effects of differences in mass transfer coefficients are small. This internal correction of pesticide response is illustrated in Table I1 for Aminocarb, showing consistent response for a series of injections of 2.5nmol quantities of Aminocarb. For a series of injections at a flow rate of 1.00 mL/min, and for potentials of 1050 mV or greater vs. Ag/AgCl, the normalized n values was 4.03 0.01, with a relative standard deviation (RSD) of 0.2% of charge before normalization. The observed absolute precision for Aminocarb as well as the remaining carbamate and urea pesticides was consistently better than was observed for ferrocene (with RSD of LO%), so that the precision of normalized n vaues was limited primarily by the precision of the ferrocene charge measurements. The normalized Aminocarb response for a flow rate of 1.0 mL/min and injection of 2.5-nmol aliquots is shown as a function of potential in Figure 3. The response is essentially
*
BHMP Metoxuron Monuron Monolinuron Chlorotoluron Fluometuron Metobromuron Diuron Difenoxuron Linuron Chlorobromuron Neburon Not a urea.
0.96 (0.50) 2.10 (0.39) 1.90 (0.57) b 1.51 (0.59) b b 1.22 (0.35) 4.01 (0.25) b b 1.22 (0.69)
0.98 (1.90) 1.76 (0.50) 2.67 (1.11) 2.22 (0.61) 2.28 (0.41) 1.20 (0.81) 2.32 (0.81) 1.75 (0.62) 3.83 (0.30) 1.17 (1.65) 1.56 (1.50) 2.03 (0.71)
1.09 (1.45) 1.84 (0.60) 2.51 (0.62) 2.04 (0.57) 2.30 (0.67) 1.58 (0.98) 2.28 (0.70) 1.78 (1.81) 4.22 (0.66) 1.42 (1.19) 1.77 (0.90) 2.20 (0.60)
Poor response.
Table IV. Coulometric Response of Selected Carbamate Pesticides
compd
response, equiv/mol (70RSD), at given applied potential 1250 mV 1300 mV
comment
Aminocarb
4.08 (0.46) 4.03 (0.40) (4.02 (0.22) at 1150 mV) Asulam 2.53 (0.30) 2.17 (0.96) a 1.0 (0.68) Carbaryl M; broad, Carbendazim 4.74 (0.97) (2.0 X unresolved signal) Chlorpropham 1.35 (0.86) 1.61 (1.0) Desmedipham 5.98 (0.21) 5.54 (1.1) (2.5 X M) 6.23 (1.5) 6.20 (0.71) (1.0 X 10" M) M) Phenmedipham 5.05 (0.72) 6.09 (0.28) (2.5 X 5.43 (2.2) 6.24 (1.1) (1.0 x 10-5 MI Chloramben 2.50 (0.62) 3.33 (0.44) Dichloran 1.62 (1.7) 1.77 (0.50) a Picloram a Poor response.
constant from 900 mV out to the positive potential limit. Error bars indicate plus and minus 1standard deviation from mean values. The slight rise at the potential limit of 1310 mV is due to the extremely high background signal, which degrades the signal-to-noise ratio and thus limits the accuracy of the background correction. The coulometric system was used to evaluate the response of selected urea and carbamate pesticides at several potentials. The results of this study are shown in Tables I11 and IV, respectiveljl. The coulometric responses of the ureas (11 compounds) and carbamates (10 compounds) parallel closely the observed detection limits of the LCEC experiments described above for the ureas and reported previously ( I , 2) for
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
the carbamates. I t is apparent that most of the ureas and carbamates investigated undergo multielectron oxidations involving a t least 2 electrons/molecule. However, in most cases the ultimate number of electrons transferred could not be ascertained with certainty because rapidly increasing background current precluded experiments at more positive potentials. The results can be divided into three general cases. The compounds showing little or no coulometric response were found to give poor LCEC detection limits. These compounds showed no discernible wave in cyclic voltammetry, indicating that their oxidation potentials lie a t potentials more positive than investigated under these conditions. Those compounds showing a rise but no plateau in coulometric yield with increasing potential gave reasonable detection limits in LCEC, but are characterized by response which depends on the applied potential, consistent with measurements taken on the rising portion of the oxidation wave. The good precision obtained for these compounds in our coulometric measurements attests to the good potential control achieved in our cell design. The compounds, as exemplified by Aminocarb, which show a stable coulometric yield over a wide range of conditions and exhibit a current plateau of the oxidation wave at attainable detector applied potentials, show good electrochemical response in the LCEC experiment as noted by ourselves, Frei et al. ( 4 ) ,and Sturrock et al. (34). Detection limits improve as the coulometric n values increase, although later eluting compounds show poorer detection limits than earlier eluting compounds due to on-column dilution, since the peak sample concentration in the eluent is inversely proportional to (1+ k'), where k'is the capacity factor for that component (35). The results obtained for the ureas and carbamates raised the question of the oxidative pathway. Kissinger et al. (36, 37) had proposed that these compounds must undergo hydrolysis to the corresponding phenol or amine to be detectable in their scheme, partially because of constraints on maximum applied potentials in their investigations and partially because they looked at compounds with rather positive oxidation potentials. The known hydrolysis rates for ureas to the amine are quite slow, ruling out this pathway as a significant direct oxidation pathway (e.g., a CE mechanism, involving hydrolysis as the chemical step prior to electron transfer). The proton NMR spectra of the amines are also well-known. We undertook a preliminary study by 'H NMR to evaluate the oxidation products of the ureas. These initial studies showed no sign of the corresponding amines, either before or after oxidation, nor were the corresponding oxidation products of the hydrolyzed species observed. These results clearly indicated that the oxidation pathway requires further investigation. In addition, the chromatographic retention times of several amines were found quite distinct from the retention times of the corresponding ureas, showing that hydrolysis is not necessary to generate the electroactive species. Studies of carbamate reaction products using NMR are described elsewhere (38). The observed detection limits obtained here for the ureas by LCEC are comparable to those obtained by other workers using other detection methodologies, as noted above. These results are certainly not as promising as those achieved for the carbamates, although they are not surpassed by alternate detection schemes. The work of Frei et al. ( 4 ) indicates that some improvement in LCEC detection limits is possible for the ureas. The flow coulometric approach proved to be effective for assessment of the electron transfer stoichiometry, using na-
2781
nomole quantities of material, with good precision. The observed precision and accuracy compare favorably with results noted in a review by Ostrovidov (39),especially considering the small sample quantities used here. The approach was quite useful in clarifying the LCEC detection limits obtained for different pesticides. Registry No. BHMF, 62524-59-8; metoxuron, 19937-59-8; monuron, 150-68-5; monolinuron, 1746-81-2; chlorotoluron, 15545-48-9; fluometuron, 2164-17-2; metbromuron, 3060-89-7; diuron, 330-54-1; difenoxuron, 14214-32-5; linuron, 330-55-2; chlorobromuron, 13360-45-7; neburon, 555-37-3; aminocarb, 2032-59-9; asulam, 3337-71-1; carbaryl, 63-25-2; carbendazim, 10605-21-7;chlorpropham, 101-21-3;desmedipham, 13684-56-5; phenmedipham, 13684-63-4; chloramben, 133-90-4; dichloran, 99-30-9; picloram, 1918-02-1;water, 7732-18-5.
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RECEIVED for review May 6,1986. Accepted July 8,1986. This project was funded in part by the Office of Research and Development, U.S. Environmental Protection Agency, under Grant R-808084-02. Additional funding was provided by the University of Georgia. The Environmental Protection Agency does not necessarily endorse any commercial products used in the study. The conclusions represent the views of the authors and do not necessarily represent the opinions, policies, or recommendations of the Environmental Protection Agency.
