The electroanalysis of organic pollutants in aquatic matrices

The Science of the Total Environment, 37 (1984) 71--81 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

THE ELECTROANALYSIS MATRICES

71

OF ORGANIC POLLUTANTS IN AQUATIC

WILLIAM FRANKLIN SMYTH and JOHN ANTHONY HEALY Department of Chemistry, University College Cork, Cork (Ireland)

ABSTRACT

The electroanalysis of organic pollutants in aquatic matrices is reviewed. Attention is directed to the application of polarography, voltammetry, stripping voltammetry and online electrochemical detection following high-performance liquid chromatographic separation to the identification and determination of trace concentrations of selected groups of organic compounds and their metabolites in aquatic matrices. These include carbonyls, simple aromatics, carboxylic acids, alkyl sulphonates, organophosphorus compounds, compounds containing endocyclic and exocyclic

"~c / = N/

groups, corn-

pounds containings - - S H groups, carbamates, halogenated anilines and benzidines. Comparisons are made between the results of these electroanalytical applications and those based primarily on alternative spectroscopic and chromatographic techniques. INTRODUCTION V o l t a m m e t r i c m e t l m d s , b a s e d o n t h e use o f t h e t e c h n i q u e s such as D C (direct current) and pulse polarography and v o l t a m m e t r y , anodic and c a t h o d i c s t r i p p i n g v o l t a m m e t r y a n d on-line e l e c t r o c h e m i c a l " d e t e c t i o n h a v e seen e x t e n s i v e e x p l o i t a t i o n a n d resulting diverse a n a l y t i c a l a p p l i c a t i o n in r e c e n t y e a r s f o r t h e d e t e r m i n a t i o n o f t r a c e c o n c e n t r a t i o n s o f m a n y organic m o l e c u l e s o f significance in e n v i r o n m e n t a l c h e m i s t r y [1, 2 ] . T h e p r o c e s s o f e l e c t r o n e x c h a n g e f o r m a n y organic m o l e c u l e s , o c c u r r i n g at a m e r c u r y or solid e l e c t r o d e - - a q u e o u s s o l u t i o n i n t e r f a c e is g o v e r n e d n o t o n l y b y t h e r a t e o f m a s s t r a n s p o r t t o t h e e l e c t r o d e s u r f a c e b u t also b y t h e r a t e o f e l e c t r o n t r a n s f e r itself. F a c t o r s such as t h e s t e r e o c h e m i s t r y o f i n t e r a c t i o n o f t h e molecule at the electrode surface and the protonation and adsorption r e a c t i o n s it, a n d / o r its e l e c t r o c h e m i c a l r e a c t i o n p r o d u c t ( s ) , u n d e r g o at this i n t e r f a c e will t e n d t o m a k e t h e overall e l e c t r o c h e m i c a l r e a c t i o n " i r r e v e r s i b l e " a n d t h u s r e d u c e t h e s e l e c t i v i t y o f p o l a r o g r a p h i c a n d v o l t a m m e t r i c analysis in t h a t b r o a d , ill d e f i n e d c u r v e s / p e a k s result. I t is f o r this r e a s o n t h a t s e p a r a t i o n t e c h n i q u e s s u c h as s o l v e n t e x t r a c t i o n , T L C a n d H P L C are f r e q u e n t l y a necessary pre-requisite for the polarographic or voltammetric determination 0048-9697/83/$03.00

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72 of trace concentrations of these organic molecules; this also decreases the effect of electro-active interferences in these aquatic matrices. Solvent extraction techniques can have the added advantage of pre-concentration of the organic molecules under investigation. Adsorption of the molecule and/ or its electrochemical reaction product(s) can give rise to post- and pre-waves which complicate DC polarography and to a lesser extent NPP (normal pulse polarography) where the potential is only applied to the electrode surface during the pulsing period. The p h e n o m e n o n of adsorption can be used to analytical advantage in stripping voltammetry where the p r o d u c t of the electrochemical reaction is adsorbed on the electrode surface by imposition of a suitable applied positive (CSV) or negative (ASV) potential for a selected time interval prior to stripping of the adsorbed species from the electrode by imposition of a potential scan in an opposite direction to the original applied potential. The sensitivity and limit of detection for a limited number of organic molecules can be improved by several orders of magnitude over the corresponding analytical parameters for differential pulse polarographic analysis (DPP) of these molecules [3]. On-line electrochemical detection with solid electrodes such as glassy carbon is inherently more sensitive than polarographic and voltammetric analysis in quiescent solution due to the convective element in addition to that of diffusion in the overall mass transport to the electrode surface. It is the purpose of this paper to c o m m e n t on the applicability of the aforementioned electroanalytical methods to selected organic molecules (and their metabolites) of significance in the analysis of aquatic matrices. Where possible, a critical comparison will be made between them and alternative analytical methods based primarily on chromatographic and spectroscopic techniques.

DISCUSSION

Carbonyl compounds Although the carbonyl group is electro-reducible, the little work that has been carried o u t on carbonyl c o m p o u n d s by direct polarographic methods has mainly been confined to aldehydes such as formaldehyde, acetaldehyde, b u t y r a l d e h y d e and furfural. Limits of detection in the low pg ml-1 region in waste waters have been reported [ 1 ]. The use of derivatization of the carbonyl group has been exploited by Afghan et al. [4] in a systematic study of these c o m p o u n d s in various supporting electrolytes, using twin cell potential sweep voltammetry. The formation of the semicarbazone was found to be most satisfactory. The formation of the semicarbazone is optimised at pH 4--6 to ensure that a significant fraction of the carbonyl c o m p o u n d is protonated and, at the same time, to ensure that the concentration of the p r o t o n a t e d amine remains low (it is the p r o t o n a t e d carbonyl c o m p o u n d and u n p r o t o n a t e d amine which react to form the semicarbazone). Excess of the electro-inactive semicarbazide is added to ensure complete reaction.

73 Using a citrate buffer with EDTA added to complex interfering heavy metals, a limit of detection of 0.25 ng ml- 1 was claimed for the determination of some carbonyl c o m p o u n d s present in natural waters and industrial effluents w i t h o u t any separation or preconcentration of the sample. The m e t h o d was, however, unable to differentiate many aliphatic and aromatic aldehydes and ketones. A recent paper [ 5] has improved upon the selectivity of this m e t h o d by the use of reversed-phase HPLC and UV detection at 3 6 0 n m . Vigh et al. [5] have achieved resolution and identification of the 2,4-dinitrophenylhydrazone derivatives of a range of aldehydes and ketones.

Simple aroma tic compounds (i) Nitrocompounds. Direct polarographic methods for nitrocompounds in waste water and effluents have been developed. For example, nitrobenzene in industrial wastes has been determined after separating nitrophenols by distillation. The supporting electrolyte consisted of aqueous ethanol containing hydrochloric acid. Nitrochlorobenzenes in water (0.005 #g ml- 1 ) have been determined following extraction with activated charcoal and polarography of the eluted acetone solution in a pyridinium hydrochloride supporting electrolyte. These and other references are collected in ref. 1. (ii) Phenolic compounds. Phenols are not reducible at the DME (dropping mercury electrode) but do exhibit anodic waves at a variety of solid electrodes [6]. Of particular electroanalytical significance is the work of Kissinger and co-workers [7, 8] who have been able to determine trace concentrations of biologically important c o m p o u n d s such as neurotransmitters in b o d y fluids following HPLC separation and electrochemical detection at a semi-micro carbon paste electrode. This work has been reviewed in a recent article. Until these developments in electrochemical detection in conjunction with HPLC, most polarographic methods for the determination of phenolic c o m p o u n d s were based on prior derivatisation procedures. In particular, the use of nitration has found many applications for the determination of these c o m p o u n d s in biological fluids, e.g., phenol in water [ 1 ] has been determined by this procedure using pulse polarography with a q u o t e d limit of detection of 1 ng ml-1. A r m e n t r o u t et al. [9] have recently selectively detected individual phenolic compounds, including the more toxic halogenated ones, at low ppb levels using HPLC with a polymeric cation exchange resin column, acidic acetonitrile/water eluent and an electrochemical detector containing a unique carbon-black/polyethylene tubular anode. The detection limits of selected phenols are given in Table 1 [9] and these compare more than favourably with those obtained by temperature programmed GLC and electron capture detection (ECD) of hepta-fluorobutyryl derivatives of phenols [10] and HPLC--UV detection of nitrophenols, solvent extracted from water samples by CH2CI 2 [ 1 1 ] .

74 TABLE 1 DETECTION LIMITS OF SELECTED PHENOLS Concentration (ppb) Phenol Monochlorophenols 2,6-Dichlorophenol 2,4-Dichlorophenol 2,4,6-Trichlorophenol 2,4,5-Trichlorophenol 2,3,4,6-Tetrachlorophenol Pentachlorophenol

0.2 0.4 0.5 0.5 0.6 0.6 0.7 0.7

(iii) Polychlorinated compounds. Organochlorine pesticides in aquatic matrices are usually determined by glass capillary GLC where the electron capture and coulometric detectors offer sensitivity and selectivity n o t usually obtained by other methods involving s p e c t r o p h o t o m e t r y or polarography/ voltammetry e.g., Babkina et al. [12] have determined a range of organochlorine pesticides (including dichlorvos, ppl-DDT, ~- and T-BHC) with sensitivities in water samples in the range 5--60 pg 1-1 . Recently, Farwell et al. [ 13] have reported on the use of interrupted sweep voltammetric analysis for the identification of polychlorinated insecticides and other polychlorinated aromatics. The ' apparatus consisted of a 3-electrode potentiostaticallycontrolled circuit with a logic-controlled interrupted linear voltage sweep mechanism. Using DMSO/0.1M TEAB as the supporting electrolyte, voltammograms for many polychlorinated aromatics have been obtained. However, at least 9 m g of pure c o m p o u n d are required for positive identification. The technique is therefore of little value in residue analysis but could find more application in the identification of isomeric products formed during the synthesis of polychlorinated agrochemicals. The peak with the most negative potential corresponds to naphthalene and the remaining reduction peaks represent the step-wise removal of C1 atoms. Carboxylic acids Materials of the amino polycarboxylic acid type can be determined polarographicaUy, by observing the reduction of a heavy metal--ligand complex, in the presence of excess metal ions. The reduction potential of this complex is shifted to more negative values, and the limiting current of this reduction is used for the quantitative determination of the ligand. Haberman [14] demonstrated how nitrilotriacetic acid (NTA), which is a detergent builder with the formula N(CH3COOH)3, could be determined using In(III). After an excess of In(III) had been added to the NTA in the aqueous solution, the free metal gave a polarographic wave and the limiting current of the complexed In(III) could be found by difference. Studies on metal--NTA complexes have shown t h a t the o p t i m u m pH for trivalent metal

75 TABLE 2 LINEAR ALKYL BENZENE SULPHONATE CONTENT OF SEWAGE DETERMINED BY SPECTROPHOTOMETRIC AND POLAROGRAPHIC PROCEDURES Day of sampling

Source of sample, a sewage stream

Concentration ( tlg ml- 1) Spectrophotometric method

Polarographic method

A B C

2.9 0.8 42.0

0.6 1.9 2.9

A B C

16.8 67.0 69.3

0.3 1.4 22.3

A

4.3

B

12,0

C

77.5

0.2 1.4 2.9

aSewage stream A -- detergent-free sewage; B ----normal sewage; C ----dosed sewage. ion c o m p l e x a t i o n is pH 2 and for divalent metal ions such as Cd(II) and Pb(II), p H 7 . Using an anion exchange col um n to c o n c e n t r a t e NTA, H a b e r m a n [14] was able t o determine 0.025 gg m1-1 when I n ( I I I ) w a s used as the complexing metal. Afghan et al. [ 1 5 ] has improved the limit of d e t e c t i o n of this m e t h o d t o 0 . 0 1 p g m1-1 using a Bi c o m p l e x and has a u t o m a t e d the p r o c e d u r e (15 samples h-1 ).

Alkyl sulphonates and alkyl benzene sulphonates Polarographic m e t h o d s such as depression of m a x i m a m e t h o d s and t e n s a m m e t r y have been developed for anionic, cationic and neutral surfactants [1] but, in general, t h e y are n o t specific t o surfactant t y p e and an effective separation is a prerequisite to d e t e r m i n a t i o n in real situations. Ion exchange is the m o s t generally used surfactant separation m e t h o d , either anionic or cationic t y p e being separated f r o m nonionic by suitable choice of resin. Hart et al. [16] have d e t e r m i n e d the linear alkyl benzene sulphonate c o n t e n t of sewage and tap water samples by an indirect polarographic m e t h o d based on nitration of the aromatic ring. The m e t h o d was proved reliable when c onc e nt r at i ons were of the order 0.5 pg m l - 1 or greater and it was f o u n d m o r e selective t han the colorimetric m e t h y l e n e blue m e t h o d which gives a measure of total anionic surfactant present in the sample (Table 2). Reversed-phase HPLC with fluorimetric det ect i on at 295 nm has been used f or the resolution and quantitative d e t e r m i n a t i o n o f sodium alkylbenzenesulphonates [ 1 7 ] . Sensitivities of 0.097 pg m1-1 in river water samples were r e p o r t e d .

76 Organophosphorus compounds GLC with flame photometric or thermionic detection is commonly employed for the determination of organophosphorus compounds in samples of environmental significance. However, "cold" methods such as HPLC, polarography/voltammetry are increasingly being investigated as alternative methods due to the instability of these insecticides under the conditions employed in GLC analysis. (i) D e t e r m i n a t i o n o f \P- and /II s

\P - s / IL

containing c o m p o u n d s . Nangniot

[18] has studied the polarographic behaviour of a wide range of organophosphorus pesticides. Although phosphoric acid esters cannot be reduced at the DME, compounds containing the above functional groups can produce sharp adsorption peaks using faster linear sweep voltammetric techniques (250mV s-l). This method could determine concentrations of selected organo-phosphorus compounds at the pM level. Cathodic stripping voltammetry (CSV), following alkaline hydrolysis of these thiophosphates to s release sulphur-containing molecules such as (R0)2e~0 capable of forming insoluble mercury salts, has been applied to the determination of the insecticides, phthalophos and benzophosphate in apples with a detection limit of 0.2pg kg-1 [1]. (ii) D e t e r m i n a t i o n o f NO2 containing c o m p o u n d s . Perhaps most applications of polarography for the determination of nitro-containing agrochemicals have come from investigations of nitrophenyl esters, e.g. parathion, fenitrothion, etc. Smyth and Osteryoung [19] have made a detailed pulse polarography study of parathion, methyl parathion, paraoxon and other structurally related organophosphorus agrochemicals and their metabolites but were unable to use the differences observed in their behaviour for quantitative purposes. Structurally related nitrophenyl esters should therefore be separated by a chromatographic procedure prior to analysis. (iii) E n z y m o p o l a r o g r a p h i c determination. Davidek and Seifert (cited in ref. 1) have developed an enzymopolarographic method for the determination of the organophosphorus pesticide intration. The method was based on the inhibition of anticholinesterase activity by the organophosphorus moiety (a reaction which parallels the in vivo biological activity of these compounds). Unreacted enzyme is then incubated with fl-naphthyl acetate and the fl-naphthol liberated measured by polarography following nitrosation. The method was applied to the analysis of intration in lettuce, cabbage, cherries and tomatoes. Naturally occurring enzymes which would be capable of hydrolysing fl-naphthyl acetate were removed by precipitation

77 with C2HsOH and subsequent centrifugation. No interference was observed in the presence of carotenes, xanthophyll, chlorophyll or anthocyanidines and the m e t h o d was f o u n d to be relatively simple and rapid to perform. (iv) Determination by derivatisation. The herbicide glyphosate (Nphosphonomethylglycine) (I) was found to be more conveniently analysed by polarography than by GLC [ 2 0 ] . For the latter m e t h o d a lengthy fourstage clean-up and two-stage derivatization procedure was necessary, whereas ion exchange followed by nitrosation only was required for polarography. The eluate from ion exchange was treated with 50% sulphuric acid and potassium bromide and sodium nitrite solutions. After 15 min, ammonium sulphamate was added to destroy excess nitrite and polarography was carried o u t after de-aerating with nitrogen. O

0

0

O

II

II

II

II

OH-C- CH2-1~I- CH2-- I~--OH H

OH --C-- CH 2- NI- CH2- P-[ OH

OH

NO

(I)

OH

(El

The N-nitroso derivative {II) gave a reduction peak of -- 0.78 V, which could be used to monitor between 35 and 210 ng ml-1 of glyphosate, in natural waters. It was suggested that one analyst could analyse t w e n t y samples a day, using the above procedure. Compounds containing endocyclic and exocyclic /.C=N \

z groups. Polaro-

graphic m e t h o d s of analysis have found widespread application for the determination of many drug substances containing the azomethine group e.g., 1,4-benzodiazepines, antidiabetic c o m p o u n d s and benzhydrylpiperazine derivatives [1]. Separation procedures involving solvent extraction, TLC and HPLC are increasingly a necessary pre-requisite for the simultaneous determination of mixtures of trace amounts of the parent c o m p o u n d and its metabolites particularly in complex matrices such as blood [ 1 ]. The triazine pesticides, terbutryne, ametryne and atrazine, which contain reducible e n d o c y c l i c /\c - - -

N/

bonds, have been determined [21] in p o n d and

canal water with a limit of detection of 5 ng ml- l following extraction with dichloromethane, evaporation of the solvent, dissolution of the residue in 50% m e t h a n o l / 0 . 0 0 5 M sulphuric acid and application of linear sweep voltammetry. The polarographic m e t h o d matched a GLC procedure in terms of time of analysis b u t was inferior with respect to sensitivity and selectivity. S m y t h and Osteryoung [22 ] have investigated the polarographic behaviour of agrochemicals, cytrolane (IV), cyolane (III), chlordimeform (V) and drazoxolon, all of which contain exocyclic /\ c = N/ groups, and r e c o m m e n d e d o p t i m u m conditions for their determination by pulse polarography.

78

O Lf (C 2 HsO) 2 - P-- N ~ c / S ~ . c H R \

~]:~ R=H I~_j R=CH 3

/

S--CH 2

CH3 /CH3 C[

N----CH--N~cH3 (~)

Compounds con taining -- SH groups

C o m p o u n d s containing sulphur in the -- SH form are particularly amenable to CSV analysis due to their ability to form partially insoluble complexes with mercury. The application of CSV to the determination of some thiourea-containing agrochemicals has been investigated by Smyth and Osteryoung [23]. Limits of detection at low ng ml-1 levels were found using the differential pulse m o d e and this m e t h o d was found to be more sensitive than polarographic methods based on anodic waves observed at the DME. Thiourea can also be determined polarographically following complexation with Cu(II) ions [1] or by liberation of the S atom and subsequent determination of H2S [1]. This latter procedure has also been used for the determination of other S-containing pesticides, e.g. diazinon, rogor and phenkapton and involved reduction of the pesticide by A1 in HC1 solutions in the presence of Ni. The H2S evolved is then determined by monitoring the decrease of Pb(II) concentration in the Pb(OAc)2 trapping solution. The m e t h o d could determine d o w n to 0.25 pg ml- 1 in pure solution. This has to be compared with a limit of detection of 1 pg 1-1 using GLC--electron capture detection (for diazinon) [24]. Brand and Fleet [25] have investigated the application of CSV to the determination of the fungicide, tetramethylthiuram disulphide (thiram) in aqueous solutions. They reported that the best results were obtained using a mercury plated platinum electrode in a solution of thiram containing an excess of ascorbic acid. This addition had the effect of chemically reducing the disulphide moiety in thiram to form free - - S H groups which were then amenable to CSV analysis. Using this method, they were able to determine thiram d o w n to 10-SM in pure solution. This m e t h o d offers a much greater sensitivity over the DC, linear sweep or AC (alternating current) techniques which have limits of detection for this c o m p o u n d of 6 × 10 -6, 3.5 x 10 -6 and 1 × 10-~M respectively. Carbamates

The reduction of carbamates at negative potentials has been investigated by polarography after derivatisation via nitration or nitrosation and recently, Anderson and Chesney [26] have applied their little investigated oxidation

79 TABLE 3 SUMMARY OF CARBAMATES

HPLC-ELECTROCHEMICAL

DETECTION

OF

SELECTED

Pesticide

Eluent compound a

Capacity factor, k'

Current Min. detectable Current b'd response, b'c quantities 10~M samples (nA) P~/ Pg Current (pA)

Aminocarb Carbendazim Chlorpropham Desmedipham Dichloran Phenmedipham

15% 10% 30% 30% 25% 30%

3.6 2.5 4 3.6 2.4 3.6

47 70 2.5 69 22 69

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN

0.035 0.01 -0.02 0.05 0.01

140 40 -120 200 60

200 100 -150 125 100

aAU eluents were mixtures of pH 6 aqueous phosphate buffer (ionic strength 0.1) with varying percent CH3CN composition by volume. b20 pl Injection. CApplied potential + 1.1V/Ag/AgC1, except + 1.15V for Desmedipham and Phenmedipham. d Obtained for signal/noise (peak-to-peak) ratio 2:1.

reactions to a reverse-phase liquid chromatographic m e t h o d with thin layer Kel-F--graphite electrochemical detection operated in the constant potential amperometric m o d e at -]- 1.1 V (vs. Ag/AgC1). Calibration curves were linear over at least three orders of magnitude with relative standard deviations of 1--2%. Detection limits in the range 40--150 pg, which correspond to sample concentrations of 2--7ng m l - ' (Table 3), were obtained and the m e t h o d compared favourably with GLC with electron capture detection following hydrolysis to the corresponding phenols or amines and reaction with halogen rich reagents. The electrochemical detector, in this case, was found more sensitive than both the UV detector, operated at 1 9 0 - - 2 1 0 n m , and the fluorescence detector in which dansyl derivatives were formed prior to injection or post-column derivatisation carried o u t with o-phthalaldehyde.

Halogenated anilines Halogenated anilines m a y enter the environment from a variety of sources, including metabolism of insecticides, herbicides, and fungicides; industrial waste, and reduction of nitrosubstituted aromatic compounds. The toxicity of anilines requires that these c o m p o u n d s be monitored in the environment. Previous m e t h o d s using gas chromatography require derivatization and are unable to separate some of the halogenated metabolites. An HPLC m e t h o d has been developed [ 27 ] that allows separation and detection of sub-nanogram quantities without derivatization. The separation of these c o m p o u n d s has been accomplished on a commercially available C-18 bonded phase column. Sub-nanogram quantities can be detected using an electrochemical detector. The detection limit of c o m p o u n d s like 2-amino-4~hlorophenol, o, m- and

80

p-chloroaniline, p-bromoaniline and 3,4-dichloroaniline is in the range from 0.2 to 0.4 ng. UV detection was found to be faster but less sensitive. Benzidines These compounds are EPA priority pollutants. Therefore, a sensitive m e t h o d for their measurement is required. A liquid chromatographic m e t h o d [28] employs a LiChrosorb RP-2 column, 45% acetonitrile (pH 5) mobile phase, UV detections at 280 nm, and electrochemical detection at + 0.70 V applied potential. The three-electrode electrochemical cell contains a tubular carbon-black/polyethylene working electrode t h a t permits detection of sub-ppb levels in a 50pl injection. UV detection is about 50 times less sensitive, but a 500pl injection permits detection of 2 ppb benzidine and dichlorobenzidine. Recoveries from fortified waste water of benzidine at 1 - - 6 p p b and dichlorobenzidine at 3 - - 1 2 p p b levels were 83% and 87%, respectively.

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81 15

16 17

18 19 20

21 22 23

24 25

26

27

28

B. K. Afghan, P. D. Goulden and J. F. Ryan, Automated method for determination of nitrilotriacetic acid in natural water, detergents, and sewage samples, Anal. Chem., 44 (1972) 354--359. J . P . Hart, W. Franklin Smyth and B. J. Birch, Polarographic determination of some linear alkylbenzene sulphonates, Analyst, 104 (1979) 853--859. A. Nakae, K. Tsuji and M. Yamanaka, Determination of trace amounts of alkylbenzenesulphonates by high performance liquid chromatography with fluorimetric detection, Anal. Chem., 52 (1980) 2275--2277. P. Nangniot, La Polarographie en Agronomie et Biologie, Duculot, Gembloux, 1966, p. 263. M. R. Smyth and J. G. Osteryoung, Pulse-polarographic investigations of parathion and some other nitro-containing pesticides, Anal. Chim. Acta., 96 (1978) 335--344. J. O. Bronstad and H. O. Friestad, Method for determination of glyphosate residues in natural waters based on polarography of the N-nitroso derivative, Analyst, 101 (1976) 820--824. C. E. McKone, T. H. Byast and R. J. Hance, A comparison of some methods for the determination of triazine herbicides in water, Analyst, 97 (1972) 653--656. M.R. Smyth and J. G. Osteryoung, Polarographic determination of some azomethinecontaining pesticides, Anal. Chem., 50 (1978) 1632--1637. M. R. Smyth and J. G. Osteryoung, Determination of some thiourea-containing pesticides by pulse voltammetric methods of analysis, Anal. Chem., 49 (1977) 2310--2314. T. M. Petrova and Yu. B. Andreev, Determination of Basudin (diazinon) and Dursban (chloropyrifos) in soil and water, Khim. Sel'sk. Khoz. Bashk., 18 (1980) 52--54. M. J. D. Brand and B. Fleet, The application of polarography and related electroanalytical techniques to the determination of tetramethylthiuram disulphide, Analyst, 95 (1970) 1023--1026. J. L. Anderson and D. J. Chesney, Liquid chromatographic determination of selected carbamate pesticides in water with electrochemical detection, Anal. Chem., 52 (1980) 2156--2161. E . M . Lores, D. W. Bristol and R. F. Moseman, Determination of halogenated anilines and related compounds by high-performance liquid chromatography with electrochemical and ultra-violet detection, J. Chromatogr. Sci., 16 (1978) 358--362. D. N. Armentrout and S. S. Cutie, Determination of benzidine and 3,3-dichlorobenzidine in waste water by liquid chromatography with UV and electrochemical detection, J. Chromatogr. Sci., 18 (1980) 370--374.