Voltammetric analysis of some carbamate pesticides

J. Electroanal. Chem., 125 ( 1 9 8 1 ) 4 3 7 - - 4 4 5

437

Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

VOLTAMMETRIC ANALYSIS OF SOME CARBAMATE PESTICIDES

G.E. BATLEY

Analytical Chemistry Section, Australian A t o m i c Energy Commision, Lucas Heights, N.S.W. 2234 (Australia) B.K. A F G A N

Analytical Methods Division, Canada Centre for Inland Waters, Burlington, Ont. L 7R 4A6 (Canada) (Received 18th December 1980; in revised form 17th February 1981)

ABSTRACT The electrochemical activity of 13 carbamate pesticides has been investigated in the potential range of a glassy carbon electrode, over a pH range from 2.5 and 10. Four compounds, pirimicarb, methiocarb, aminocarb and zectran, give oxidation waves; however, only those for aminocarb and zectran are suitable for analytical electrochemical detection in association with high-pressure liquid chromatography. The oxidation reaction has an ECE mechanism, proposed to involve formation of a carbamate cation radical. This hydrolyses to form a phenol which is in turn ox!dized, the end product being a substituted dimethylaminobenzoquinoneimine.

INTRODUCTION

With the increasing use of carbamate pesticides as alternatives to the more persistent and toxic organochlorine and organophosphorus pesticides, the need has grown for selective trace analysis techniques for these compounds in waters, soils and plant and animal tissue. Both gas chromatographic and colorimetric methods have been used, with Some success, for carbamates in environmental samples [1,2] ; however, lengthy clean-up and extraction procedures are required, and the built-in preconcentration and separation of high-pressure liquid chromatography (HPLC) offers a preferable alternative. The latter has been applied to the analysis of selected carbamates using UV [3], fluorimetric [4] or multi-detection systems [5]. Although electrochemical detection systems are now finding greater use with HPLC, there have been no reports of their application to carbamate pesticides. Indeed, the literature is bereft of data on the voltammetric behaviour of these compounds, although their indirect polarographic determination, via the nitroso derivatives, has been reported by Benson and Gajan [6]. This paper examines the electrochemical activity of a range of carbamate pesticides and discusses the potential of the signals obtained, either for direct voltammetric analysis or analysis via electrochemical detection in HPLC. 0022-0728/81/0000---0000/$02.50 © 1981 Elsevier Sequoia S.A.

438 EXPERIMENTAL

Apparatus and reagents Voltammetric studies were carried out in buffered aqueous and methanolic solutions with a glassy carbon electrode (Metrohm EA286) using a P.A.R. Model 174 polarographic analyser coupled to a Houston Model 2000 recorder. Standard solutions (1000 mg 1-1 in acetonitrile) of aldicarb, aminocarb (matacil), carbaryl, carbofuran, chloropropham, dimetilin, IPC, metalkamate, methamyl, methiocarb, zectran (mexacarbate), primicarb and p r o p o x u r were obtained from Nanogens Co. (Watsonville, CA, U.S.A.). 3-Methyl,4-dimethylaminophenol was obtained from Chemago Co. (Kansas City, MO, U.S.A.). Buffer solutions were prepared from analytical reagent grade chemicals. Acetate buffers were used below pH 7, and phosphate buffers for higher pH values. RESULTS AND DISCUSSION

Electroanalysis of carbamates The electrochemical behaviour of the 13 carbamate pesticides was examined in buffered aqueous solutions at a glassy carbon electrode (GCE). None of the c o m p o u n d s studied exhibited reduction peaks within the potential range of the electrode, and only four c o m p o u n d s -- aminocarb, zectran, methiocarb and p i r i m i c a r b - gave measurable oxidation waves (Table 1). These c o m p o u n d s are phenyl and pyrimidyl esters of carbamic acids, having either methylthio or dimethylamino substituents on the conjugated ring system (Table 2). The complex oxidation mechanism involving these substituent groups will be discussed in more detail later. In aqueous solutions at pH 6.6, the anodic potential range of the GCE is limited to a b o u t +1.3 V vs. SCE by the evolution of oxygen. The exact potential at which this occurs is a function of electrode preparation [7]. The oxidation waves for both methiocarb and pirimicarb appeared as measurable shoulders on the oxygen evolution wave, but their resolution varied as a function of electrode age. Because of this large and variable background current, these

TABLE 1 Differential pulse v o l t a m m e t r i c o x i d a t i o n o f c a r b a m a t e s in 0.16 M a c e t a t e , p H 6.6, at a glassy carbon electrode a Compound

Concentration/ m g 1-1

Ep IV vs. SCE

ip/uA

b 1/2/mV

Zectran Aminocarb Pirimicarb Methiocarb

2 2 2 2

+0.65 +0.74 +1.15 + 1.20

2.54 1.58 1.30 1.35

82 95 100 102

a Pulse m o d u l a t i o n 25 m V , 0.5 s r e p e t i t i o n , 5 m V s - l scan rate.

439 TABLE 2 Structure formulae of electrochemically active carbamates Compound

R--O--C--NR' R" II O R

Aminocarb

CH3 C~N__~- ~

cH3"/ ~

CH3

CH3

Zectran

R'

R"

CH3

H

CH3

H

CH 3

H

CH3

CH3

CH3 . ~

~/

CH 3 CH3

cH3--s-~

Methiocarb

CH3 Pirimicarb

CH3/%~.~3CH3~ H3

waves are unsuitable for electroanalytical determinations in flowing streams and were therefore not examined in further detail. The oxidation waves for aminocarb and zectran, on the other hand, offer excellent analytical potential, being well resolved from the oxygen wave. At pH 6.6, linear calibration plots were obtained for both compounds, for concentrations in the range 0.5--10 mg 1-'. The estimated limit of detection, using differential pulse voltammetry, with a 25 mV pulse modulation and 0.5 s pulse duration, was 30 pg 1-' . For analyses using HPLC, a mixed solvent system is required for the elution of carbamates [4]. Table 3 shows that the effect of methanol on the oxidation of aminocarb was to reduce the height of the differential pulse voltammogram, while increasing the half-width, with a shift to more positive potentials. A linear concentration response was still maintained, making electrochemical detection of HPLC effluents in 20--50% methanol/water an attractive measuring technique. The pH dependence of the oxidation peak potentials, illustrated in Fig. 1, shows two slopes, and is typical of that observed for aromatic amines [8]. The intercept pH value corresponds to the pKa for the protonation of the dimethylamino group (eqn. 1): R

CH3,~..~"~ ~ ~-CH'3 /N"~ ~/L-O--C--N,~.H "t- H+ ~ CH3

R

CH3 H~//~k " ~J ~-CH3 ,~-o-c-._. H CH3"~' 3 ~

(1)

440 TABLE 3

Effect of methanol on aminocarb oxidation at pH 6.6 a % Methanol

Ep/V vs. SCE

ip/t~A

b 112/mY

0 20 40 60

0.740 0.742 0.742 0.775

1.58 1.56 1.25 1.10

95 95 118 125

a Differential pulse voltammetry at GCE, 25 m V m o d u l a t i o n , 0.5 s repetition.

The measured intercept pH values were 5.8 and 6.1 respectively, for aminocarb and zectran. Current responses for both compounds increased to a maximum near pH 6. This behaviour will be governed both by the protonation of the carbamates and by the rates of any ensuing chemical or electrochemical reactions. Although the peak potentials of aminocarb and zectran are separated by almost 100 mV in alkaline solution, it was not possible to resolve the peaks obtained for mixtures of the two, using either linear sweep or differential pulse voltammetry. In practice, this would not present a problem, as the c o m p o u n d s are readily separated by HPLC [3]. It should also be noted that replicate voltammograms on these c o m p o u n d s will not reproduce the initial peak current because of accumulation of reaction products at the electrode. Reproducibility can be achieved by stirring the solution either mechanically or with nitrogen gas between runs or, for the purpose

.90

.80 Ep/v SCE

.70

\

.60 ]

I

I

I

I

'1

I

I

I

2

3

4

5

6

7

8

9

10

11

pH

Fig. 1. Effect of pH on peak potential for the differential pulse voltammetric oxidation of zectran ( i ) and aminocarb (e), at a GCE. Pulse modulation 25 mV, pulse repetition 0.5 s.

441 of amperometric detection, carrying out the measurement in a continuously stirred solution. Oxidation mechanism The cyclic voltammetric behaviour of aminocarb and zectran was studied over a range of sweep rates and pH values in an a t t e m p t to elucidate their oxidation mechanisms. Typical voltammograms (Fig. 2), recorded at 50 mV s -1 sweeps in acetate-buffered solutions at pH 6.6, show a single, apparently irreversible, oxidation peak on the first anodic scan. On the reverse scan, this peak gives no companion reduction peak; however, several waves appear at more

2.C

-1£

-OA

0 "

OA

0.8

E/Vv~ SCE

b

2.0

/

t

1.0

J/~A

-!

-0.4

0

0.4

0.8

E / V vs SCE

Fig. 2. Cyclic v o l t a m m o g r a m s for (a) zectran and (b) aminocarb (4 mg 1- l in 0.16 M acetate buffer pH 6.6) at a GCE. Scan rate 50 m V s -1 .

442

negative potentials. On the second scan, new anodic peaks, one for zectran and t w o for aminocarb, appear ahead of the primary oxidation. These and the cathodic peaks increase in height on subsequent scans while the primary oxidation peak diminishes. This behaviour is consistent with an ECE mechanism, where a fast chemical reaction is interposed between two electron-transfer reactions, the second of which occurs at a more negative potential than the first [9,10]. The following reactions appear to be more complex for aminocarb, where three cathodic and three anodic peaks were obtained on the second and subsequent cycles, compared to only two cathodic and two anodic peaks for zectran at pH 6.6. Similar behaviour was observed at pH 8.9, while at pH 3.5 additional waves appeared for both aminocarb and zectran. At all three pH values studied, the width of the primary oxidation wave, as measured by Ep --Ep/2, approximated to 60 mV for both c o m p o u n d s at a scan rate of 100 mV s -1 . This is the expected value for a reversible one-electron oxidation [9,10], b u t could also be obtained for a reversible one-electron oxidation, as the first step in an ECE mechanism in which the second oxidation occurs more easily than the first. The primary charge-transfer step is most probably the removal of an electron from the dimethylamino nitrogen attached to the conjugated system (eqn. 2): R

CH3 .~ ? /CH3 ~N'--
N--~

CH~ ~

?--o-c--N..

H

+

(2)

This t y p e of reaction has been proposed for N,N-dimethylaniline and other aromatic amines [11,12], while an analogous cation radical formation has been suggested for methylthiobenzenes [ 13] and would be expected for methiocarb oxidation. The intermediate carbamate cation radicals are unstable and, as found for dimethylaniline, will undergo immediate chemical reaction. Further diagnostic criteria for reaction t y p e are the effects of voltage scan rates on peak potentials and peak currents. Scans were varied from 5 to 500 mV s -1, using the P.A.R. 174 with a chart recorder. At pH 8.9, the peak potentials for aminocarb and zectran were both shifted towards more positive potentials by 34 and 33 mV respectively, for a 10-fold change in scan rate, with similar behaviour occurring at lower pH values. This is consistent with the predicted behaviour [9,10] for a reversible one-electron transfer in the proposed ECE scheme. The effect of scan rate on peak heights is shown in plots of ip/V i n vs. log v (Fig. 3). For pH 8.9, these plots show a decrease in the current function, which is approximately linear with respect to log v, and having a slope, for aminocarb, which is 35% greater than that for zectran. At lower pH values, the current functions decrease more rapidly with increasing scan rates, again with aminocarb exhibiting the greater slope. These results are indicative of a following chemical reaction, the rate of which is pH dependent and is consistently greater for aminocarb. Peak currents for the cathodic and anodic waves obtained on the second cyclic scans were difficult to measure precisely; however, it was possible to show that the current functions (ip/V ~n ) for both the anodic and cathodic

443

8of

89

v/mVs

-1

Fig. 3. Effects of scan rate on the current function for zectran (e) and aminocarb (A) at different pH values (4 mg 1-1 solutions in 0.16 M acetate or phosphate buffers).

waves were independent of scan rate in the range 5--500 mV s -1 as expected for an ECE mechanism [10]. The hydrolysis reactions of phenyl N-methylcarbamates (eqn. 3): R

R

CI~N__~OJ__N~CH3 CH.,~ - -

oH__=

H

CH3~

OH+ CH3NH2 ÷

FK~O~ ÷ H20

(3)

have been discussed by several authors [14--17] .The rates are slow, a typical half-time for zectran in river water at pH 8 being nine days [17]. Fujita et ai. [ 16] have discussed the dependence of hydrolysis rate constants on Hammett substituent constants and the pKa values of the substituent phenols, from which we may predict more rapid hydrolysis for aminocarb than zectran. It is proposed that the unstable carbamate cation radical undergoes rapid hydrolysis, with the formation of a dimethylaminophenol cation radical and methylamine according to eqn. (4), R

R

CH~,...+ / ~ - ~ .~j ./CH 3 /N"--~' "~--O-'-C--N~ + OHCH~ ~

,

H

CH~L ..4.,/~'-~ H 3 ~ N ~ _~_ OH+CH3NH2 + HCO; '+ H20

~

C

(4)

the rate again being faster for aminocarb than for zectran. The product phenol will undergo a second spontaneous oxidation to give the dimethylbenzoquinoneimine (eqn. 5): R

~_

R

%'-, ~~- -

CH3

H+

(5)

On the basis of the above reactions, one would expect to see waves for the aminophenol--quinoneimine couple in the cyclic voltammograms. This oxidation is itself a complex ECE mechanism. The oxidation of p-N,N-dimethylaminophenol has been discussed in detail by Marcus and Hawley [18]. In addi-

444

2.G

-2.C 2

-0:4

'

6

0:4 E/

0:s

V vs 5CE

Fig. 4. Cyclic voltammogram of 3-methyl,4-dimethylaminophenol (4 mg 1-1 in 0.16 M acetate buffer pH 6.6) at a GCE. Scan rate 50 mV s-1 .

tion to the oxidation and reduction peaks due to the parent aminophenol, the cyclic voltammetry of this c o m p o u n d shows waves for benzoquinone, formed by hydrolysis of the primary oxidation product, p-N,N.dimethylbenzoquinoneimine, as well as those due to 2,4-bis(dimethylamino)phenol. The latter is formed via a Michael 1,4-addition of a hydrolysis product dimethylamine to the primary oxidation product. The possibility of similar reactions exists with the quinoneimines formed from the cation radical oxidation products of aminocarb and zectran, but modified by the presence of methyl substituents on the aromatic ring. For zectran, where m e t h y l groups block the two reactive sites adjacent to the dimethylamino group, the cyclic voltammograms show fewer waves than aminocarb, which has one methyl substituent in the meta position. Cyclic voltammograms for 3-methyl,4-dimethylaminophenol, the proposed product from aminocarb at pH 6.6, showed two cathodic waves and one anodic wave, directly compatible with scans for the parent carbamate (Fig. 4). A similar compatibility was observed at lower pH values. Coulometric oxidations were also performed at a glassy carbon electrode, on 30 pg 1-1 solutions of aminocarb and zectran in acetate buffer at pH 6.6. Values for n of 1.8 and 2.0 respectively, were obtained, as expected for an overall reaction leading to the function of substituted N,Nodimethylbenzoquinoneimine products.

445 REFERENCES 1 R , C . H a l l a n d D.E. H a r r i s , J. C h r o m a t o g r . , 1 6 9 ( 1 9 7 8 ) 2 4 5 , C R C Press, C l e v e l a n d , O h i o , 1 9 7 3 . 2 T. C y r , N. C y r a n d R . H a q u e in G. Z w e i g a n d J. S h e r m a ( E d s . ) , A n a l y t i c a l M e t h o d s f o r P e s t i c i d e s a n d P l a n t G r o w t h R e g u l a t o r s , Vol. IX, A c a d e m i c Press, N e w Y o r k , 1 9 7 7 , C h . 3. 3 C.M. S p a r a c i n o a n d J . W . H i n e s , J . C h r o m a t o g r . Sci., 1 4 ( 1 9 7 6 ) 5 4 9 . 4 R . J . A r g a u e r , J. A s s o c . O f f . A n a l . C h e m . , 5 3 ( 1 9 7 0 ) 1 1 6 6 . 5 B.K. A f g h a n , J . F , R y a n a n d R . J . W i l k i n s o n , P i t t s b u r g h C o n f e r e n c e o n A n a l y t i c a l C h e m i s t r y a n d A p p l i e d S p e c t r o s c o p y , C l e v e l a n d , 1 9 7 9 , E x t e n d e d A b s t r . , p. 4 2 . 6 W . R . B e n s o n a n d R . J . G a j a n , J. O r g . C h e m . , 31 ( 1 9 6 6 ) 2 4 9 8 . 7 W.E. V a n d e r L i n d e n a n d J . W . D i e k e r , A n a l . C h i m . A c t a , 1 1 9 ( 1 9 8 0 ) 1. 8 R . N . A d a m s , E l e c t r o c h e m i s t r y a t S o l i d E l e c t r o d e s , M a r c e l D e k k e r , N e w Y o r k , 1 9 6 9 , C h . 5. 9 R . S . N i c h o l s o n a n d I. S h a i n , A n a l . C h e m . , 3 6 ( 1 9 6 4 ) 7 0 6 . 10 R . S . N i c h o l s o n a n d I. S h a i n , A n a l . C h e m . , 37 ( 1 9 6 5 ) 1 7 8 . 11 T. M i z o g u c h i a n d R . N . A d a m s , J. A m . C h e m . S o c . , 8 4 ( 1 9 6 2 ) 2 0 5 8 . 1 2 E.T. S e o , R . F . N e l s o n , J . M . F r i t s c h , L . S . M a r c o u x , D.W. L e e d y a n d R . N . A d a m s , J. A m . C h e m . S o c . , 88 (1966) 3498. 1 3 A. Z w e i g , W.G. H o d g s o n , W . H . J u r a a n d D . L . Marille, T e t r a h e d r o n L e t t . , 2 6 ( 1 9 6 3 ) 1 8 2 1 . 1 4 T. V o n t o r a n d M, V e c e r a , C o l l e c t . C z e c h . C h e m . C o m m u n . , 38 ( 1 9 7 3 ) 5 1 6 . 1 5 T. V o n t o r a n d M, V e c e r a , C o l l e c t . C z e c h . C h e m . C o m m u n . , 3 8 ( 1 9 7 3 ) 3 1 3 9 . 1 6 T. F u j i t a , K . K a m o s h i t a , T. N i s h i o k a a n d M. N a k a j i m a , A g r i c . Biol. C h e m . , 3 8 ( 1 9 7 4 ) 1 5 2 1 . 17 E.W. M a t t h e w s a n d S . D . F a u s t , J. E n v i r o n . Sci. H e a l t h , B 1 2 ( 1 9 7 7 ) 1 2 9 . 1 8 M . F . M a r c u s a n d M.D. H a w l e y , J. E l e c t r o a n a l . C h e m . , 1 8 ( 1 9 6 8 ) 1 7 5 .