I:ree~nius Zeitschrift liir Fresenius Z Anal Chem (1989) 334:550-553

�9 Springer-Verlag 1989

Determination of tetramethrin (neo-pynamin) by differential pulse voltammetry with a carbon paste electrode modified with sepiolite Pedro Hernandez, Jose Vicente, and Lncas Hernandez Departamento de Quimica, Universidad Autonoma de Madrid, E-28049 Madrid, Spain

Bestimmung von Tetramethrin (Neo-Pynamin) durch Differential-Puls-Voltammetrie unter Verwendung einer mit Sepiolit modifizierten Kohlepaste-Elektrode

Summary. A method is described for the determination of tetramethrin (neo-pynamin) by differential pulse voltamme- try with a carbon paste electrode modified with 10% (w/ w) sepiolite. Preconcentration was carried out under open circuit conditions in 0.01 mol/1 acetic acid/potassium acetate medium at pH 5.3 over 10min, recording the voltammogram in 0.01 mol/1 of KH2PO4/K3PO4 at pH 12. This led to the appearance of a peak at - 1.32 V against SCE at 40 mVs- a and a pulse amplitude of 100 inV. Under these conditions determination limits of 45 ng ml-1 were achieved. The method was applied to the determination of tetramethrin in soil and water samples.

Introduction

Neo-pynamin or tetramethrin is a synthetic insecticide belonging to the pyrethroid group. Natural pyrethroids are of limited use for crop protection, although they have been employed to give protection near harvest time and during transit and storage [1].

The action of tetramethrin on insects is based on absorp- tion through contact with the exoskeleton; it attacks the central nervous system, producing the characteristic symptoms of convulsions with a high degree of efficiency over short periods of time.

The insecticide has low toxicity for mammals; however, assays in rats with a level of 2.0 gg ml- 1 in their atmosphere causes them to exhibit a marked degree of hostility and hyperexcitability, which disappears after 24 h [2].

The determination of tetramethrin has mainly been carried out with GLC by UV spectroscopy after previous TLC purification [3].

The present paper reports on the electrochemical deter- mination of the insecticide with an electrode modified with sepiolite magnesium silicates containing OH groups and water molecules. The structure of sepiolite consist of contin- uous tetrahedral planes. The basal oxygen atoms have the capacity to adsorb organic compounds and have been used

Offprint requests to: P. Hernandez

by Hernandez et al. in electroanalytical determinations, in oxidation or reduction processes of such organic molecules [4-8].

Experimental

Apparatus and reagents

The electrode was constructed of spectroscopic graphite (Trade Mark, Union Carbide) with a grain size of less than 45 gm. This was used to construct the carbon paste with nujol oil as the non-electroactive agglutinant.

Gel-sized powdered sepiolite (provided by Rio Rodano) was added to the carbon paste, keeping the latter humid. The percentage of sepiolite added was proportional to the spectroscopic graphite used. The paste thus obtained was packed in a polyethylene tube, with a geometric surface of 1.6 mm 2. The height of the carbon paste was 0.5 cm. Contact was achieved directly in the carbon paste with a copper wire.

A Metrohm E-506 polarograph with two cells was employed; one for preconcentration and the other for mea- surement; the latter contained the supporting electrolyte, the reference electrode (saturated calomelanous) and a platinum counterelectrode.

Stock solutions of tetramethrin (180 gg ml-1) were pre- pared from 99.0% pure solid product by dissolution in ethanol; these were used to obtain solutions of lower concen- tration by dilution in deionized water (Millipore, MilliRo, MilliQ).

The other reagents employed were of analytical grade.

Procedure

The modified electrode is placed in the preconcentration cell containing 20.0 ml of a solution of tetramethrin for a prefixed time, with constant stirring in the solution and under open circuit conditions.

Following this, the electrode is removed from the pre- concentration cell and placed in the measurement cell (pre- viously washed with water and dried), recording the voltammogram between -0 .8 and - 1 . 6 V, using differ- ential pulse voltammetry as the electroanalytical technique.

Peak intensity (ip) is calculated by the tangent-fit mode in all measurements.

Regeneration of the electrode is achieved, without modifying its surface, by keeping it polarized at - 1 . 6 V for 1 min. The electrode is conserved simply by keeping its surface humid with deionized water.

551

3.0

2,0 .< =L

1.0

2 6 10 pH

-1.4

Ep(V)

-1.2

-1.0

F i g . 1. Influence o fpH on ip. a Preconcentration cell; b measurement cell; c Ep in measurement cell

Table 1. Effect of concentration of acetic acid/potassium acetate buffer solution

mol/1 Buffer ip (gA)

0.001 2.4 0.01 3.96 0.1 2.8 0.5 2.48 1.0 2.40

Table 2. Effect of 0.01 mol/1 electrolyte at pH 12. ip values (gA)

Na K

82

"~4.0

I [ I i 5 10 15 20

min

Fig. 2. Variation in ip on modifying accumulation time. a in KNOa medium; b acetic acid/potassium acetate

Results and discussion

Figure 1 a shows the variation in ip when pH is changed (with HN03 and KOH) in the preconcentration cell using solution of 1.8 ~tg m l - t , with an electrode modified with 10% (w/w) sepiolite; a 5 rain residence time of the electrode in the preconcentration cell and 0.01 mol/1 KNO3 in the measurement cell, pH 5.3. The instrumental variables used in this work were 40 mVs -1 as the sweep rate (V) and - 1 0 0 mV as the pulse amplitude (A E).

It was found that the maximum degree of preconcentra- tion (higher ip values) occurs between pH 5.4 and 6.2, the peak potential (Ep) remaining at -1 .36 V. For pH-values lower than 1.2 and above 9 no reduction wave of tetramethrin appears.

This phenomenon is due to the fact that the tetramethrin molecule undergoes acid and alkaline hydrolysis and at these pH-values adsorption of the hydrolysis products into the modified electrode does not occur, such as the species adsorbed in the tetramethrin molecule.

This can be seen in Fig. l b on modifying the pH of the measurement cell, keeping pH constant and also the remaining variables in the preconcentration cell at 5.3. As the pH increases, so does ip, which remains constant between pH 4 and 10 with a sharp rise occurring at pH 12, where the highest value is reached.

These changes in the values of ip are accompanied by a variation in Ep (Fig. 1 c), the value at pH 12 being - 1.31 V.

Figure 2 shows the effect of the accumulation time of tetramethrin, using 0.01 mol/1 buffer solution in the mea- surement cell at pH 12. When preconcentration is carried out at pH 5.3 modified with HNO3, an adsorption maximum (Fig. 2 a) is seen after 12 rain of residence time, ip remaining constant at 4.0 ~tA for longer times. When the tetramethrin solution is prepared in 0.01 tool/1 acetic/potassium acetate buffer at pH 5.3, for the same preconeentration times as in the above case, an increase in preconcentration is observed

NOs 2.01 3.99 C1- 1.51 2.85 PO~ 2.27 3.99 C1047 1.67 1.81 CH3-COO- 2.1 2.55

(Fig. 2b) manifested in higher ip. In this medium, after 20 rain of preconcentration, a cons tan t ip is not reached, pointing to saturation of the electrode. This indicated that the presence of the buffer solution induces a greater degree of accumulation on the modified electrode.

On studying the effect of concentration of acetic acid/ potassium acetate, it was observed that the one producing the greatest ip is 0.01 mol/1 (Table 1) using an accumulation time of 5 rain.

Modification of the percentage of sepiolite in the electrode led to decreases in ip for values lower than 10% (w/w) in sepiolite and an increase in residual current and resistance for higher proportions, which causes distorsions in the voltammogram.

Study of the different electrolytes (Table 2) at a concen- tration of 0.01 mol/1 in the measurement cell, using sodium or potassium salts in each at pH 12 (with accumulation times of 5 min) showed that the highest ip values are obtained with phosphates. In all the electrolytes studied, not only are higher ip values obtained with potassium salts, but also a change occurs in the shape of the wave (Fig. 3), but no changes in Ep, which persists at - 1.32 V.

This seems to show that the presence of sodium salts in the measurement cell causes a rapid change, replacing the tetramethrin. This occurs in an identical fashion on studying the adsorption of ethanol and acetone by montmorillonite [9].

When the concentration of the phosphate buffer solution was altered (K2HPO4/K3PO4) at pH 12 between 1.0 and 10- 3 mol/1, no important changes occur in ip, which remains at 3.98 gA, except that as the concentration of buffer solu- tion is increased, this is paralleled by a rise in the relative standard deviation (8.93% for 1.0mol/1 to 2.22% for 0.01 mol/1 for ten measurements).

Figure 4 a shows the behaviour of the electrode when the sweep rate is modified, with A E equal to - 1 0 0 mV; the greatest ip is seen to be produced with 40 mVs -~. The modification in A E using 40 mVs -1 as the sweep rate (Fig. 4b) leads to a linear increase in i v, the highest value being reached with 100 mV.

Having chosen the most suitable conditions in the pre- concentration cell and the measurement cell together with

552

T 0.5,~,A

I

I I I I I -1.6 -1.4 -1.2 -1.0 V

Fig. 3. Modification in voltammograms of tetramethrin in phosphate medium at pH 8:1 using potassium salts; 2 sodium salts; R residual current

- 1 I

0.1 0.3 0.5 ...,7,,,"0'71-~'9

/s 4

3

I I I I -1,6 -1.4 -1.2 -1.0 V -1.6

10 ~ 90.8~,,u.A

7

t

-1.4 -1.2 -%0 V

Fig. 5. Calibration graphs for solutions of tetramethrin of (1) 45; (2) 90; (3) 180; (4) 225; (5) 270; (6) 315; (7) 405; (8) 495; (9) 765; (10) 885 ng ml- 1. R = residual current

3.0

~,2.0

1.0

20 60 100 A E I I I

G /

f y'b Z / /

I I I 10 30 50 mVS -1

Fig. 4. a Influence of pulse amplitude on ip. Conditions: accumula- tion time 5 rain in KNO3 medium, pH 2 and measurement at pH 12 modified with KOH; b influence of rate

the instrumental variables, as described above, a study was made of the behaviour of the electrode on modifying the concentrat ion of te t ramethrin using accumulat ion times of 10 rain. This behaviour shows a linear response (Fig. 5) for concentrat ions lower than 0.33 gg m l - 1 ; least squares fitting of the da ta yields a straight line ip (~tA) = 0.11 x8.19 (lag ml-1) , with a correlat ion coefficient of r = 0.995 for concentrat ions higher than 0.33 ~tg m l - ~ i v (laA) = 0.97 x 4.70 (llg m l - l) and r = 0.998.

Under these conditions, the determinat ion limit (10 a) is 45 ng ml -~ [10].

Statistical study of 5 solutions of 0.18 lag ml -1 and 10 determinat ions each with 4 different electrodes revealed rela- tive errors of 4.0 % and relative s tandard deviat ions of 6.3 %.

ip

< 2"0 t

d i ~ 30 60min

1 0.5#A

t

I I I I -1.6 -1.4 -1.2 -1.0 V

Fig. 6. Effect of accumulation time for concentrations lower than 45 ngml-1, a = 20;b = 30;c = 45;d = 60rain

553

It is possible to determine concentrations below 45 ng ml-1 using longer accumulation times, since with the ac- cumulation time and the concentration employed, (1.8 pgml- 1) no saturation of the electrode was obtained in any of the studies. This phenomenon also becomes gradually more obvious with lower concentrations. On studying the effect of the accumulation time in a solution of 45 ng m l - 1 (the determination limit found) a linear relationship among the times employed was observed (Fig. 6).

The method described was applied to the determination of tetramethrin in soils used for cultivation. The procedure employed was as follows: a solution was prepared by adding 900 pg oftetramethrin to irrigation water; this was gradually poured onto the soil samples over one week, collecting the effluent water. Three days after adding the whole solution of tetramethrin the content of the compound in the effluent water was determined by the above-described procedure.

For the determination in soil, 6 g of soil were treated with a water-methanol mixture (1 : 2) with vigorous stirring, the sample was filtered and the pesticide determined with the described procedure.

It was found that the water-soil recovery was 97%, 16.6% corresponding to the effluent water and 77.7 to the soil.

Acknowledgements. The authors would like to thank the CICYT for sponsoring Project PA86/0367 in which this work is included.

References

I. Hassan KA (1982) The chemistry of pesticides. Verlag Chemie, Weinheim, p 152

2. Acofar no 227 (1988) p 348 3. Miyamoto O (1973) J Anal Methods Pestic Plant Growth Regul

7:345 4. Hernandez L, Hernandez P, Lorenzo E, Sosa Z (1988) Analyst

113:621 5. Hernandez L, Hernandez P, Sosa Z (1988) Fresenius Z Anal

Chem 331:525 6. Hernandez L, Hernandez P, Sosa Z (1988) Fresenius Z Anal

Chem 329: 756 7. Hernandez L, Gonzalez E, Hernandez P (1988) Analyst

113:1715 8. Lorenzo E, Alda E, Hernandez P, Blanco M (1988) Fresenius

Z Anal Chem 330:139 9. Rausell JA, Serratosa, JM (1987) In: Newman (ed) The

chemistry of clays and clay minerals, p 371 10. Manson JM (1980) Anal Chem 52:2241

Received March 11, 1989