Bull. SOC. Chim. Belg. vd. 102 / no 6 / 1993

EUROPEAN SECTION 0037-9646 I92 I $2.00 + 0.00

0 1993 Cornit6 van Beheer van het Bulletin V.Z.W.

SIMULTANEOUS DETERMINATION OF FOLPET AND FENAMIPHOS IN WATER AND PESTICIDE FORMULATIONS BY DERIVATIVE SPECTROPHOTOMETRY

D. Cervantes OcaAa, M.D. Gil Garcia, M. Martinez Galera and J.L. Martinez Vidal Department of Analytical Chemistry, Faculty of Experimental Sciences of Almeria, 04071 Almeria (Spain)

Received : 07/07/1993 - Accepted : 16/08/1993

KEYWORDS : Pesticide, water, formulation, derivative spectrophotometry.

SUMMARY A new method is described to analyse the binary mixture of folpet and fenamiphos, using the first derivative spectrophotomet obtained by mathematical treatment of the data. Calibration graphs were linear between 1 .O to 15.Opg.mL of folpet and between 1.2 to 27.9pg.mL-1 of fenamiphos. The method has been applied for determining both pesticides in commercial formulations and groundwater at ng.mL-' levels after a preconcentration step with Cis.

INTRODUCTION Several authors have reviewed recently the subject of derivative spectrophotometrylS2. The use of this

technique in conjunction with the possibility of numerical treatment processes of the absorption spectra - such as multiplication or division of spectra -, has improved the potential of the UV-VIS spectrophotometry, increasing the information level available from an absorption spectrum. In this way UV-VIS spectrophotometry is a fast and inexpensive competitive analytical technique.

A number of reports combines the derivative spectrophotometry with simultaneous multiwavelength measurement techniques for the determination of organic compounds, specially in the pharmaceutical area', but the ones about the potential of derivative ratio spectra spectrophotometry for the pesticide determinations in samples containing a few ana~y tes~-~ are scarce.

Fenamiphos [ethyl 3-methyl-4-(methylthio) phenyl isopropyl- phosphoramidate] is a broad-spectrum non volatile nematicide- insecticide and it is commonly used for nematode control in horticultural crops. Folpet [N-(trichloromethylthio) phtalimide] is a systematic fungicide for the treatment of fruits and vegetables. Fenamiphos and folpet are usually analyzed by GC9-14 or HPLC15"7, but the chromatographic techniques require an expensive equipment and the analyse is costly. In this work we present the results obtained in the study of a new spectrophotometric method easy and fast to determine fenamiphos and folpet in commercial formulations and groundwater samples from an horticultural area of Almeria (Spain), with intensive practices of agricultural exploitations in greenhouses.

Y

EXPERIMENTAL Apparatus

A Milton Roy 3OOO diode array spectrophotometer fitted with a Spectronic 3OOO Array system software V 1.14 was used for spectral acquisition and storage of data. An Olivetti PCS 486 fitted with a Grams/386 software'' was used for manipulation and analysis of the data.

Chemicals and solvents Folpet and Fenamiphos standard solutions of 50 pg.mL-' in acetonitrile were prepared from the

products Riedel de Haen (Pestanal). Commercial formulations of fenamiphos (Nemacur 40) and Folpet (Foltene) were supplied by Bayer and lnagra respectively. All organic solvents used were of spectrophotometric grade. Cartridges Sep-Pack C18 Cromlab. Distilled water was obtained from a Millipore water purification Milli-Q system.

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Procedure for determining folpet and fenamiphos in synthetic mixtures Samples were prepared in 25 mLflasks, containing between 25-375pg of folpet and/or between 30-700

pg of fenamiphos and suitable volumes of acetonitrile and methanol to give a final 1:l (v/v) mixture. To determine folpet, the amplitude of the first derivative spectrum of the sample is recorded between

200 en 350 nm, smoothed through 11 experimental points, and the first derivative is calculated with M = 11 nm. The concentration of folpet was proportional to the amplitude of the signal at 228.7 nm ('Da8.7).

To determine fenamiphos, an analogous procedure was followed. The concentration of fenamiphos is proportional to the amplitude of the signal at 257.1 nm (1D257.1).

Procedure for determining folpet and fenamlphos In water 500 mL of water samples were passed through Sep-Pak C18 disposable cartridges (Cromlab) at a rate

of 5-10 mL.min-'. The cartridges were preconditioned with 5 mL of methanol followed by 5 mL of ultrapure water. The samples concentrated were extracted with 5 mL of acetonitrile : methanol (1 :1) (v/v) and folpet and fenamiphos were determined as described above.

Procedure for determining folpet and fenamiphos in commercial formulations Weigh 10 g of commercial products, dissolve in acetonitrile and transfer to a 25 mL volumetric flask

with suitable volumes of acetonitrile-methanol to give a final 1 :l (v/v) mixture. Aliquots containing not more than 375 pg of folpet and 700 pg of fenamiphos are transfered to a 25 mL volumetric flask, eluted with acetonitrile-methanol 1 : 1 (v/v) and both pesticides determined as described above.

RESULTS AND DISCUSSION The absorption spectra corresponding to folpet, fenamiphos and a mixture of them can be seen in

Figure 1. The absorption maximum of folpet is situated at 224 nm when it is operated with acetonitri1e:methanoI (1 :1) (v/v) solution; this maximum does not experience any change with the pH between

1.20 ,

U

FIG. 1.

Absorption spectra of (1) folpet (5.0 pg.rnL-'), (2) fenamiphos (7.7 pg.rnL-'), and (3) their mixture in acetonitri1e:methanol (1 :l).

1 .O and 8.7; however the folpet solutions are hydrolysed for pH > 9.0. The absorption maxima of fenamiphos, at the same operatory conditions, are encountered at 208 and 252 nm. The maximum at 252 nm does not experience any change with the pH between 1.0 and 12.0. Overlapping of the absorption spectra of both pesticides makes impossible their simultaneous determination. The absorbances are constant for both pesticides during 10 days at least. We realized a comparative study to determinate folpet and fenamiphos by derivative spectrophotometry and derivative ratio spectra.

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Derivative spectrophotometry Figure 2 shows the first derivative of the spectra of folpet, fenamiphos and their mixture. The

zero-crossing of fenamiphos and folpet occurs at 228.7 nm and 257.1 nm respectively. Then, folpet can be determined by measuring at 228.7 nm (1D228.7) and fenamiphos at 257.1 nm ('D257.1). Several values of the experimental points were tested for smoothing the absorption spectra and 11 points were selected as optimal". We also optimized the value of M to obtain the first derivative spectra from smoothed spectra and M = 11 nm was selected as the optimum. Calibration graphs were obtained to determine folpet and fenamiphos by measuring the first derivative signal at the selected wavelengths and they gave straight lines to 15.Opg.mL-' folpet and 27.9pg.mL.' of fenamiphos. The statistical data for calibrations graphs of folpet and fenamiphos are summarized in Table 1.

o'o'loo 7

Pesti- Range cide (ug.mL1)

Folpet 1.0-15.0

0.0100

2

a '+ Y

'E -0 .0100 9 Y

v1 I

-0.030U ;2

St.dev. St.dev. St.dev. Detec. 2 (nm) Equation r Lim. Lim.

228.7 'D228.7 = 0.001-0.007C 0.9975 0.001 0.002 0.003 0.1 0.2

(Sb) (Sa) (Sr) (ug,mL-l)(ug,mL-')

0

Fena- miphos

First derivatives for : (1) 5.0pg.mC' of folpet, (2) 7.7pg.mC' of fenamiphos, and (3) their mixture in acetonitri1e:methanoI (1 :l).

1.2-27.9 257.1 'D257.1 = -0.001C 0.9987 0.001 0.001 0.001 0.1 0.4

TABLE 1.

Statistical data for calibration graphs in the determination of folpet and fenamiphos by first derivative spectrophotometry (C = pg.mC').

Derivative ratio spectra Figures 3 and 4 show the ratio spectra of folpet standard (spectra of folpet divided by the spectrum of

a 3.7pg.mL-1 of fenamiphos solution) and their first derivatives. The ratio spectra first derivative amplitudes at 218.1 nm and 229.1 nm are proportional to the folpet concentration. To determine the other component (fenarniphos) an analogous procedure was followed. In the same figures can be seen the ratio spectra of

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FIG. 3.

Ratio spectra : (1) 5.0 pg.mL-' of folpet when fenamiphos (divisor) was 3.7 pgmC'. (2) 5.2 pg.mr' of fenamiphos when folpet (divisor) was 5.0pg.mL'.

FIG. 4.

Derivative of the ratio spectra : (1) 5.Opg.mC' of folpet when fenamiphos (divisor) was 3.7pg.mL-' (2) 5.2pg.mL-' of fenamiphos when folpet (divisor) was 5.0pg.mC'.

fenamiphos standard divided for 5pg.mC' of folpet and their first derivatives. The ratio spectra first derivative amplitudes at 242.4 and 258.3 nm are proportional to the fenamiphos concentration. The ratio spectra were smoothed through the use of 11 experimental points, and the first derivatives calculated with M = 11 nm.

The concentration of the divisor was modified in both cases and different calibration graphs were obtained (Table 2). It can be seen that the estimated standard deviation decreases with the increase of the divisor concentration in all cases. Comparing the obtained results with the ones obtained by the derivative spectrophotometry method, it can be observed how these last ones obtain lower values for Sb, Sa and Sr. The detection and determination limits2' are analogous for both methods. In this way, we considered best to determine folpet and fenamiphos by the derivative spectrophotometry method due to the best values obtained for the statistical criteria Sb, Sa and Sr.

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TABLE 2.

Statistical data for calibration graphs in the determination of folpet and fenamiphos by using derivative ratio spectra.

Folpet Divisor

2.0

Range L St.dev. St.dev. St.dev. Det.Lim. Determ. Fenami-

phos @g.mL-') (nm) Equation (Sb) @a) Divisor

(sr) hg.mL-l) Lim. (ug.rnL

1.2-27.9 242.4 lDD242.4 = 0.002 0.034 0.9988 0.070 0.1 0.3 -0.019 +0.129c

5.0

2.0

5.0

1.0-15.0 218.1 1DD2i8.1 = 0.008 0.066 0.9970 0.094 ! 1 I ~0.012+0.194C 1 ~ ! ! 1 O3

1.2-27.9 242.4 'DD242.4 = 0.001 0.009 0.9988 0.019 0.1 0.3 0.005 +O.O34C

1.2-27.9 258.3 'DD258.s = 0.002 0.023 0.9992 0.048 0.1 0.2

1.2-27.9 258.3 'DD258.3 = 0.004 0.006 0.9992 0.013 0.1 0.2

0.019 +0.112C

0.005-0.029C

3.1

4.8

1.1

1.0-15.0 218.1 'DD218.1 = 0.004 0.031 0.9971 0.044 0.1 0.3 0.0oO +0.100c

1.0-15.0 218.1 1DD218,1 = 0.002 0.019 0.9971 0.027 0.1 0.3 0.0oO + 0.062c

1.0-15.0 229.1 'DD229.1 = 0.008 0.070 0.9978 0.100 0.1 0.2 0,044-0.259C

The results were obtained from 10 points for each calibration graph Sb = Standard deviation of the slope. Sa = Standard deviation of the intercept. Sr = Residual standard deviation.

3.1

4.8

Determination of folpet and tenamiphos in binary mixtures Binary synthetic mixtures were prepared in the ratio folpeWfenamiphos 5:l to 1:12.5 and resolved by

the derivative spectrophotornetry proposed method. The results obtained are summarized in Table 3. The recoveries of folpet are between 85.0 and 103% (r.s.d. 1.2-2.7%), measuring at 228.7 nm ('D228.7) and the recoveries of fenamiphos are comprished between 99.2 and 120.0% (r.s.d. 1.1-3.5%), measuring at 257.1 nm ('~257.1).

1.0-15.0 229.1 'DD229.1 = 0.004 0.033 0.9980 0.047 0.1 0.2

1.0-15.0 229.1 'DD229.1 = 0.002 0.020 0.9980 0.029 0.1 0.2

0.022-0.125C

0.01 3-0.077C

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TABLE 3.

Results obtained in the determination of folpet and fenamiphos in synthetic mixtures by first derivative spectrophotometry.

Composition of mixture I FolDet recovery (%1 I FenamiDhos recovery I%) 1

4.0

2.0

1 Folpet (ug.mL-') 1 Fenamiphos dug.mC') I 'D228.7 I 0257.1 I

2.0 90.3 110.1

2.0 98.2 100.6

10.0 ! 2.0 87.0 I 120.0 I

2.0

2.0

I 8.0 I 2.0 I 88.6 I 114.2 I

15.0 91.7 99.8

25.0 85.3 99.2

I 4.0 1 4.0 1 102.3 1 99.5 I I 2.0 I 10.0 I 90.0 1 100.5 I

The results are average of three determinations

INTERFERENCES According to the proposed method, 5pg.mC' of dieldrin, captan and tetradifon; 4pg.mC' of buproforin

and ethefon; 2,5pg.mL-' of methomyl and 1,2pg.mL-' of vinclozoline, do not interfere in the determination of 5 pg.m~-' of folpet.

On the other hand, 5pg.mC' of dieldrin and captan; 4pg.mL-' of ethefon; 2,5pg.mC1 of vinclozoline; 1,2pg.mC1 of methomyl and 0,5pg.n1L'~ of tetradifon and buproforin, do not interfere in the determination of 6.2pg.mL-' of fenamiphos.

APPLICATIONS Slmultaneous determination of toipet and fenamiphos in water

The proposed method was applied to determine folpet and fenamiphos in groundwater samples using a solid-phase extraction step with C18 cartridges preconditioned with 5 mL of methanol followed by 5 mL of ultrapure water. The study was carried out operating with spiked samples of ultrapure water with both pesticides. A flow rate between 5-10 mL.rnin-' through the cartridge was found to be optimal. The samples concentrated were eluted with 5 mL of acetonitrile : methanol (1:l) (v/v). Cartridges with silica gel, florisil and C18 were used for the preconcentration of both pesticides. The best recoveries were obtained with C18 - Table 4 -. The recoveries are 72.5% (r.s.d. 2.3%) for folpet and 77.4% (r.s.d. 2.0%) for fenamiphos with florisil as sorbent and 2.3% (r.s.d. 8.3%) for folpet and 38.7% (r.s.d. 4.5%) for fenamiphos with silica gel, operating at a spiked level of 5pg.mL-'. Using a solid-phase extraction step with CIS the calibration curves were linear between 20 - 100 ng.mL'l of folpet and 10 - 200 ng.mC1 of fenamiphos. The correlation coefficients are higher than 0.9954. The determination limits are 13 ng.mL-' for folpet and 7 ng.mC' for fenamiphos.

Operating on not contaminated groundwater samples at a spiked level of 50 ng.mC1 of folpet and fenamiphos, the recoveries encountered by the proposed method were 70.2 and 85.3% (r.s.d. 2.3% and 3.5%) respectively.

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TABLE 4.

Results obtained in the determination of folpet and fenamiphos in spiked ultrapure water

2.0

2.0

5.0

5.0

10.0

[Folpet] (ug.mL-l) x 1 [Fenamiphos] (ug.mL-l) x lo-* 1 1.7 85.0 1.2 1.4 116.6

1.7 85.0 6.2 7.0 112.9

4.6 92.0 3.1 3.3 106.4

4.5 90.0 6.2 6.5 104.8

9.4 94.0 6.2 6.6 106.5

I Spike I Found I Recovery(%) 1 Spike I Found 1 Recovery(%) 1

The results are average of three determinations

The method was applied for determination of folpet and fenamiphos in groundwater of Almeria (Spain) without detecting them.

Slmultaneous determination of folpet and fenamlphos In commercial formulations In addition, folpet and fenamiphos were determined in commercial formulations mixtures by the

proposed method. The suitability of the method has been evaluated by standard addition method. It can be observed - Table 5 -that both pesticides are recovered quite satisfactory.

TABLE 5.

Results obtained in the determination of folpet and fenamiphos in commercial formulations using the proposed method.

The results are average of three determinations

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The folpet and fenamiphos concentrations in the comercial formulations, Foltene (Bayer) and Nemacur 40 (Inagra), were found, 373% (mN) and 44.5% (mN) respectively.

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