Stability of pesticides in plant extracts used as calibrants in the gas chromatographic analysis of residues
Introduction
Contrary to “classic” organochlorine pesticides such as DDT, aldrin etc., which are very persistent and relatively stable during analytical procedure, “modern” pesticides are significantly less stable; their degradation can be catalysed by many physicochemical factors. Although common organic solvents themselves are not reactive, some decomposition of pesticides can occur, especially when traces of impurities are present.
In many multiresidue methods, ethyl acetate is used for the extraction of pesticide residues from grains, fruits and vegetables 1, 2, 3, 4, 5, 6. Filtered extracts are often stored for several days in this solvent prior to the further determinative step. Some testing laboratories entirely omit sample clean-up and this routine practice results in excessively high recoveries depending on analysed materials and concentration levels. The use of matrix-standards can be (in some instances) the cost-effective way to compensate for matrix effects. To compensate for matrix-induced-enhancement of recoveries, standard solutions in residue-free sample extracts are used for the gas chromatographic calibration in some testing laboratories 7, 8.
Our study was aimed at the evaluation of pesticide stability in plant extracts in order to consider suitable conditions for their storage and handling in testing laboratories prior to gas chromatographic determination. To test the influence of coextractives on the pesticide stability, different types of matrices represented by wheat, oranges and cabbage were employed as important food commodities currently analysed for pesticide residues within Europe. Each of these matrices represents a broader group of commodities as to the content of moisture, lipids, pigments and many other components potentially influencing the stability of pesticides.
Samples were homogenised with ethyl acetate and anhydrous sodium sulphate and “crude” extracts were spiked with standard solution of pesticides mixture.
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Materials
Ethyl acetate, cyclohexane and toluene, all organic trace analysis grade SupraSolv, were supplied by Merck (Darmstadt, Germany). Sodium sulphate anhydrous (analytical grade) was obtained from Lachema Brno (Brno, Czech Republic) and heated 4 h at 600°C before use. Silanized 2-ml amber ampoules (Supelco, Bellefonte, PA, USA) were used for the storage of extracts.
All 38 pesticide standards with purity >95% were supplied by Dr. Ehrenstorfer (Germany). Two stock solutions were prepared in toluene
Results and discussion
A modified analytical method according to the European standard prEN 12393 (method “P”) was used [6]. As a part of the intra-laboratory method validation, both recovery and repeatability for relevant pesticides/matrices were determined for sample extracts spiked at concentration levels corresponding to those used in further stability study. Data in Table 1 illustrate the performance characteristics of the employed multiresidual method. Recoveries were evaluated for all residues on spiked sample
Conclusions
Most of the examined pesticides were stable in ethyl acetate plant extracts at least 40 days if stored at temperatures below 20°C. No degradation of synthetic pyrethroids was observed even at 40°C, whereas decomposition of most organophosphates in orange or cabbage extracts become significant at elevated temperatures. If stored at a sufficiently low temperature, the stability of all pesticides in wheat extracts is comparable with that in pure extraction solvent. Pesticide standard solutions
Acknowledgements
This study was supported by Copernicus Project, contract No. CIPA CT94-0144 coordinated by Dr. R. de Vos, TNO, Zeist, Netherlands and also by the National Agency of Agriculture Research, contract No. 6522.
References (11)
- et al.
J. Chromatogr. A
(1995) - et al.
J. Chromatogr. A
(1996) - et al.
J. Chromatogr. A
(1993) - et al.
Fresenius J. Anal. Chem.
(1991) - et al.
J. Agric. Food Chem.
(1994)