Immunoassays for the detection of organophosphorus pesticides on stored grain

Abstract

Rapid semi-quantitative assays for grain protectant pesticides based on immunological methods have been studied at the Central Science Laboratory (MAFF) in order to provide a comparison with established chromatographic methods. The latter can provide precise and reliable data but tend to be expensive, time consuming and require highly skilled analytical staff. Increased public awareness and the recent reduction of maximum residue levels (MRLs) will place greater demands on analytical laboratories. In many instances when grain is traded it is only necessary to know that a fixed limit, often the MRL, has not been exceeded and in this situation immunoassays may be able to provide a cost effective, rapid screening technique which will identify those samples requiring detailed analysis using conventional instrumental techniques.

Commercial tube and plate ELISA kits were obtained from Millipore Corp. (USA) for the detection of organophosphorus pesticides and their performance was assessed, initially using laboratory pesticide standards. A small change in the response was observed when the kits were tested in the presence of substances co-extracted from the grain matrix and it was shown that this effect could be partially corrected by using a blank which also contains grain matrix. The kits were also assessed for cross-reactivity with other organophosphorus pesticides and the common metabolites which are formed when these pesticides degrade. No cross-reactivity of practical importance was found. A direct comparison was made with the established analytical method, gas chromatography and between 88 and 98% correlation was obtained, indicating that the kits possessed a high degree of accuracy. The standard curves were found to be reproducible as shown by the low coefficients of variation.

The ease-of-use of the kits in both laboratory and field situations was appraised and it was found that in order to achieve optimum results, some degree of user training was required. The instructions supplied with the kits were not easy to follow and could lead to erroneous results, hence a protocol for practical use on stored grain was developed which will allow grain store keepers, millers and maltsters to obtain reliable estimates of pesticide residues.