RAPID FIELD-TESTS FOR THE ORGANOPHOSPHORUS PESTICIDES, FENITROTHION AND PIRIMIPHOS-METHYL - RELIABLE ESTIMATES OF RESIDUES IN STORED GRAIN

In order for grain handlers and traders to reliably estimate residues of grain protectants in the field, antibody-based tests were developed for the organophosphorus pesticides, fenitrothion and pirimiphos-methyl. To complement the rapid analysis, rapid but efficient extraction techniques were developed. In these tests, a pesticide-containing methanol extract of the grain sample and an enzyme-labelled component are added dropwise to precoated tubes containing buffer. After a brief incubation, the tubes are rinsed out in tap water and a substrate/chromogen for the enzyme is added. The colour developed is stabilized by acidification and the test result read either by eye or in a portable field photometer. Significant levels of the particular pesticide result in a pale colour compared to a dark green pesticide-free control. No calculations were required to provide a quantitative estimate of residue as this could be read directly from a graph of colour yield (absorbance) vs logarithm of pesticide concentration, using standard solutions of pesticide. For fenitrothion, the test had a limit of detection of 4 ng/ml (0.1 ppm in grain) and gave quantitative estimates in the range 0.5-15 ppm (in the grain), while the pirimiphos-methyl test had a limit of detection of 1 ng/ml (0.03 ppm in grain) and gave quantitative estimates over the range 0.1-15 ppm. Thus both tests can be used to segregate ''pesticide-free'' grain, with residues below 0.1 ppm. Data obtained using the field test correlated well with those obtained using laboratory methods, including both gas-liquid chromatography and immunoassay using microwell plates. The field immunoassay reagents were formatted into a small prototype test kit, and the components stabilised for field use.