Colorimetric method for field-screening above-tolerance parathion residues on and in citrus fruits

A colorimetric technique has been developed which is suitable for use as a field-screening method for detecting above-tolerance levels of parathion on and in citrus fruits. By using this method, a grower should be able to postpone harvesting a crop until parathion residues are below tolerance level, so that the crop is safe to market. The method depends on the reaction of parathion with 4-(p-nitrobenzyl)-pyridine. Parathion is extracted by mixing chopped citrus rind with acetone in a hand-operated homogenizer. The extract is partially cleaned by a partitioning step before final cleanup with a Sep-Pak Florisil cartridge. The colored reaction solution is read at 560 nm by using a portable, rechargeable spectrophotometer. A single test can be completed in about 75 min; the average time per test when several are conducted sequentially is considerably shorter. The analytical system responds readily to less than 5 ppm parathion on or in 1 g navel orange rind, which corresponds to less than 1 ppm in the whole fruit. The present U.S. tolerance for parathion on or in citrus is 1 ppm on a whole fruit basis. Preliminary work indicates that the method should also be suitable for apples.