Studies of Losses of Pesticides During Sample Preparation

The loss of pesticide residues during sample preparation after extraction was studied. Filtration, partitioning and washing, concentration, and evaporation were examined separately, using 14C-labeled γ- BHC, p,p'-DDT, and dieldrin as representative insecticides. Washing-out of polar solvents was also investigated, using 14Clabeled methanol, acetone, and acetonitrile. Although losses of insecticide residues during filtration and partitioning and washing are relatively small, they are cumulative and cannot be neglected. Polar solvents are not completely removed by normal washing procedures. The possible significance of the presence of these trace amounts of polar solvents in column cleanup is discussed. Evaporation to dryness caused the most significant loss; it is not recommended even in the presence of soil co-extractives because they do not completely prohibit loss. A recommended standard procedure for soil extracts is given which yields quantitative recoveries of insecticides with five different solvent systems. The procedure should also serve as a guide for plant and animal extracts.

Thin-Layer Chromatography as a Rapid Screening Method for the Determination of Carbaryl and 1-Naphthol Residues Without Cleanup

A rapid, sensitive, screening method has been developed in which thin-layer chromatography is used for the detection and estimation of carbaryl and l-naphthol residues in apple, lettuce, and tomato extracts without prior cleanup. A procedure giving a convenient screening range of 0.1—30 ppm is described, but 0.02 ppm in apples and lettuce and 0.03 ppm in tomatoes may be detected without cleanup. By use of a partitioning cleanup 5 ppb of carbaryl may be detected.

Thin-Layer Chromatography for Chlorinated Pesticide Residue Analysis without Cleanup

Thin-layer chromatography on silica gel may be used for the determination of chlorinated pesticide residues in some plant extracts without prior cleanup, thus eliminating the possible loss of pesticides during the cleanup procedure. Rapid screening of a large number of samples is thus possible with consequent saving of time and labor. When necessary, thin-layer chromatography may be used as a prior cleanup for gas chromatography.