EXTRACTION OF PESTICIDE RESIDUES FROM PLANTS

Abstract This research was designed to evaluate the extraction efficiency of 14 C pesticides applied in commercial formulation to crops. Experimental variables included: Ten pesticides representing three chemical classes (organophosphates, organochlorines and carbamates); three extracting solvents (acetone, acetonitrile and methanol); two blenders (Polytron Homogenizer and Lourdes blender); three plant types (radish, strawberry and mustard greens); and three harvest intervals post application. Extractions were performed by blending followed by leaching and then Soxhlet extraction. 14 C content was measured in each fraction collected as well as in the remaining plant tissue. Subsequently the plant tissues were subjected to acid hydrolysis (0.25N HCl, reflux, 1 hour) in an effort to release any bound residues. Thin layer chromatography separations were made of both organic and aqueous soluble extracts to separate metabolites and quantify actual toxicant levels. A comparison of blenders revealed no statistical difference in percent 14 C extracted or toxicant levels. The three solvents often showed statistically significant differences in extractions of 14 C. Although variability did exist, the most consistent and efficient solvent was methanol, followed by acetonitrile, with acetone being the least effective. One example of this phenomenon was dimethoate applied to mustard greens; at the 7 day harvest interval, 97% of the 14 C was extracted using methanol, 91% using acetonitrile and 88.3% using acetone. Another difference between solvents was noted. After extracts were collected, the extracts were concentrated using a rotary evaporator to remove the organic solvent, and the remaining aqueous phase was then partitioned with methylene chloride. In a number of cases, the partitioning of the acetonitrile extracts resulted in a smaller proportion of the 14 C in the methylene chloride compared to extracts made with acetone and methanol. When one compares the three plant species, strawberries were generally extracted most efficiently, mustard greens were next and radishes were the most difficult to extract. One example of this may be seen in the case of propoxur applied to the three crops, all being harvested 14 days post application; the percent 14 C extracted from strawberries was 98%, from mustard greens, 94% and from radishes, 86%. Generally, the unextractable or bound 14 C increased with increasing harvest interval. For example, 15, 34 and 42% were bound at 3, 10 and 21 days, respectively, after carbofuran had been applied to radishes. Acid hydrolysis of solvent extracted tissues released varying quantities of 14 C. In the case of carbofuran applied to radishes and harvested 21 days post application, 12% of the bound 14 C (42% of the total 14 C was bound) was solubilized; for captan applied to radishes and harvested at 14 days, 62% of the bound material (11% total 14 C was bound) was released.