T. F. idlem man' and C. P. Rice Department of Food and Resource Chemistry University of Rhode Island Kingston, 02881
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TLC Analysis of Carbaryl Insecticide on Sprayed Foliage A student experiment
Although most pesticide residues are determined hy gasliquid rhromnto,vraphy. rhe carbama~esare difficult to ann1rw hv this mc2thod hecause of their thermal initabilir!. In -a-the past few years thin-layer chromatography (tlcl-has emerged a s a n important analytical method for this group of pesticides. In 1974, certain areas of Rhode Island were aerially sprayed with carbaryl(1-naphthyl N-methyl carhamate) t o control a gypsy moth infestation. T h e day followine the treatment of our area. we collected a l a r ~ auane tity 07 leaves and froze them f& carbaryl analysis b; students in our eraduate-level ~ e s t i c i d echemistry course. Our extraction-cleanup was adapted from Johnson , ~ t h e foliage exand Stanhury's crop residue r n e t h ~ d and tract was chromatographed on thin-layer plates along with standards of carharyl and 1-naphthol (a breakdown product of carharyl). After development, the plate was sprayed with sodium hydroxide, which hydrolyzed carbaryl to produce the intensely hlue fluorescent anion of 1-naphthoL3 -2
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Procedure
Blend 25 g of leaves, containing -100 ppm carbaryl, with two 150-ml volumes of dichloromethane (reagent-grade solvents are used throughout). Filter the extracts through a plug of glass wool and concentrate to a sticky residue on a flash evaporator or steam bath. Dissolve the residue in 20 ml of acetone and precipitate the ~ l a nwaxes t with 20 ml of 0.1% ammonium chloride in 0.2% phosphoric aeid. Allow the precipitate to settle for about 15 min and filter with suction through a fritted glass filter funnel. The filtrate should he yellow-orange or pale green; a dark green color indicates insufficient removal of chlorophylls. Wash the residue in the funnel with 20 ml of a 1:l mixture of acetone and ammonium chlorideaeid solution, and transfer the filtrate and washings to a separatory funnel. Add 100 ml of distilled water and extract the carbaryl with two 50-ml portions of dichloromethane, shaking the funnel gently with a swirling motion to minimize emulsion formation. Draw off the organic layer through glass wool and concentrate to 10 ml. Spat the extract along with standards3 of carbaryl and 1naphthol on a 20 X 20-cm Silica Gel G tlc plate (without fluorescent indicator), and develop the plate in 2:l ethyl ether-hexane.
The solvents should be added to the chromatography tank at least
k' hr before developing the plate to ensure chamber saturation. Allow the solvent front to travel 10 em above the spotting line, remove the plate, and spray with 1.0 M sodium hydroxide until the layer is saturated but not dripping (Caution! Avoid skin contact with the spray mist; spray the plate in a well-ventilated hood). Observe the spots immediately under long-wavelength uv light. For development in 21 ethyl ether-hexane, we found
R j s of 0.5 and 0.9 for carbaryl and 1-naphthol respectively, whereas Locke4 reported values of 0.34 and 0.48 for development in 4:1 ethyl ether-hexane. This difference can easily be attributed t o variations in tank atmosphere saturation and activity of the tlc adsorbent. Carbaryl and 1-naphthol can he quantitatively determined by measuring the fluorescence in situ with a scanning f l ~ o r i m e t e r Lacking .~ this instrument, we asked each student to visually compare the size and intensity of sample spots with those of 5-6 spots of a carharyl standard, covering the range 50-500 nglspot. Six students each selected and analyzed 25 g of leaves a t random from a bag containing approximately 1 kg of sprayed foliage. Their results, which reflect sampling a s well as analytical variations, were 63, 80, 102, 140, 144, and 349 mg carbarylikg leaves (mean = 146 i 104 s.d.). None of the students was able to detect conversion of carharyl t o 1-naphthol in the leaves, even in a sample which was allowed t o stand a t room temperature for several days before analysis. We do not think t h a t this was a fault of the analytical method, a s we obtained a n 80% recovery of both carbaryl and l-naphtho1 from a sample fortified with each a t a 40-ppm level. Johnson and Stanbury2 were unable t o find this hydrolysis product of crops treated with carbaryl, although most of t h e pesticide disappeared within seven days of application. Perhaps 1-naphthol, if formed a t all on leaf surfaces, decomposes or vaporizes more quickly than does carbaryl. 'Present address: Department of Chemistry, University of South Carolina, Columbia, S.C. 29208. Johnson, D. P., and Stanbury, H. A,, J. Agr. Food Chem., 13, 235 (1965). We prepared standards of carharyl from Sevin 50W, a commercially available wettable powder containing 50% carharyl, by shaking the powder with dichloromethane and allowing the insoluble material to settle before use. Sevin formulations can be used to "spike" samples if no sprayed foliage is availahle .' Locke, R. K., J. Agr. Food Chem., 20,1079 (1972). 5Frei, R. W., Lawrence, J. F., and Belleveau, P. E., Z. Anal. Chem., 254,271 (1971).
Volume 53, Number 3. March 1976 / 173
