Abstract
A method for the determination of resmethrin (5-benzyl-3-furylmethyl-dl-cis, trans-chrysanthemate) in mosquito coils has been developed by gas chromatography.
The recovery of resmethrin from mosquito coils was not sufficient with common organic solvents alone by a Soxhlet extractor and the filler extracted from coils interfered with the stability and the recovery of resmethrin. So, several absorbents were mixed with coils for the removal of the filler and it proved that the recovery was the highest when powdered and activated charcoal were mixed with powdered coil and the mixture was refluxed with acetone. Furthermore, the addition of n-hexane to acetone was effective not only to diminish the extracts from coils blank but also to raise the recovery of resmethrin from coils and the recovery was more than 96% with the coefficient of variation of 1.34% when mixture of acetone and n-hexane (1+1) was used. However, the gas chromatographic peaks of the extracts from coils blank overlapped with the peak of resmethrin and disturbed the determination, and the extract was cleaned up by thin layer chromatography before gas chromatography.
The recommended procedure was as follows. Eight gram of powdered coil and 2 g of powdered and activated charcoal were mixed well and the mixture was refluxed with 120 ml of the mixture of acetone and n-hexane (1+1) for 3 hours by a Soxhlet extractor in water bath of 80°C. The solvent was removed in vacuo and residual matter was dissolved in chloroform to adjust the concentration of resmethrin in the range of 0.75 to 2.0 mg/ml. Two milliliter aliquot of this solution was spotted on a silica gel plate with thickness of 0.5 mm and dimension of 20×20 cm. The plate was developed with the solvent system consisting of n-hexane: ethyl acetate (4+1) in an ascending manner until the solvent front migrated about 13 cm above the starting line. The plate was air-dried completely and resmethrin (Rf value: about 0.85) was visualized by a UV lamp. The spot of resmethrin was scraped from the plate, transferred to a glass filter (JIS 11 G-4) and eluted four times with about 10 ml portion each of chloroform. Dicyclohexylphthalate was added as an internal standard, the solvent was removed in vacuo and the residual matter was dissolved in 1 ml of chloroform. Resmethrin was determined by gas chromatography under the following operational condition.
Apparatus: Yanagimoto GCG-550F with FID, column: glass cclumn with 0.75 m in length and 3 mm in inner diameter, column packing: 6080 mesh, acid-washed and silanized Chromosorb W coated with 5% FFAP (free fatty acid phase), column temperature: 200°C, injection temperature: 220°C, detector temperature: 230°C and carrier gas: nitrogen 20 ml/min.
This method could be applied to the determination of resmethrin in the coils containing both resmethrin and allethrin and allethrin could likewise be determined.
