Analytical methods
Samples from the newly submitted supervised trials were analysed by HPLC with on-line reaction and fluorometric detection. The method determines both propoxur and its metabolite 2-hydroxy-propoxur (2-OH-propoxur). The homogenized sample is extracted with dichloromethane and the dichloromethane evaporated. The residual material is transferred to an Extrelut cartridge with 20 ml of saturated aqueous sodium chloride solution and 2 x 25 ml of dichloromethane. The cartridge is eluted with a further 50 ml of dichloromethane and the eluate evaporated to dryness. The dry residue is dissolved in 2 ml of methanol for analysis. A reverse-phase HPLC system is used with an acetonitrile/water gradient. The column eluate is passed successively through an on-line hydrolysis reactor to produce methylamine and a derivatization reactor to form a fluorophore with o-phthalaldehyde and 2-mercaptoethanol. Fluorometric detection is at 340 nm excitation and 455 nm emission.
The mean recoveries from potatoes were 86% for propoxur and 84% for 2-OH-propoxur at fortification levels of 0.02-0.2 mg/kg, and from lettuce 95% for propoxur and 93% for 2-OH-propoxur at 0.04-1.0 mg/kg. The limits of determination were 0.02 mg/kg for potatoes and 0.04 mg/kg for lettuce (Blass, 1990).
In the older trials samples were analysed by a colorimetric method (lettuce in the 1960s) or by GLC (lettuce and potatoes in the 1970s). The colorimetric method is based on measurement of the reaction product of o-isopropoxyphenol and aminoantipyrine at 490 nm. In the GLC method the compound determined is methyl N-methylcarbamate which is formed in the injection port by transesterification of propoxur with methanol.
