Reactive pre-concentration of trace amounts of pesticides in environmental analysis procedures. Part I. Determination of Carbamates by indirect spectrophotometry

Abstract

A method is described for the indirect spectrophotometric determination of aldicarb (Temik)[2-methyl-2-(methylthio)propionaldehyde O-methylcarbamoyl oxime] at trace levels in aqueous samples. The procedure starts with nucleophilic cleavage of aldicarb on a small column packed with a strongly basic anion exchanger, also providing quantitative sorption of the resulting oximate. Subsequent addition of sulphuric acid results in the in situ formation and elution of hydroxylamine, later used to reduce quantitatively an equivalent amount of Fe^III to Fe^II. Fe^II is determined spectrophotometrically via the formation of the red Fe^II-bathophenanthroline complex. Elution recoveries were virtually 100% with standard deviations of less than 1% for the detection of 2.5 × 10^–6M aldicarb, pre-concentrating 200 ml of a 1 µg ml^–1 analyte solution. Larger sample volumes permit pre-concentration factors of up to 100. Thus, 500-ml water samples were spiked to contain 25 ng ml^–1(initial concentration) of either aldicarb, aldicarb sulphoxide, aldicarb sulphone or methomyl. After 25-fold pre-concentration, analyte recoveries were 95% or better using the indirect spectrophotometric determination.