Determination of ethylenethiourea in food commodities by a two-step derivatization method and gas chromatography with...

Ethylenethiourea (ETU) is a decomposition product from ethylene-bis-dithiocarbamates (EBDCs), the most widely used class of fungicides in the world. ETU has been classified as a possible human carcinogen. The maximum permitted residue level (MRL) in the European Union was set at 0.05 ppm. Gas chromatographic determination of ETU can be achieved only after derivatization. ETU is extracted from food samples and cleaned up by a combination of two-step derivatization and liquid-liquid partitioning. In the first step, ETU is derivatized with benzyl chloride to form S-benzyl ETU, which is then trifluoroacetylated to form the final product, which is amenable to GC. The determination is carried out with capillary gas chromatography using electron-capture (ECD) and nitrogen-phosphorus detection (NPD) as selective detection methods in parallel. The responses of ECD and NPD were found to be of the same order of magnitude. Therefore, the parallel response was found to be a useful criterion for peak identification down to the limit of detection. Reproducibility of the two-step derivatization of ETU to form trifluoroacetylated S-benzyl ETU was found to be satisfactory. The recoveries from apple, pear, tomato and a common baby food, at various concentration levels, were found to be between 82-92%, with a limit of detection of less than 1 ppb. Commercial samples, submitted for routine monitoring of dithiocarbamates (DTC) were also monitored in our laboratory for the presence of ETU. Four of the twenty samples found positive for DTC were also found to be contaminated with ETU in the range of 0.01 to 0.37 ppm. Three of these food samples were found to contain ETU residues above the MRL of 0.05, while those food samples containing DTC residues between 0.2 and 0.8 ppm were all below the MRL of DTC. No relation exists between the DTC residues concentration and the level of ETU. The screening data were further confirmed by electron impact mass spectrometry in selected ion monitoring mode. Chromatograms of ETU residue analyses are presented to demonstrate the extremely sensitive detection method with real food samples.