Analysis of Dithiocarbamate Fungicide Residues by Liquid Chromatography/Mass Spectrometry and Isotope Ratio Mass Spectrometry

On the basis of the earlier experiences, the goal was to develop a simple, rapid, and
sensitive LC/MS method for the simultaneous determination of 'intact' DTC fungicide residues in fruits and vegetables that eliminates the most time-consuming steps, i.e. derivatization and clean-up steps. The following issues were addressed:

- Extensive screening of diverse LC columns from different suppliers to evaluate a suitable column for separating the DTCs with consideration that DTCs are only stable under strong alkaline conditions;
- Development of a simple volatile MS-tolerant eluent with consideration that DTCs are only stable under strong alkaline conditions;
- On the basis of the existing extraction buffers, a simple extraction buffer had to be developed to increase the stability of very unstable DTCs in the presence of plant juices; the stability tests must be performed;
- Due to the high instability of DTC residues extracted from plant samples, a stable isotope dilution assay (SIDA) had to be applied, i.e. a stable isotope-Iabeled DTCs as internal standards are necessary;
- Validation of the new LC/MS method on different fruits and vegetables with regard to reproducibility and recovery.

Concerning the fact that generally LC/MS/MS systems are used in pesticide residues analysis, it was additional aim to develop a more sensitive method of LC/MS/MS as an improvement of LC/MS method. The validation of the new LC/MS/MS method on different fruits and vegetables had to be performed and proven by parallel analyses using the LC/MS method and the common CS2 digestion/distillation method, and so to check the routine capability of LC/MS(/MS) for the residue analysis ofDTC fungicides.