Methods for determining the vapour pressure of active ingredients used in crop protection. V. Thermogravimetry combined with solid phase MicroExtraction (SPME)

The well-established evaporation rate method for determining the vapour pressure of active ingredients in crop protection down to the order of 10(-12) mbar can be supplemented by the new sample preparation technique of Solid Phase MicroExtraction (SPME). With this technique, it is possible to identify evaporating substances by analysis after partitioning into the polymer coating of a thin fibre in the outlet-gas flow of thermogravimetric equipment. The active ingredients fenpropimorph, kresoxim-methyl, metolachlor, clomazone and (Z)-9-dodecenyl acetate were used in this study, which showed that, despite the relatively small amount of collected material, an analytical identification of the evaporating compound by SPME/GC can be successfully achieved. In particular, the experiments have demonstrated a clear correlation between the linearity of the weight loss curve and the evaporation process of a pure compound. In the case of organic compounds that are unstable to heat, the SPME method can also be utilized to show whether, and at what temperatures, decomposition of the sample into fragments of higher volatility occurs. For example, the insecticide dimethoate showed a clear temperature dependence of both evaporation behaviour and in the SPME/GC analysis.