Metalaxyl is an important chiral acetanilide fungicide, and the activity almost entirely originates from the R-enantiomer. Racemic
metalaxyl has been gradually replaced by the enantiopure R-enantiomer (metalaxyl-M). In this study a chiral residue analysis method for
metalaxyl and the metabolite metalaxyl acid was set up based on high-performance
liquid chromatography tandem mass spectroscopy (HPLC-MS/MS). The
enantioselective degradation and chiral stability of metalaxyl-M in tomato fruits in two geographically distinct regions of China (
Heilongjiang and Hunan
Province) were evaluated and the
enantioselectivity of metalaxyl acid was also investigated. Tomato plants grew under field conditions with a one-time spray application of metalaxyl-M wettable powder. It was found that R-metalaxyl was not chirally stable and the inactive S-metalaxyl was detected in tomato fruits. At day 40, S-metalaxyl derived from R-metalaxyl accounted for 32% and 26% of the total amount of
metalaxyl, respectively. The metabolites R-metalaxyl acid and S-metalaxyl acid were both observed in tomato, and the ratio of S-metalaxyl acid to the sum of S- and R-metalaxyl acid was 36% and 28% at day 40, respectively. For both
metalaxyl and metalaxyl acid, the half-life of the S-enantiomer was longer than the R-enantiomer. The results indicated that the enantiomeric conversion should be considered in the bioactivity evaluation and environmental pollution assessment.
Chirality 28:382–386, 2016.