Verification of the need for optical purity measurement of chiral pesticide standards as agricultural reference materials

In this study, optical purity measurement was performed on eight kinds of commercially available pesticide, namely, Dimethenamid-P, Dichlorprop-P, Fluazifop-P butyl, Indoxacarb, Metalaxyl-M, Mecoprop-P, Quizalofop-P ethyl, and Uniconazole-P, in both enantiomer and racemate forms. Chiral separation of each pesticide was achieved by using HPLC with a photodiode-array (PDA) detector and a circular dichroism (CD) detector, which were connected in series. The chiral column used for the enantiomeric separation was a normal phase Chiralpak AD-H column. The mobile phase was n-hexane, with various alcohols added as polar modifiers. The study included investigation of the effects on enantiomeric separation of the percentage of alcohol used, the presence or absence of acid in the mobile phase, and the column temperature, and the optimum detection wavelengths of both PDA and CD detectors. Enantiomeric excess (ee) was calculated as an expression of optical purity. As a result, the ee of some pesticides investigated was approximately (over 95%) in accord with the data provided by manufacturers. However, the ee of Indoxacarb, Uniconazole-P, Quizalofop-P ethyl, and Fluazifop-P butyl was in the range 34.1–94.5%. These results suggest that there is a need to conduct optical purity tests, in addition to a chemical purity test, for optically active pesticides.