Differential Roles Of Trace Water In Enantiomeric Resolution Of Chiral 2-propionate Derivatives Under Normal-phase Conditions

Separation of chiral transformation products greatly complements the understanding of stereochemistry of chiral pollutants. Transformation products usually differ in environmental and toxicological properties compared to the parent contaminants, thus causing potential and unknown environmental risks, particularly, some of which are detected in higher concentrations and/or in higher frequency. So they are recommended to be included in risk assessment of the parent contaminants. Chiral 2-propionate compounds are widely used in agriculture, metalaxyl, quizalofop ethyl, diclofop methyl, benalaxyl,2,4-drop propionate and their metabolites have been studied.In this study, direct enantiomeric resolution of metalaxyl and its main degradation product metalaxyl acid, often co-occurring in the environment, was carried out in normal phase HPLC with a Chiralcel OJ-H column. (R)-metalaxyl acid and (S)-metalaxyl, which were almost paralleled bonding to the chiral stationary phase, tended to separate, start to overlap, co-elute, and separate again with subtle changes of the mobile phase consisting of n-hexane,2-propanol, acetic acid, and trace water. Their competition above hampered an acceptable direct separation in fresh mobile phases. Aged mobile phases with a storage period of 3 to 5 days, however, significantly improved their separation, in which trace water from moisture air diffusion was found to play a major role. Trace water differentially affected peak width and retention times and then induced enhancing peak separation, confirmed by deliberate addition of water to fresh mobile phases. Furthermore, none of the studied factors, involving temperature, concomitant analytes, and trace water, could cause changes of the configuration of the chiral stationary phase.To elucidate differential chiral recognition of aryloxypropanoates herbicide quizalofop ethyl (QE) and its primary product (quizalofop acid, QA), their enantiomeric separation and biological impacts to two freshwater algae were investigated. Addition of trace water which selectively affects retention of analyte induced simultaneous enantio-separation for the two compounds owning to intrinsically water-specific resolution mechanisms, although they both possessed a chiral center in the 2-position of propionates. Uptake and/or transformation of QE and QA were found a lack of enantioselectivity and isomer inversion. Eight variants of chiral 2-propionate derivatives were chosen as well, which included diclofop methyl, diclofop acid, dichlorprop-methyl, dichlorprop acid, ibuprofen, napropamide, benalaxyl, and benalaxyl acid. The results demonstrated selective retention of trace water in separation. And trace water affected the resolution of 2-propionate derivatives in different degree, and the acid derivatives were more sensitive to the race water than the ester analogues.