ENANTIOMERIC PURITY DETERMINATION OF (+)-2-(4-CHLORO-2-METHYLPHENOXY)-PROPANOIC ACID AND (+)-2-(2,4-DICHLORO-PHENOXY)-PROPANOIC ACID USING A HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY DIODE-LASER-BASED POLARIMETRIC DETECTOR

A method is reported for determining the enantiomeric purity of the phenoxy herbicides (+)-2-(4-chloro-2-methylphenoxy)- propanoic acid ((+)-CMPP) and (+)-2-(2, 4-dichlorophenoxy)- propanoic acid ((+)-2, 4-DP) that is suitable for quality-control assays at the 1% precision level. An achiral, reversed-phase high performance liquid chromatography (HPLC) procedure using a C18 stationary phase and methanol-acetonitrile-0.1 M aqueous acetic acid (42:30:28) as the mobile phase separates the analyte from trace chemical impurities; and the ratio of responses of polarimetric and absorbance detectors gives the (/?)-(+)-enantiomer mole fraction. Relative to pure standards supplied, samples of commercial products of (+)-CMPP and (+)-2, 4-DP are found to have mole fractions of (+)-enantiomer 0.920 ± 0.005 and 0.940 ± 0.008, respectively (95% confidence limits, seven replicate measurements). Assays are carried out with 200-(ig sample loading. The limit of detection for CMPP using the polarimetric detector is 2.4 jig, in agreement with theoretical calculations. The wavelength dependence of optical rotation conforms to a single-term Drude plot with the dominant chirophore being the benzenoid1Lbband around 285 nm, and specific rotations at 820 nm (the wavelength of the diode-laser-based polarimetric detector) are 7.1 ± 0.1 and 6.2 ± 0.2 ° mL g-1dm-1for (/?)-(+)-CMPP and (R)-(+)-2, 4-DP, respectively. © Oxford University Press 1993.