RAPID ESTIMATION OF OCTANOL-WATER PARTITION-COEFFICIENTS OF PESTICIDES BY MICELLAR ELECTROKINETIC CHROMATOGRAPHY

Micellar electrokinetic chromatography (MEKC) was evaluated as a new t echnique for the rapid estimation of octanol-water partition coefficie nt (logK(ow)). Retention measurements for more than 40 reference pesti cides with varied structural characteristics and hydrophobicity were c arried out in two MEKC systems, based on sodium dodecyl sulfate (SDS) and sodium cholate (SC), respectively. To enable an accurate determina tion of capacity factors in the SC-MEKC system, cypermethrin (a synthe tic pyrethroid insecticide) was utilized instead of Sudan III as the S C micelle tracer, since a few highly hydrophobic pesticides were found to elute after Sudan III. The linear correlation between logarithmic capacity factor (log (k) over tilde') and logK(ow) in the two systems was examined. It was found that, under the typical buffer condition (1 0 mM sodium phosphate with 60 mM surfactant, pH 7.0), the SDS-MEKC sys tem provided a somewhat wider dynamic range for hydrophobicity (logK(o w) from -1.0 to 4.5). However, the correlation of log (k) over tilde' with logK(ow) was not very high when all the reference pesticides were included in one single calibration set. For the SC-MEKC system, the d ynamic range for logK(ow) was in the range of 1.0-5.5, and a good line ar correlation existed between log (k) over tilde' and logK(ow), even when all reference pesticides were incorporated into a single calibrat ion group. By comparing the regression line of the reference pesticide s with that of a group of simple aromatic derivatives, it was discover ed that molecular size and functionality posed a less significant effe ct on the measurement of logK(ow) in the SC-MEKC system than in the SD S-MEKC system. Thus, SC-MEKC shall be the system of choice for the est imation of logK(ow). The typical error on logK(ow) determination using the current MEKC technique was within 0.5 units, suggesting that MEKC can be a valuable complement to reversed phase high performance liqui d chromatography (RP-HPLC) for the indirect determination of logK(ow). Besides maintaining all the advantages of the HPLC approach, the MEKC technique showed some unique benefits, such as better inter-column re producibility, higher throughput, and less handling of toxic pesticide s and solvents.