Repeatability of n-octanol/water partition coefficient values between liquid chromatography measurement methods.

The n-octanol/water partition coefficient (K) is a physical/chemical property that is extensively used for regulatory and environmental risk and exposure assessments. The K value can estimate various chemical properties such as water solubility, bioavailability, and toxicity using quantitative structure-activity relationships which demands an accurate knowledge of this property. The present investigation aims to compare outcomes of three commonly cited methods of K measurement in the literature for six hydrophobic chemicals with insecticidal functions as well as highly volatile petroleum constituents. This measurement has been difficult to obtain for the selected pyrethroid insecticides, cypermethrin, and bifenthrin and is a novel measurement for the latter: polycyclic aromatic sulfur heterocycles, dibenzothiophene (DBT), and three of its alkyl derivatives except for DBT. The K values were obtained using two liquid chromatographic methods with isocratic and gradient programming, and the slow-stirring method following OECD 117 and 123 guidelines, respectively. The mean log K values of bifenthrin, cypermethrin, DBT, methyl-DBT, dimethyl-DBT, and diethyl-DBT were 8.4 ± 0.1, 6.0 ± 0.3, 4.8 ± 0.0, 5.4 ± 0.1, 6.0 ± 0.1, and 6.8 ± 0.0 using the HPLC method with gradient programing. The K values were significantly reproducible within a method, however, not between the methods. Results suggest assessing a chemical's property and environmental risk and exposure solely based on the K value should be practiced with caution.