Determination of n-octanol/water partition coefficient for DDT-related compounds by RP-HPLC with a novel dual-point retention time correction

n-Octanol/water partition coefficients (P) for DDTs and dicofol were determined by reversed-phase high performance liquid chromatography (RP-HPLC) on a C18 column using methanol–water mixture as mobile phase. A dual-point retention time correction (DP-RTC) was proposed to rectify chromatographic retention time (tR) shift resulted from stationary phase aging. Based on this correction, the relationship between log P and log kw, the logarithm of the retention factor extrapolated to pure water, was investigated for a set of 12 benzene homologues and DDT-related compounds with reliable experimental P as model compounds. A linear regression log P = (1.10 ± 0.04) log kw – (0.60 ± 0.17) was established with correlation coefficient R2 of 0.988, cross-validated correlation coefficient Rcv2 of 0.983 and standard deviation (SD) of 0.156. This model was further validated using four verification compounds, naphthalene, biphenyl, 2,2-bis(4-chlorophenyl)-1,1-dichloroethane (p,p′-DDD) and 2,2-bis(4-chlorophenyl)-1,1-dichloroethene (p,p′-DDE) with similar structure to DDT. The RP-HPLC-determined P values showed good consistency with shake-flask (SFM) or slow-stirring (SSM) results, especially for highly hydrophobic compounds with log P in the range of 4–7. Then, the P values for five DDT-related compounds, 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1,1-trichloroethane (o,p′-DDT), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethane (o,p′-DDD), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethene (o,p′-DDE), and 2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol (dicofol) and its main degradation product 4,4′-dichlorobenzophenone (p,p′-DBP) were evaluated by the improved RP-HPLC method for the first time. The excellent precision with SD less than 0.03 proved that the novel DP-RTC protocol can significantly increases the determination accuracy and reliability of P by RP-HPLC.

Research highlights
► A novel dual-point retention time correction (DP-RTC) was proposed.
► Excellent linear relationship of logP and logkw was obtained by the rectification.
► LogP values for five DDT-related compounds were measured by the improved RP-HPLC.