Determination of hydrophobic parameters for pyridazinone herbicides by liquid—liquid partition and reversed-phase high-performance liquid chromatography

Abstract

The retention behaviour of eight herbicidal pyridazinones in a reversed-phase high-performance liquid chromatographic (RP-HPLC) system has been examined. Using methanol—wateras the mobile phase, a linear relationship between the volume fraction of the organic modifier and the logarithm of the capacity factor (log k') over a limited range was established for every solute. A comparison of the resulting curves showed that the separation system is selective with respect to the lipophilic trifluoromethyl substituent with otherwise the same structure. The influence of such a selective effect on the correlation between the log k' and the partition coefficient, P, obtained using the standard n-octanol—water system, is demonstrated. This effect can be eliminated if log P is related to the extrapolated k' value with pure water as eluent. The resulting curve accommodates the two sets of data ( r = 0.992). It is concluded that the organic modifier in RP-HPLC exerts subtle effects on the retention behaviour of pyridazinones, a discriminative feature which may also be important in biological membranes.