HYDROPHOBICITY PARAMETERS DETERMINED BY REVERSED-PHASE LIQUID-CHROMATOGRAPHY .2. DEPENDENCE OF RETENTION BEHAVIOR OF PYRAZINES ON MOBILE PHASE-COMPOSITION

The retention behavior of monosubstituted pyrazines was investigated by comparing the hydrophobic contribution of each substituent (kappa) to the capacity factor obtained from reversed-phase high-performance liquid chromatography (HPLC), using a C18 capsule-type column and methanol-water eluents of various compositions. The mobile phase dependency of kappa showed that the retention mechanism varied with the substituent, especially in mobile phases with very high water content, due to the intervention of the selective solute-solvent interactions. Under such circumstances, the capacity factor (log k') obtained from HPLC methods correlated very poorly with the octanol-water partition coefficient (log P). The log k' for pyrazines with only alkyl and/or alkoxy groups was measured and the correlation with log P was examined. An excellent linear correlation was obtained with the eluent containing about 30% methanol. However, at other methanol concentrations, the log k' values of the alkyl and alkoxy derivatives correlated with log P by two separated linear equations. The use of log k(w) (log k' at 100% water), which has so far been considered to be the best index of hydrophobicity, failed to simulate log P.