Retention Studies for Large Volume Injection of Aromatic Solvents on Phenyl-Silica Based Stationary Phase in RP-LC

The use of a large volume injection of hydrophobic solvents as diluents for less hydrophobic solutes has already been proven for C18 and C8 stationary phases in reversed-phase liquid chromatography. The same possibility is investigated for a phenyl-hexyl stationary phase using aromatic solvents (benzene, toluene, ethylbenzene and propylbenzene) as diluents for several model analytes also containing aromatic rings. Both hydrophobic interaction and π-π stacking account for the competitive interaction of both the diluent and model analytes with the phenyl-hexyl phase. A linear decrease in analyte retention factor was observed with an increase of injection volume in the range of 1-100 µL. A moderate peak efficiency decrease was also observed, but peaks of model analytes remained undistorted with minimum band broadening up to 100 µL injection volume. A very small retention decrease was observed when changing the sample diluent in the homologous series: benzene, toluene, ethylbenzene and propylbenzene. The critical conditions for a successful large volume injection of analytes dissolved in studied hydrophobic solvents are for the analyte to have lower hydrophobicity and for the specified solutes to have proper solubility.