What Drives a Separation — the Stationary Phase or the Mobile Phase?

influence the driving force. Thermodynamic quantities determined from homologous series retention throughout the entire composition range may lead to a better understanding of the driving force and fundamental retention mechanism that results from the inherent differences in chromatographic solvation between acetonitrile and methanol. Tan and co-workers (11) showed that stationary-phase composition also has been important in determining a separation’s driving force; therefore, I investigated bonding densities ranging from high to low surface coverage. Many have investigated the various hydro-organic solvent interactions that occur in reversed-phase LC (2,8,9,12– 19). Differences in intermolecular interactions of the solute between mobileand stationary-phase components are the driving force of a separation. Tan and Carr (9) found that interactions between the stationary phase and solute are affected by the composition of the mobile phase. How do differences in interactions alter retention and possibly change the fundamental retention mechanism in a reversed-phase LC separaThe driving force of a liquid chromatography separation is determined by solute partitioning between the mobile and stationary phases. Chromatographers must study a wide range of mobile-phase compositions and stationary-phase bonding densities to determine the thermodynamic driving force. The author investigated methanol and acetonitrile as organic modifiers throughout a wide range of bonding densities (1.74–4.4 mol/m2) with nitroalkanes and alkylbenzenes as homologous solute series. She found that acetonitrile solvated the alkyl chains more efficiently than methanol and that she needed approximately 20% organic modifier to sufficiently wet the stationary phase and solvate the alkyl chains. The chromatographic system showed nonlinear enthalpy and entropy of solute transfer throughout the entire composition range for both organic modifiers, but she obtained a linear free energy of solute partitioning at organic modifier concentrations greater than 20%. All solute partitioning was enthalpically driven on a monomeric C18 stationary phase. What Drives a Separation — the Stationary Phase or the Mobile Phase?