Mobile-Phase Contributions to Analyte Retention and Selectivity in Reversed-Phase Liquid Chromatography: 1. General...

Analyte retention in reversed-phase liquid chromatography is manipulated via the elution strength of the water-organic solvent (W-OS) mobile phase, whereby raising the OS volume fraction or eluent strength (methanol < acetonitrile) lowers retention. We investigated this effect at the molecular level through molecular dynamics simulations in a slit-pore model of a C₁₈ stationary phase, using the solute benzene to trace the composition and occupation of the immediate analyte environments involved in solute partitioning into the bonded-phase region and solute adsorption to the interfacial region. Spatially resolved contact analysis revealed that the number of bonded-phase contacts per analyte molecule decreases from the bonded-phase region over the extension of the interfacial region while the number of solvent contacts increases. The analyte density distribution in the stationary phase is sensitive to the local W density, which is controlled by the mobile-phase parameters. With increasing mobile-phase elution strength, the W density recedes from the interfacial region, favoring the occupation of analyte environments closer to the bulk liquid region. The ensuing redistribution of analyte density within the stationary phase results in an overall loss of bonded-phase contacts, tantamount to loss of retention. The retentivity of the stationary phase therefore depends on its solvation by the mobile phase.