Part I. A thermodynamic study of acetonitrile + water mixtures in reversed-phase liquid chromatography. Part II. A gas chromatographic study of the effect of temperature and column pressure on solute retention on a bonded phase using helium and carbon dio

In the first part of this study, we develop a theory of the elution behavior of solvent disturbance peaks (DP) and the isotopically labeled components (ILC) of the eluent binary mixture under isocratic and isothermal conditions in reversed phase liquid chromatography (RPLC). The relevant equations of this theory relate the retention volumes of the DP and ILC to the absolute sorption isotherms (ASI) of the binary liquid mixture. Two models are used to obtain the ASI using the DP and ILC data. We observe a significant difference in the enthalpic ({dollar}Delta{lcub}bf H{rcub}{dollar}) and entropic ({dollar}Delta{lcub}bf S{rcub}{dollar}) contributions to solute retention of some n-alkylbenzenes in acetonitrile (ACN) + water and methanol (MeOH) + water mixtures (B. J. Barman, PhD dissertation, Department of Chemistry, Georgetown University, Washington D.C., 1986). We find that {dollar}Delta{lcub}bf H{rcub}{dollar} is fairly constant over a wide mobile phase composition range and {dollar}Delta S{dollar} exhibits a maximum in ACN + water mixtures. From the relative comparison of the solute activity coefficients in the mobile phase with solute retention data we conclude that solute retention and selectivity is mainly governed by processes taking place in the mobile phase.; Using the results of the ASI of the ACN + water mixtures two simple models are developed for the stationary phase. According to these models, we conclude that the sorbed solvent participates in solute retention but does not significantly affect solute selectivity in RPLC. We also conclude that the sorbed solvent partially penetrates into the bonded phase.; In the second part we report the specific retention volumes, V{dollar}sb{lcub}rm g{rcub}spcirc{dollar}, of some n-alkanes and n-alkylbenzenes measured at several average column pressures ({dollar}langle{dollar}P{dollar}rangle{dollar}) on a C18 bonded phase using He and CO{dollar}sb2{dollar} as carriers. We find that the experimental slope of ln V{dollar}sb{lcub}rm g{rcub}spcirc{dollar} against {dollar}langle{dollar}P{dollar}rangle{dollar} is at least twice the theoretical slope predicted purely from virial effects. This suggests that the sorption of CO{dollar}sb2{dollar} may play an important role in solute retention at low {dollar}langle{dollar}P{dollar}rangle{dollar}. The comparison of retention data on the bonded phase and on liquid n-octadecane suggests that the structure of the bonded phase and the substrate (silica) play important roles in solute retention in gas chromatography.