Regression against Temperature of Gas–Liquid Chromatography Retention Factors. Van’t Hoff Analysis

Abstract

The present work deals with the precise experimental determination of the gas–liquid chromatography (GLC) retention factors (k) in a sufficiently large temperature range to allow the calculation of the thermodynamic quantities associated with the sorption process. Once isothermal retention factors of three homologous series members of the type H-(CH2)n-Y (Y = CH3, OH, CN) were measured over a temperature range of about 110 K on a low-polar PDMS (HP-1) capillary column and checked for accuracy and precision by “arc plot” representation, the data were analyzed by applying different forms of the van’t Hoff relationship. We compared the linear versus nonlinear van’t Hoff plots representing situations characterized by ΔsolvCp° = 0, ΔsolvCp°≠ 0 = constant, and ΔsolvCp° = f(T), respectively (ΔsolvCp° represents the difference in isobaric heat capacity associated with movement of the analyte between the mobile and the stationary phase). The “logarithmic” and “quadratic” nonlinear van ‘t Hoff equations were shown to be more appropriate than the linear van‘t Hoff equation for determining enthalpy and entropy of solvation. Special attention was devoted to the fitting performance and extrapolation capability of models with nonzero ΔsolvCp°. By several metrics, the quadratic model exhibits better behavior in extrapolations yielding reasonable accuracy for retention time and/or enthalpy of solvation predictions at temperatures located below the experimental range.