Correct specific retention volume determination in inverse gas chromatography

Inverse Gas Chromatography (IGC) is an important technique for characterization of solids. Determining the specific retention volume of the injected probe molecule is the basis of the analysis for all the physico-chemical properties that the technique can determine, most importantly in Heat of Sorption, Glass Transition Temperature, Gibbs Adsorption Free Energy. Two equations have been used in the literature to calculate the specific retention volume; one normalizes the retention volume to 0 °C (standard temperature), which was previously proven to be thermodynamically incorrect, while the other calculates the retention volume at the measurement temperature. Here, we compare the heat of sorption for a series of alkanes on two substrates, micro crystalline cellulose and natural graphite, calculated using these two equations. This study shows that the specific retention volume is strongly dependent on the column temperature. Using the retention volume values normalised to 0 °C consistently overestimates the heats of sorption by up to 10%. Most importantly, correcting the retention volume to standard temperature will misrepresent the effect of temperature on the retention volume and the thermodynamic parameters derived from it.