Reversal of elution order in a single second dimension by changing the first column nature in comprehensive two-dimensional gas chromatography

This paper explains how one single stationary phase can involve two different elution orders for linear alkanes, cyclic alkanes, aromatics and phenols using comprehensive two-dimensional gas chromatography. For this purpose, a coal-derived middle distillate was injected in nonpolar×semipolar and polar×semipolar configurations implying the same second dimension stationary phase (trifluoropropyl). Results show that even if the same column is utilised as a second dimension, the group-type elution order is reversed from one combination to the other. This can be explained as follows:for the polar×semipolar combination, each fraction eluting from the first dimension contains species that differ so much in terms of boiling points, that volatility plays a key role in the second isothermal separation. This is exemplified by the separation of a phenol and demonstrated using the proportional relationship between retention times, vapour pressures and activity coefficients. Moreover, van't Hoff plots (plots of ln k vs. 1/T) demonstrated the influence of the elution temperature from the first dimension on the second dimension separation. Therefore, available choice of stationary phase's combinations is much higher considering that one single column leads to very different retentions for similar compounds. Finally, this can explain why a reverse orthogonality approach is usually proficient for the separation of polar compounds.