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.

Towards comprehensive hydrocarbons analysis of middle distillates by LC-GCxGC.

The detailed characterization of middle distillates is essential for a better understanding of reactions involved in refining processes. Owing to a higher resolution power and an enhanced sensitivity, but especially to a group-type ordering in the chromatographic plane, comprehensive two-dimensional gas chromatography (GCxGC) offers unsurpassed characterization possibilities for petroleum samples. However, GCxGC fails to totally discriminate naphthenes from unsaturates occurring in hydrotreated diesel samples. This article aims at promoting the implementation of LC-GCxGC for the quantitative determination of hydrocarbon distribution in middle distillates, including naphthenes. In this configuration, liquid chromatography (LC) enables the separation of hydrocarbons into two fractions (viz., saturated and unsaturated) before the subsequent analysis of each fraction by GCxGC. In this paper, the choice of GCxGC conditions in order to achieve the separation and identification of hydrocarbons by chemical class is discussed; under these conditions, naphthenes are separated according to the number of saturated rings. For the first time, the presence of di-, tri-, and tetra-naphthenes resulting from the hydroconversion of aromatics can clearly be evidenced. A quantitative procedure for the determination of the distribution of hydrocarbons, including the distribution of naphthenes according to the number of saturated rings, is also proposed and discussed in detail. LC-GCxGC is found to provide an unequalled degree of information that will widely contribute to a better understanding of hydroconversion processes.