Shape-shifting molecule behaviour during comprehensive two-dimensional gas chromatography analysis—A review

This research considers a number of molecular structural changes that have been reported in gas chromatography (GC), which take place during the chromatographic separation process of a molecule, and leads to an observation of an elevated baseline between the molecule and its product in a single column (1D) GC analysis. This phenomenon has been termed dynamic gas chromatography (DGC) in the case of interconverting compounds or reaction chromatography for a reaction process (RGC), and has also been referred to as forming a ‘bridge’. This is distinct from the conventional (dynamic) equilibrium process that defines chromatography. In order to observe this effect, the precursor (PRE) and product (PRO) must have resolvable peaks i.e., they must have a sufficient magnitude difference in their retention factors. Comprehensive two-dimensional gas chromatography (GC×GC) offers particular advantages to examine the nature of this elevated baseline phenomenon in the 2D space of GC×GC, provided the relative retentions of the respective PRE and PRO of the transformation are now resolved in the second dimension. This leads to the observation of what may appear as ‘streaking’ effects in the 2D plot, with theoretical terminal positions due to both the PRE and PRO components. These streaking effects should not be confused with adsorption (e.g., tailing) effects. The elevated baseline may comprise a single species, such as for a decomposition process, PRE → PRO, or may comprise more components such as when a reversible or interconversion process occurs, PRE ⇌ PRO. If degradation occurs in the heated injector, PRO may generate an elevated peak at its characteristic first and second column retention times (¹t_R and ²t_R) as more PRO is produced. Considerations of different shapes arising from the different processes and their kinetics will also be discussed. This review introduces general aspects of dynamic GC, with a range of applications reported for this phenomenon. The manner in which this on-column process may be illustrated in GC×GC analysis to produce certain patterns in the 2D space are highlighted. Two recent examples of rearrangements in essential oil samples and irreversible changes in pyrethrins are considered in more depth, and experimental details are provided for these two cases.