The observation of band shapes for shape-shifting molecule transformations in comprehensive two-dimensional gas...

Selected components in essential oils are able to undergo on-column thermally-mediated structural changes (Cope rearrangements) during gas chromatography (GC) analysis. In this study, focus is on the germacrene B-to-γ-elemene and furanodiene-to-curzerene transformations. The technique of comprehensive two-dimensional gas chromatography (GC×GC) is shown to be especially useful in analysing the progressive on-column transformation; it yields a plateau - also called a 'bridge' - in 2D space which demonstrates the continuous 'reaction' process, plus importantly generates a band largely free from interferences and overlap with other compounds. It is demonstrated that even very low abundance processes can be readily observed. Isothermal operation in GC×GC generates a band at a constant retention time on the second dimension. For temperature programming, by contrast, ²t_R progressively moves to shorter retention as the oven temperature increases. Changes in the band shape arising from varied isothermal temperature (130, 140, 150, 160 and 170 ⁰C), changes in column flow rate (0.5, 0.8, 1.0 and 1.2 mL/min), and variations in temperature programming rate (0.5, 1.0 and 2.0 ⁰C/min) are described. These may be ascribed to changes in the rate at which the transformation proceeds. The band arising from the germacrene B-to-γ-elemene transformation increases in intensity as temperature programming rate increases, and reaches a maximum at the germacrene B first dimension (¹D) retention time. However the furanodiene-to-curzerene band is shown to decrease in intensity, eventually apparently reducing to zero. This is interpreted as the complete depletion of the precursor furanodiene at some position along the capillary column, and thus is unable to produce further curzerene. None of the conditions tested allowed the furanodiene peak to be observed in the chromatogram.