Decoding Shapeshifting Molecules: Band Shape Insights from 2D Gas Chromatography

Researchers from Monash University (Victoria, Australia) and Universiti Sains Malaysia (Penong, Malaysia) observed how band shapes can change in molecules during comprehensive two-dimensional gas chromatography (GC×GC). Their findings were published in the Journal of Chromatography A (1).

The injection of a single compound in GC typically leads to the resulting chromatogram containing a single, ideally symmetrical, peak. Variation in this peak shape may occur if an excessive amount of compound is injected such that non-linear conditions arise where the relationship K = C_S/C_M deviates from a constant value (in isothermal GC) and leads to overloading in GC, among other phenomena. If there are occasional extra non-chromatographic interactions, tailing behavior may occur. The first can be overcome by injection of less analyte or other physical parameters that produce linear conditions, whilst the latter may be improved by use of a thick film column, a new column entirely, or derivatizing the analyte. Irreversible thermal degradation of an injected analyte may cause anomalous peaks to appear in a chromatogram, which may assist in interpretation of the degradation process. As such, peak shapes in GC can reveal details of different phenomena related to the injector, sample mass, or on-column effects.

One notable phenomenon related to molecular processes is referred to as “shapeshifting,” where a molecule can undergo a change in structure (A → B, or A ⇌ B) under prevailing conditions during the GC analysis. Typical provisions state that the energetics of the process should allow independent observation of both of the species A and B, plus the interconversion plateau or bridge.

In this research, two transitions in myrrh oil, namely germacrene B-to-γ-elemene and furanodiene-to-curzerene, were investigated further with notable interest in the respective band shapes produced under various chromatographic operating conditions. Consideration of the implications on the relative abundances of the precursor molecule on the suspected shape of the band is proposed.

GC×GC is shown to be especially useful in analyzing the progressive on-column transformation; it yields a plateau—also called a bridge—in a 2D space, which demonstrates the continuous reaction process, and generates a band largely free from interferences and overlap with other compounds. Even very low abundance processes could be readily observed, with 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 can be ascribed to changes in the rate at which 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, reducing to zero. This could be interpreted as the complete depletion of the precursor furanodiene at some position along the capillary column; as such, the column is unable to produce further curzerene.

References

(1) Lim, R.; Amaral, M. S. S.; Wong, Y. F.; Marriott, P. J. The Observation of Band Shapes for Shape-Shifting Molecule Transformations in Comprehensive Two-Dimensional Gas Chromatography. J. Chromatogr. A 2025, 1758, 466189. DOI: 10.1016/j.chroma.2025.466189