Researchers compared two widely used equations to identify pitfalls in normalizing specific retention volume for inverse gas chromatography (IGC) analysis.
A study published in the Journal of Chromatography A demonstrated discrepancies in the relationship between specific retention volume and column temperature in the process of inverse gas chromatography (IGC), showing that thermodynamic quantities can be affected by normalization of retention volumes where temperature is a variable (1).
Researchers from Surface Measurement Systems in Allentown, PA, the Surfaces and Particle Engineering Laboratory at Imperial College in London, UK, and the Institute of Material Chemistry and Research at the University of Vienna, Austria examined two equations accepted in previous literature to compare the heat of sorption for multiple alkanes on two substrates, namely micro crystalline cellulose and natural graphite.
Heat of sorption is one of the physicochemical properties, also including glass transition temperature and Gibbs adsorption free energy, that is determined by inverse gas chromatography (IGC), the primary objective of which is to characterize solids such as powders, particulates, and fibrous materials. Parameters measured during IGC analysis are inlet and outlet pressure (pi and po, respectively), pressure drop (△p), ambient pressure (pa), column temperature (Tc), ambient temperature (Ta), dead time (t0), retention time of elute (tr), and carrier gas flow rate (F), where po = pa if no restrictor is installed at the column outlet and the pressure drop within the detector is negligible. The specific retention volume of the injected probe molecule, expressed as Vg, forms a basis for the analysis of the properties identified by IGC. To express it another way, this is considered to be the volume of carrier gas necessary for the molecule to be eluted.
IGC is a technique used for the characterization of the surface and bulk properties of solid materials. It involves the injection of a non-polar gas into a chromatography column packed with the material of interest. The gas interacts with the surface of the material, and the resulting changes in the gas properties are measured. The data obtained from IGC can be used to determine various surface and bulk properties of the material, such as surface energy, polarity, acid-base properties, and porosity.
The errors found by the researchers confirmed the thermodynamic inaccuracy of one equation which they termed misleading. Namely, it normalizes retention volume at a standard temperature of 0 °C, multiplying specific retention volumes by the ratio 273.15/Tc. Using this formula, the researchers found that heats of sorption were consistently overestimated by up to 10%. More importantly, they said, a correction to a standard temperature misrepresents the effects of temperature on retention volume, while significantly distorting the relationship between retention volumes measured at differing temperatures.
While these were seen as significant pitfalls, other shortcomings in other prior studies were identified. One did not account for compressibility of the mobile phase, meaning retention volume was not corrected with the pressure drop occurring on the sample column. Other studies failed to correct the measured flow rate with regard to column temperature. In publishing this study, the researchers not only endeavored to prove that long-held equations may contain inaccuracies when it comes to consistent analysis, but also by doing so, aimed to change some of the nomenclature associated with IGC itself.
(1) Kondor, A.; Burnett, D.J.; Bismarck, A.; Williams, D.R. Correct specific retention volume determination in inverse gas chromatography. J. Chromatogr. A 2023, 1700, 464009. DOI: 10.1016/j.chroma.2023.464009