Fast temperature programmed gas chromatography coupled to supercritical fluid chromatography (SFC×GC)

Abstract

The topic of this thesis is the development of comprehensively coupled (supercritical fluid × gas) chromatography and its application to the chemical analysis of biodiesel and biodiesel blends. A future low-carbon economy might still have a need for large, high-efficiency diesel engines fueled with a high-quality carbon neutral fuel such as biodiesel. The quality of fuels are judged according to technical standards: documents that detail the requirements of compliance. Liquid fuels are complex mixtures that challenge the separation science used to ensure compliance. Chromatographic separations that use different separation mechanisms can be combined to meet those challenges, culminating in comprehensive coupling, where every fraction of a first separation is subjected to a second separation. The fundamental difference between the separation mechanisms of supercritical fluid chromatography (SFC) and gas chromatography (GC) allows for a powerful, technically feasible, coupled technique. To make the coupling practical the GC separation must be fast and temperature-programmed. A coaxial resistive heater for short capillary columns with active cooling by liquid carbon dioxide was constructed, with which hundreds of consecutive fast temperature-programmed GC separations were performed at a rate of four per minute. These chromatograms of fractions of SFC separations were combined to construct two-dimensional SFC×GC chromatograms. When biodiesel and biodiesel blends are analysed by SFC×GC, separation in the first dimension is by polarity and degree of unsaturation and in the second dimension by volatility. The resulting chromatograms contain powerful patterns of peaks, with the aromatic hydrocarbons separated from the alkanes, and the fatty acid methyl esters (FAMEs) of the biodiesel separated from the hydrocarbons of the petrodiesel. Because the flame ionization detector (FID) is used, quantification should be straightforward and reliable. The FID remains compatible with SFC×GC even when organic modifiers are added to the SFC mobile phase, because the volatile modifiers elute on the GC as a solvent peak, separate from the less-volatile analytes of interest. SFC×GC can be used for research and quality control in the liquid fuels and vegetable oil industries.