Optimisation and application of pneumatic modulation techniques for comprehensive two-dimensional gas chromatography
Creators: name: Poynter, SDH
Publisher: name: University Of Tasmania
Year: 2023

description: Gas chromatography is the logical choice for the separation of volatile and semi-volatile mixtures and can provide exceptional peak capacity, particularly when used in a comprehensive two-dimensional configuration. Furthermore, coupling with a mass spectrometer provides a powerful tool for identification of components in complex mixtures. The key component in a comprehensive two-dimensional GC system is the modulation interface between the two separation dimensions, the primary purpose of which is to trap and release fractions of primary column effluent onto the secondary column whilst conserving the resolution of separated components from the first dimension throughout the two-dimensional separation. This thesis explored the use of pneumatic modulation interfaces for comprehensive two-dimensional gas chromatography coupled to a quadrupole mass spectrometer. Little research had been conducted into the use of pneumatic modulation with mass spectrometric detection due to a conflict between the high carrier gas flow required for pneumatic modulation and the requirement for high vacuum in the mass analyser. To commence, a series of one-dimensional separation approaches were investigated. A wide bore column was used under low pressure conditions for the analysis of moderately complex essential oils. Previous reports suggested that this technique was unsuitable for anything but simple mixtures, yet satisfactory separations of essential oils were possible. A high-temperature wax column was investigated subsequently, to determine if the claimed enhanced temperature stability would be beneficial for the second-dimension column in a comprehensive two-dimensional separation approach. This stationary phase was only available in narrow-bore dimensions and was highly satisfactory for cryogenic modulation but the internal diameter was not appropriate for pneumatic modulation due to the high head pressure required for operation. Next, a long, narrow-bore column was used to evaluate the conditions at which optimum efficiency could be generated for a translatable method. Use of fundamental relationships revealed a simple technique for determining a suitable column length based on a desired head pressure which provides the best trade-off between speed and efficiency optimisation. To conclude these one-dimensional investigations, an application is presented. Metabolite extracts from hop (Humulus lupulus L.) were obtained and separated using a GC‚Äö-MS approach. Simple statistical analysis techniques were performed to establish the suitability of the separation approach for generating sufficient information to allow varietal classification of the samples. A range of modulation interfaces were explored to describe the suitability of such devices for GC‚àöv=GC separation techniques. To commence, a commercial pneumatic modulation interface was used for the enantioseparation of a moderately complex essential oil. Results were critically evaluated in comparison to a thermally modulated approach, and suggested that the technique displayed significant potential as an alternative to traditional methods. Following this, a microfluidic Deans switch modulator was prepared. This utilised a switching system that only sampled a portion of the first-dimension column effluent for secondary separation. While exemplary separations of essential oil samples were achieved, it was clear that a total transfer method could provide superior results by improving sensitivity. To investigate this, a valve based pneumatic modulator was constructed and used in conjunction with the optimum column dimensions for the final investigation presented herein. Dual secondary columns were utilised to facilitate both flame ionisation and quadrupole mass spectrometric detection. This approach provides a powerful analytical tool for both qualitative and quantitative separation of complex samples such as those investigated in this study and would satisfy the requirements of many potential applications.