Resistive heating for multidimensional gas chromatography
thesisposted on 2023-05-27, 11:50 authored by Jacobs, MR
Gas chromatography is a critical technique used for the separation of volatile and semi-volatile samples prior to analyte detection for qualitative identification and quantitation. The separation of samples prior to detection provides useful information on the physical properties of analytes based on their retention within the chromatographic system, and separating sample components prior to their detection vastly simplifies the identification and quantitation of each component due to the reduction in sample complexity at the point of signal transduction. The optimisation of GC methods is crucial for ensuring high quality, timely and repeatable separations. Often a goal of method optimisation is to minimise the time required for an analysis while ensuring that adequate separation of target analytes is obtained. Such optimisations are achieved through the selection of column dimensions, stationary phase coating, carrier gas flow rate, and column temperature. While changing the column dimensions and stationary phase properties are very effective methods for adjusting separations, they are not programmable and require changes in hardware configuration. The carrier gas flow rate can be manipulated via pressure control of the carrier gas; unfortunately high carrier gas flow rates are detrimental to separation efficiency, without offering proportional gains in the speed for analysis. Another control parameter that is potentially very useful for high speed GC is the temperature of analysis. Changing the temperature of analysis can vastly increase the speed of analysis while maintaining separation efficiency. Temperature control of GC columns has been achieved using convection ovens for over 50 years. Convection ovens are simple to operate, safe and provide accurate control of column temperature during moderate temperature programming rates compared to classical alternatives such as liquid bath heating. Convection ovens simplified the installation of injectors, GC columns and detectors due to the connectivity provided by the heated oven cavity, which minimised solute condensation in the analyte flow path. The main limitation of convection ovens is their large thermal mass, which can cause thermal hysteresis during fast temperature programming. Secondly large amounts of electrical power are required to facilitate fast temperature programming, which can be detrimental in applications such as portable analysis. The separation column is normally the only component that needs to be temperature-programmed during GC analysis, while the injector and detector components are held at high isothermal temperatures to prevent solute condensation; therefore convection ovens present a source of instrument inefficiency. The resistive heating of capillary columns is an alternative to convection oven based heating that is significantly faster since only the mass of the capillary columns and associated heating elements need be heated. There are a number of commercially available GC instruments that offer the option of resistively heated capillary columns however these instruments still maintain the legacy convection oven for column to injector and detector connectivity. Commercial GC instrumentation is almost entirely limited to bench-top laboratory analysis, with few options for portable GC analysis systems. Portable analysis offers significant benefits over laboratory-based analysis. Performing an analysis at the point of sampling abolishes the need for transporting samples back to a laboratory-based facility, eliminating the time delay between sampling and analysis. Removing the sample transport step also minimises the risk of sample degradation or cross contamination arising from the analyte storage procedure, time, or sample preparation steps back at the laboratory. This allows portable analysis to provide greater confidence in the quality of measurements made compared to laboratory analysis. Portable analysers must be robust, repeatable, simple to use, and relatively low in cost to facilitate their deployment in the field. The Falcon Calidus‚Äövë¬¢ GC was selected as the analytical platform in this research due to its use of resistive column heating for fast GC analysis, excellent power efficiency, small form factor and low cost that would make it ideal for deployment in the field for portable GC analysis. Since the separation of complex mixtures using one-dimensional GC is generally unfeasible due to instrument and time constraints, multi-dimensional (MD) separation techniques were investigated as a strategy for maximising the separation capabilities of the Calidus‚Äövë¬¢ GC instrument. Specifically comprehensive two-dimensional gas chromatography (GC ‚àöv= GC) was investigated due to its potential to separate a large numbers of components without substantially increasing the time of analysis. Resistively heated thermal modulation and flow modulation strategies using PMDs were explored as a means to incorporate MD GC analysis techniques into the Calidus‚Äövë¬¢ GC. The Calidus‚Äövë¬¢ GC instrument was evaluated and found to be capable of providing comprehensive two-dimensional GC ‚àöv= GC analyses for the characterisation of a range of complex samples, while maintaining the capability of portable analysis unlike conventional bench top GC instruments. Additionally a novel single-stage thermal modulator was characterised, optimised and applied for GC ‚àöv= GC and found to be very effective at modulating a range of solutes without the need for the cryogenic focusing common in commercial GC ‚àöv= GC modulators. PMDs were also investigated as a low cost means of controlling injection bandwidths and providing effluent modulation in the Calidus‚Äövë¬¢ system and were found to be similarly effective compared to thermal focusing strategies.
Rights statementCopyright 2016 the Author Chapter 1, section 2 appears to be the equivalent of a post-print version of an article published as: M. R. Jacobs, E. F. Hilder, R. A. Shellie, 2013, Applications of resistive heating in gas chromatography : a review, Analytica chimica acta 803, 2-14. Chapter 4, sections 1-4 appears to be the equivalent of a post-print version of an article published as: M. R. Jacobs, R. Gras, P. N. Nesterenko, J. Luong, R. A. Shellie, 2015, Back-flushing and heart cut capillary gas chromatography using planar microfluidic Deans' switching for the separation of benzene and alkyl benzenes in industrial samples, Journal of chromatography A 1421, 123-128