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Resistive heating for multidimensional gas chromatography
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Abstract
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™ 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™ GC instrument. Specifically comprehensive two-dimensional gas
chromatography (GC × 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™ GC.
The Calidus™ GC instrument was evaluated and found to be capable of providing
comprehensive two-dimensional GC × 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 × GC and found to be very
effective at modulating a range of solutes without the need for the cryogenic focusing
common in commercial GC × GC modulators. PMDs were also investigated as a low cost
means of controlling injection bandwidths and providing effluent modulation in the
Calidus™ system and were found to be similarly effective compared to thermal focusing
strategies.
| Item Type: | Thesis - PhD |
|---|---|
| Authors/Creators: | Jacobs, MR |
| Keywords: | Resistive heating multidimensional gas chromatography |
| Copyright Information: | Copyright 2016 the Author |
| Additional Information: | 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' |
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