In this review, recent technological advances in classical heart-cutting two-dimensional gas chromatography (GC–GC) are outlined. These developments are then illustrated by application to analysis of important flavor compounds at trace levels in very complex matrices. The technology can even be extended to three-dimensional capability (GC–GC–GC) for the most complex challenges.
Single column capillary gas chromatography (GC) offers high separation efficiency that, when combined with a suitable choice of stationary phase and detection system, is sufficient for most practical demands. However, real-life samples such as fragrances, fuels, and environmental extracts can often overwhelm the separation capabilities of even the most efficient single columns. A solution is to perform two-dimensional (2D) GC, where a second capillary column is added, and effluent is transferred from the first column to the second column for further separation. The more dissimilar the two chromatographic phases, the greater the gain or increase in the selectivity factor (α). It is critical when performing 2D GC that attention is given to how columns are connected and the corresponding mechanisms for transferring compounds between columns. This is key to success and is where most development has taken place over the years.
There are two fundamentally different approaches for the transfer of first column effluent to the second separation space. The most simple and direct is heart-cutting GC (GC–GC), where subsets of compounds or retention-time segments are passed to the second column for further separation. In an alternative approach (GCxGC) all compounds eluting from the first column are passed “on-line” to the second column; however, unlike in GC–GC, the speed of the secondary separation can be two orders of magnitude faster than the first column separation. GCxGC can therefore require expensive fast-scanning mass spectrometers and elaborate techniques for construction and interpretation of the two-dimensional data. This area is constantly developing, but is perhaps not yet ready for acceptance into routine analysis. The final goal is the evolution of a system that can offer on-demand routine two-dimensional capability to practitioners with maximum use of current routine mass spectrometry (MS) equipment and selective elemental detectors.
In this review, a 2D heart-cutting system is described that incorporates recent technological advances together with an olfactory detection port (ODP) for research and problem-solving in the flavor industry.
Applications studied are as follows:
- Methional in daily food products.
- Geraniol in brewed green tea.
- Total profiling of sulphur compounds in tobacco smoke.
Classical 2D GC with heart-cutting is continually making significant strides in terms of flexibility, applicability, and software-driven ease of access. Systems can be assembled for coupled 1D and 2D investigations without the need for intermediate instrumental reconfigurations. Multiple detectors, including human olfactory perception, can be incorporated into the various cycles to offer powerful capability for investigation and detection of flavor and environmental compounds.
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