Comprehensive two-dimensional gas chromatography (GCxGC)
What is it?
Gas chromatography (GC) is used by analytical chemists to separate an organic mixture prior to identification and quantitation. In a simple example, an individual chemical component elutes from the GC as a single discrete peak. With environmental and biological samples, multiple chemical components are eluting at the same time, resulting in overlapping peaks. Comprehensive two-dimensional gas chromatography (GCxGC, GC x GC, 2DGC, GC × GC, GC×GC) is a multidimensional chromatography technique used to improve the number of separated peaks in a single analysis. The GCxGC set-up includes a long first dimension column (typically 30 m – 60 m) that is connected to a short second dimension column (typically 0.5 m – 1.3 m), which is then connected to a detector. The phase chemistries of the two GC columns must be different in order to maximize the separation of the mixture. In between the two columns is a high-frequency modulator. The modulator is designed to quickly collect and “inject” fractions from the primary column onto the secondary column. The rapid process of modulation (every 2-5 sec) preserves the peak separation of the primary column and enables the secondary separation. There are two general categories of modulators, thermal and flow. A thermal modulator alternates cold and hot jets to trap and inject the effluent onto the secondary column. This type of modulator diverts the entire sample onto the secondary column and is best for trace analysis. A flow modulator uses valve switching to send small bands of the primary effluent onto the secondary column. This type of modulator is best for low-cost applications where ultimate sensitivity is not required. A single detector records both the first and second dimension retention time. Example detectors include flame ionization detectors (FID) which allow good quantitative analysis and linearity for petroleum hydrocarbons while time of flight mass spectrometry (TOFMS) can be used to provide a fourth dimension of information in full scan mass spectrometry for identification of unknown compounds. The GCxGC chromatogram (contour plot) is similar to a topographic map where the first dimension retention time is on the x-axis, the second dimension retention time on the y-axis, and peak intensities are on the z-axis (Figure 1).
Why is it important?
The analysis of complex environmental, biological, and petroleum samples using single dimension gas chromatography (1DGC, 1D GC) often results in a large area of unresolved components. Some of these compounds can be separated through the use of mass spectrometry (MS). However, the power of MS diminishes when high concentrations of matrix components interfere with low concentration target analytes (e.g. polychlorinated biphenyls, ignitable liquid residues, polycyclic aromatic hydrocarbons). Mass spectrometry is also unable to differentiate between structural isomers. There are many benefits to using GCxGC for the analysis of complex samples. The high concentration matrix components can be separated from analytes of interest with chromatography prior to MS detection. Separating interferences from low concentration analytes gives a higher quality match when using MS library database searching when compared to 1D GC analysis. A peak with a high MS library match increases the confidence of positive identification. The use of GCxGC is also beneficial in simultaneous targeted and non-targeted analysis. The GCxGC contour plot is structured in a way that is helpful for non-targeted analyte identification. Compounds with similar chemical and physical properties elute in clusters in a GCxGC analysis. This means that identifying one component in the cluster can provide clues as to the identity of neighbouring peaks. Complex samples contain thousands of individual analytes; by using GCxGC, the number of identifiable peaks compared to a 1D GC analysis increases exponentially. Detecting and identifying more peaks in a sample can give meaningful information that would otherwise be impossible. Analyzing a sample by GCxGC can increase the confidence of decisions required in a variety of environmental, biological, and petroleum applications.
Chemistry Matters Consulting Services and Expertise
The Chemistry Matters team has been working with this technology for over 15 years and pushing the adoption of GCxGC in a number of areas of work. Through strategic partnerships, we provide our clients with cutting-edge technology to solve complex issues in environmental forensics case studies, wildfire arson investigations, and petroleum hydrocarbon fingerprinting. Chemistry Matters can provide full service GCxGC expertise from analysis to data interpretation to communication of results.
When a chemical is released into the environment it is important to determine the source of the chemical release. Source identification can be especially difficult when there are multiple source inputs and when these inputs are of similar origin. Statistical analysis and receptor modelling is used to differentiate between sources; however, data provided by routine environmental monitoring is often too simplistic to conclusively determine the origin of the release. The use of GCxGC in environmental forensic investigations is beneficial to provide a detailed and comprehensive fingerprint of the chemical contaminant(s) and potential sources. Important diagnostic indicators are often at low concentrations compared to the bulk of the sample constituents. GCxGC fingerprinting has been used successfully in the analysis of petroleum hydrocarbons, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and volatile organic compounds (VOCs). GCxGC fingerprinting has also been used to determine sources and allocate responsibility on mixed free product plumes of condensate, gasoline, and diesel, including the ability to determine multiple sources of the same petroleum product.
The identification of ignitable liquid residues (ILRs) in wildfire arson investigations is considerably more complex compared to a typical structural fire. The vast area of investigation, the comparably lower concentrations of ILRs, and the presence of natural interfering compounds can all obscure the positive identification of ILRs. Through the use of GCxGC the Chemistry Matters team significantly reduced the number of false negatives in a wildfire arson investigation compared to a 1D GC analysis (Kates et al., under peer-review). GCxGC not only increased the confidence of positive samples it also expanded the capability to fingerprint gasoline and petroleum sources to further enhance an arson investigation.
The ability of GCxGC to separate multiple compound classes in a single analysis is well suited for biomonitoring studies. Reducing sample pre-treatment is made possible through the enhanced separation power of GCxGC. This is beneficial to a biomonitoring study as it decreases the cost per analysis and reduces a potential source of bias. Through GCxGC, it is also possible to simultaneously monitor targeted analytes and non-targeted analytes. Non-targeted analysis is becoming increasingly important as the number and complexity of chemical contaminants in the environment is not fully understood. GCxGC provides more data points in a biomonitoring study for statistical analysis that can better characterize exposure.
The use of GCxGC in the courtroom is still in its infancy. Outdated analytical methods that no longer reflect the current technology are still heavily relied on in litigious cases. Demonstration of equivalency to conventional methods is necessary to establish the credibility of GCxGC in the court of law. The Chemistry Matters team is actively involved in publishing research in peer-reviewed journals that highlight the advantages of using GCxGC compared to 1D GC in litigious investigations. In addition, effective communication of complex scientific findings to non-scientific stakeholders is critical to the success of any legal case. Visual communication is an effective means at describing results to a broad audience. Chemistry Matters use of GCxGC plots and statistical analysis through our partnership with Statvis Analytics Inc provide compelling visuals that can be understood by attorneys, judges, regulators, and the general public (Figure 2).
Kates, Lisa N; Richards, Philip I; Sandau, Court D., under peer-review. The application of comprehensive two-dimensional gas chromatography to the analysis of wildfire debris for ignitable liquid residue. Submitted to Forensic Science International.