Multidimensional Gas Chromatography

Two-dimensional gas chromatography (2-D GC) is a logical extension of gas chromatography (GC) carried out on a single column. It provides physical resolution of components which are difficult to analyze by other means. Two different arrangements are possible in 2-D GC. In the conventional mode, effluent fractions from the primary column are switched onto a secondary column of different polarity. In the comprehensive mode, the entire effluent from the primary column is examined by the secondary column. The major advantage of 2-D GC is the enhancement of peak capacity. Statistical methods of overlap theories predict severe limitations of the resolution of components in multicomponent mixtures. The problems are exacerbated when components form clusters, such as in the analysis of isomers. The determination of target analytes at trace levels also poses unique challenges. Selective detectors, in particular mass spectrometry (MS), can provide answers when merged components are capable of producing distinctive spectra. MS can compensate for poor chromatographic resolution in many cases but it cannot be applied for the deconvolution of isomers and closely related substances. In this situation, 2-D GC is the only choice. Instrumentation in conventional 2-D GC is based on one of two different effluent switching technologies. A valve can be used to divert the effluent from the primary column to a secondary column of different selectivity. In this arrangement, the sample inevitably comes into contact with the metal of the valve body. The other design is based on fluidic switching by application of a difference in pressure in the interface between the columns. This technology, generally referred to as Deans switching, can avoid metals in the sample flow path but it is technically more difficult than valve-based technologies. Comprehensive 2-D GC involves a focusing step. The effluent from the primary column is periodically injected into a high speed column in the form of narrow chemical pulses. The separation on the secondary column must be complete (or nearly complete) before the next pulse can be injected. The secondary column is polar, i.e. substances that have similar boiling points but differ in polarity can be resolved in the second dimension. The separations process is continuous and resembles data acquisition in routine gas chromatography/mass spectrometry (GC/MS). One of the true advantages of comprehensive 2-D GC is that the separations mechanisms on both columns can be decoupled, leading to true orthogonality. Components with similar properties fall into clearly defined data spaces. The chromatograms show a measure of order which is useful in substance identification and group type analysis. Comprehensive 2-D GC is still in its infancy. There is much room for improvements, including in the data acquisition/display area. Conventional 2-D GC, however, is mature. Because of the complexity of 2-D GC instrumentation, selective detection, i.e. GC/MS, is usually preferred where it is applicable. Many applications based on 2-D GC have been published. The analysis of flavors/fragrances, petrochemicals and persistent organohalogens in the environment by 2-D GC offers distinctive advantages over competing technologies.