Figure 1 shows a schematic diagram

Figure 1 shows a schematic diagram of a MDGC system (ref 209). Note that this can also operate as a 1D system, with the 1D column directed to the first detector. The usual mode of MDGC is straightforward. A valve or selection device (S – switch mechanism) allows some small region of components that emerge from the first column (1D) to be switched to the second column (2D). This can be called a ‘heart- cut’. The selected zone can be (usually) a narrow cut, or a large fraction of the 1D effluent can be selected. The selected compounds then elute on the 2D column, where the aim is to attain better resolution or separation. This may be, for instance, where certain peaks must be analysed in a sample, and each region where they are expected is heart-cut to the second column. The second column then provides the improved separation performance that allows each compound to be resolved. This method might be commonly used for samples that contain many compounds (a
‘complex sample’) such that most peaks are unresolved. The peaks are better resolved on the second column by virtue of the fact that we use a column of different
‘selectivity’ or phase coating. This shifts the peaks around compared to the first column, and so – hopefully – any interfering peak are separated from the desired components on the second column. We use the terms peak capacity ratio to indicate how much more separation is achieved on the 2D column compared to the 1D cut.

An obvious application of MDGC is chiral analysis. Here, a chiral column is used as the 2D column. Enantiomers are unresolved on an achiral column, and so appear as one ‘peak’ to be heart-cut to the enantioselective column. On this 2D column, they are then resolved into their component enantiomers.

Figure 1 also shows that a variety of detectors, including spectroscopic techniques, can be used to provide additional characterisation of compounds that elute for the column. We will not discuss detection methods here, however a detectors used in

comprehensive two-dimensional gas chromatography (GC×GC; see later) is provided in ref 144 and ref B18.