Single column gas chromatography (GC) does not have the separation power to analyze samples containing more than 150-250 compounds.^[1] The use of comprehensive two-dimensional gas chromatography (GC x GC) has recently been investigated as an improved means of separating such complex mixtures.^[2] GC x GC has been used in such diverse fields as petrochemical, food, environmental, forensic and essential oils.^[2][3] GC x GC offers increased peak capacity, structured chromatograms, and an increased signal height when compared to traditional gas chromatography.^[2][3] Even more benefits include enhanced sensitivity and improved identification power.^[2]

Two-dimensional chromatography is a type of chromatographic technique in which the injected sample is separated by passing through two different separation stages. This is done is by injecting the eluent from the first column onto a second column. GC X GC uses the second column as a second separation device to produce a series of chromatograms from the sample portions eluted from the first column.^[1] This allows bands which are poorly resolved from the first column to be completely by the different separation mechanism of the second column.

The second separation must operate with a higher speed than the first to produce a series of usually many hundreds of analyses. The relative speeds of the two columns must be specifically selected to produce second column chromatograms fast enough to keep up with peaks eluting from the fist column.^[1] The first column separation provides the second column with sub sample. These are relatively simple and are composed of substances of the same volatility. As such, the number of components on any one secondary chromatogram is limited making large peak capacities non-essential and as all the substance of the sub sample are of the same volatility, temperature programming is not required.^[1] The rate of peak generation is determined by the high-speed second column while the duration of the analysis, as well as how complex a mixture can be analyzed, is determined by the first column. GC X GC is faster for all but the simplest mixtures, and for truly complex mixtures, is the only method of separation that can be done within a practical period of time.^[1]

Two-dimensional gas chromatography is an orthogonal method as the separations that take place in the two columns are independent of each other; this is achieved by changing the retention of the second column as a function of the first.^[1] Typically, the first column has a non-polar stationary phase, and thus it retains compounds in proportion to their relative volatility. The second column usually has a more polar stationary phase, thus it retains compounds that are polar more strongly than those that are less polar.^[2][3] The overall net retention in the second column is due to the difference between the reaction mechanisms of the first and second columns.

Although the practicality of two-dimensional gas chromatography to certain applications has been studied, any sample that contains more than one hundred components can certainly be better performed by GC X GC than by traditional one-dimensional methods. Factors that determine whether the use of GC x GC is practical is the specific nature of the sample and analytes and as well as their dimensionality. Samples whose dimensionality is two or three are typically analyzed by GC x GC because the chromatograms are so well ordered.^[1]

Two-dimensional gas chromatography is primarily used for the separation of complex mixtures; however, it can also be used as a method of screening for simple mixtures. The two-dimensional plot produced may provide a quick overview of the components in unknown samples which can direct the analyst to the most suited column and conditions to analyze the sample.^[1] It may also be used to speed up simple separations, as discussed above, as the first dimension may be so fast that not all of the components are separated. The second dimension can thus be used to unravel overlapping peaks.

Petroleum products being complex mixtures of controlled composition are primarily tested by two-dimensional gas chromatography.^[1][3] Petroleum products are more explicitly complex mixtures of similar components, thus they have low dimensionality, typically two or three.^[1] GC X GC has been used to separate aromatics in gasoline, marine diesel fuel, kerosene, as well as the mixtures of toluene, ethylbenzene, meta-and¬-ortho-xylene and propylbenzene in white gas to investigate the use of generalized rank annihilation methods (GRAM).^[1]