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Two-Dimensional Gas Chromatography
Two-dimensional gas chromatography (abbreviated GC x GC) is a chromatographic technique in which a sample is separated into its component compounds (the individual compounds that make up the sample) by passing through two separation stages. It differs from traditional or single column gas chromatography (GC) by the number of separation stages and by the complexity of the samples that can be analyzed. GC does not have the power to analyze samples containing more that 150-250 compounds. The use of GC x GC has been shown to separate such complex mixtures more efficiently. GC x GC has been used to separate and analyze petrochemicals, check the quality of food samples and essential oils, analyze environmental samples, and identify forensic evidence. Two-dimensional gas chromatography offers increased peak capacity, structured chromatograms, and an increased signal height (see chromatography) when compared to traditional gas chromatography. Even more benefits include enhanced sensitivity allowing analytes (compounds of interest that are contained within a samples) at lower concentrations to be detected and more accurately identified.

Theory
Separation through two different separation stages is accomplished 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. This allows peaks which are poorly resolved from the first column to be completely separated 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, normally consisting of 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. The first column separation provides the second column with sub samples. Sub samples are composed of compounds from the original sample that have similar volatility. The number of components on any one secondary chromatogram is limited making large peak capacities non-essential and as all the compounds of the sub sample are of the same volatility, temperature programming is not required. 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 than GC for all but the simplest mixtures, and is the only method of separation that can be done within a practical period of time to analyze complex mixtures.

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. Typically, the first column consists of a non-polar stationary phase, and will retain compounds in proportion to their relative volatility. The second column consists of a more polar stationary phase, and will retain compounds that are polar more strongly than those that are less polar. The overall net retention in the second column is due to the difference between how the compounds react chemically with the first and second columns.

Applications
The practicality of two-dimensional gas chromatography to certain applications has been studied, but any sample that contains more than one hundred components can certainly be better performed by GC X GC than by traditional one-dimensional GC methods. Factors that determine whether the use of GC x GC is practical are the specific nature of the sample and analytes 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.

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

Applications to Forensic Science
Two-dimensional gas chromatography has several different applications in forensic science and when coupled to powerful detecting techniques such as mass spectrometry, results obtained, can be admissible in courts of law. GC x GC is used in forensic toxicology to detect drugs of abuse in urine and blood.4 Medical examiners have also used this technique to confirm pain management drugs (analgesics), like fentanyl, in hairs specimens collected at autopsy. GC x GC, due to its efficient separation ability, is also used by arson investigators to improve the detection and identification of ignitable liquid residues, which is necessary in identifying arsons.

Applications to Environmental Testing
Petroleum products, being complex mixtures of controlled composition, are primarily tested by two-dimensional gas chromatography. Petroleum products are mixtures of similar components, having a dimensionality of two or three. GC X GC has been used to separate aromatics in gasoline, diesel, kerosene, as well as separate mixtures of toluene, ethylbenzene, and meta-and-ortho-xylene in white gas.

Two-dimensional gas chromatography is also used to track the weathering of oil spills. GC x GC was used to investigate the Bouchard No. 120 oil spill in April of 2003. By employing GC x GC, samples can be taken from the site of an oil spill at later dates in an effort to study remediate oil spills. GC x GC is also useful in identifying the source of an oil spill.

GC x GC is also useful in the identification of dense non-aqueous phase liquids (DNAPLs). DNAPLs are formed by hazardous waste leaks from underground storage tanks and seep through the soil, collecting in plumes. Trichloroethylene, dichloromethane, and many other chlorinated solvents are examples of DNAPLs.