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Technical Note KT 030505-1: Introduction to GC x GC
Abstract Comprehensive two-dimensional gas chromatography (GC x GC) is a means of multiplying the resolving power of one gas chromatograph (GC) by that of another, resulting in an order-of-magnitude increase in the number of chemical species that can be separated in a given amount of time. An important byproduct of techniques used to achieve the multiplicative effect is a significant increase in the sensitivity of the GC x GC instrument, as compared to that of a conventional GC. The Basics of GC x GC Practitioners of conventional one-dimensional gas chromatography are familiar with the three main components of a gas chromatograph, the injector, detector, and column, and many varieties of each (Figure 1).
The secondary column is coated with a stationary phase of composition different from that of the first, and therefore, employs a retention mechanism different from that of the first. Thus molecules are separated on the basis of independent chemical properties in the first and second columns, e.g., volatility in the first column and polarity in the second. For example, a first column coated with a common non-polar stationary phase such as methysilicone will separate on the basis of molecular size, i.e., volatility, or boiling point; whereas a second column coated with a carbowax stationary phase will separate on the basis of polarity. It is important to understand that the molecular property “polarity” is largely independent of the molecular property “size,” or volatility. Polar moeities can be attached to compounds of any size. We say, then, that these properties are “uncorrelated”, “independent”, or “orthogonal,” these terms being interchangeable. An important consequence of the fact that the separation criteria of the two columns are independent, or orthogonal, is that the retention time axes of the two columns may be thought of as lying perpendicular to one another. This is a somewhat subtle aspect of GC x GC, which entails interactions between the two columns. [4,5] The interface between the two columns is the second key to GC x GC. This so-called “modulator” performs three tasks in a repetitive fashion: accumulation, focus, and launch. The modulator accumulates sample eluting from the first column for a period of time equal to one third to one fifth, typically, of the duration of an individual peak from the first column. Thus if a first column peak is nine seconds wide at the base, the modulator will accumulate material every three seconds, thereby “chopping” the peak eluting from the first column into three “cuts”. The modulator focusses the material collected from each cut into an extremely narrow “band”, “plug”, or “chemical pulse,” these terms being interchangeable. The final step is to “launch” or inject the sharp chemical pulses sequentially onto the second column, on which a series of high speed gas chromatographic separations occur, one separation for each chemical pulse launched onto the second column.
Because the modulator repeats the above described actions every three seconds in the case discussed above, a secondary chromatogram is registered at the detector every three seconds. In general, the raw signal of a GC x GC experiment is a time-ordered series of secondary chromatograms, each having a duration of about 1/3 (or less) that of a peak eluting from the first column.
Each of the secondary chromatograms can be thought of as lying perpendicular to the retention-time axis of the first column. When drawn that way, lying side-by-side in time order, they form a so-called “retention plane” analagous to the physical retention planes of flat bed separators, such as two-dimensional thin layer plates or electrophoretic gels. When false color is used to represent peak intensity, a “GC x GC image,” or “comprehensive two-dimensional gas chromatogram” results (Figure 4).
Most of the colored spots -- the chromatographic peaks -- in images such as Figure 4 are believed to represent one, or a very few, of the chemical species present in the sample. In the case of the diesel oil appearing in Figure 4, some 5,000 peaks are discernable. Even with this very high peak count, coelutions still occur at the higher carbon number region on the right hand side of the image. Nonetheless, valuable information is still available from rich and chemically significant peak patterns. In the example of Figure 4, chemical classes are clearly visible. Column bleed products eluting from the first column are also clearly distinguishable from the sample matrix. Diagonal sub-bands appear throughout the chromatogram, corresponding to groups of isomers – the so-called “roof-tile” effect. [13]
The relationship between the GC x GC image and the conventional gas chromatogram is apparent in Figure 4. If each vertical column of the image is integrated and plotted as a function of elution time, the conventional gas chromatogram, or “first-dimension chromatogram,” appears. Therefore, scanning downward from each apparent peak in the first dimension chromatogram, one can count the number of coeluents, visible as discernable two-dimensional peaks, of which the conventional one-dimensional peak actually consists.
Conclusion
The GC x GC technique has revealed that the world is far more complex, chemically speaking, than one-dimensional separation techniques have revealed. Perfumes, fuels, oils, flavor extracts, indoor and outdoor air, fly ash, biological tissues, soils and sediments, plant tissue extracts, pyrolysates such as cigarette smoke or arson samples, purified chemical solvents – almost any matrix the gas chromatographer encounters, is found to contain hundred to thousands more chemical species than have been detected previously. It is fair to say that, given the astonishing chemical complexity that this new technique permits us to visualize, GC x GC is truly a new window on the chemical world.
Literature References
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