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Technical Note KT 030505-1:  Introduction to GC x GC


Edward B. Ledford, Jr.*, Joel R. TerMaat, and Chris A. Billesbach Zoex Corporation, 2611 West M Court, Suite D, Lincoln, NE 68522 (402)475-7640
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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).


Most users of conventional instruments believe that gas chromatographs can resolve on the order of hundreds of compounds, and that complex mixtures, such as petroleum liquids, are composed of hundreds of chemical species. These impressions reflect experience with conventional gas chromatograms, which seem to suggest that only a few hundred compounds are to be found in a mixture such as diesel oil (Figure 2). The simplicity of the one-dimensional diesel chromatogram is deceptive, however.  There are in fact several thousand chemical species in this sample. What appear to be single peaks in the chromatogram are actually coelutions of ten or more chemical species, on average. No one-dimensional GC is capable of resolving all the organic species present in diesel oil, and this is true of most complex mixtures.


By adding two pieces of hardware to a conventional GC, namely, a second column and an appropriate interface between the second column and the first, we obtain a GC x GC (Figure 3). The keys to this device are 1) the speed of the second column, which is very fast, and 2) the action of the interface, which repetitively samples the effluent from the first column, and injects it onto the second. [1,2,3]
 

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. 


The modulator interface has been implemented with a variety of physical devices [6-11]  some of which are commercially available [12].
 

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

 

  1. John B. Phillips and Jingzhen Xu.  "Comprehensive multi-dimensional gas chromatography," Journal of Chromatography A. 703 (1995) 327-334.
  2. John B. Phillips and Zaiyou Liu. "Chromatographic Technique and Apparatus"  U.S. Patent No. 5,135,549 ; August 4, 1992
  3. John B. Phillips and Zaiyou Liu. "Apparatus and Method fo Multi-dimensional Chemical Separation"  U.S. Patent No. 5,196,039
  4. C.J. Ventrakanami, Jingzhen Xu, and John B. Phillips.  "Separation Orthogonality in Temperature-Programmed Comprehensive Two-Dimensional Gas Chromatography", Analytical Chemistry,  68, No. 9 (1996) 1486-1492
  5. Jan Blomberg. "Proper tuning of comprehensive two-dimensional gas chromatography (GC x GC) to optimize the separation of complex oil fractions," in Multidimensional GC-based Separations for the Oil and Petrochemical Industry, A Ph.D. Dissertaion. Vrije University of Amsterdam. Printed by Universal Press, Veenendaal, The Netherlands, October 22, 2002.
  6. Phillip J. Marriott and Russell M. Kinghorn. "Longitudinally Modulated Cryogenic System. A Generally Applicable Approach to Solute Trapping and mobilization in Gas Chromatography." Analytical Chemistry, 69, No. 13 (1997) 2582-2588.
  7. Edward B. Ledford, Jr. and John B. Phillips. "Apparatus and Method for Chemical Modulation."  U.S. Patent No. 6,007,602 ;  December 28, 1999
  8. J.B. Phillips, R.B. Gaines, J. Blomberg, F.W.M. van der Wielen, J.-M. Dimandja, V. Green, J. Granger, D. Patterson, L. Racovalis, H.-J.de Geus, J. de Boer, P. Haglund, J. Lipsky, V. Sinha, and E.B. Ledford, Jr. "A Robust Thermal Modulator for Comprehensive Two-Dimensional Gas Chromatography." Journal of High Resolution Chromatography, 22, No. 1 (1999) 3-10
  9. Edward B. Ledford, Jr., Chris A. Billesbach, and Joel R. Termaat.  "Transverse Thermal Modulation," U.S. Patent No. 6,547,852 B2 ; April 15, 2003
  10. J. Beens, M. Adahchour, R. J. J. Vreuls, K. van Altena, and U.A. Brinkman. "Simple, non-moving modulation interface for comprehensive two-dimensional gas chromatography", Journal of Chromatography A 919, No.1 (2001) 127-132
  11. Edward B. Ledford, Jr. "Method and Appartus for Measuring Velocity of Chromatogrpahic Pulse," PCT Patent Application No. PCT/US02/08488
  12. Zoex Corporation of Lincoln, NE offers a GC x GC system based on "Loop Modulation" (see above reference);  LECO Corporation of St. Joseph Michigan offers a system based upon "Quad Jet" modulation (see reference 9 above).
  13. Jan Blomberg.  "GC x GC vs. GC-MS: Comparison of comprehensive two-dimensional gas chromatography (GC x GC) and gas chromatography-mass spectrometry (GC-MS) for the characterization of complex hydrocarbon mixtures," in Multidimensional GC-based Separations for the Oil and Petrochemical Industry, A Ph.D. Dissertation.  Vrije University of Amsterdam.  Printed by Universal Press, Veenendaal, The Netherlands, October 22, 2002. p. 191

 
 
 
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