letter to the editor
Potentials and Limits of Comprehensive GCGC A
t the 25th International Symposium on Capillary Chromatography in Riva del Garda, Italy, May 13–17, 2002, I presented a lecture entitled, “Comprehensive GCGC: Metrics, Potential, Limits” (1). The lecture sparked many debates. Unfortunately, and contrary to the facts, the report in Analytical Chemistry (2) implies that my position on all aspects of the performance of comprehensive GCGC (briefly, GCGC) was negative. In this letter, I would like to reiterate what I have actually stated. In the reports on GCGC published since its invention by the late J. B. Phillips in 1991 (3), many researchers experimentally demonstrated the advantages of GCGC over onedimensional (1-D) GC. Unfortunately, not all the claims regarding the superiority of GCGC were substantiated by sufficient analyses or experimentation. In many cases, a justification of the superiority of GCGC was based on a comparison of its best possible performance with the performance of 1-D GC operating under severely suboptimal conditions. The highlights of my lecture are presented below: 2-D Pattern. GCGC undeniably has a very important and well-known advantage over 1-D GC. The 2-D peak distribution pattern in GCGC chromatograms makes it possible to extract valuable information about the composition of an analyte. The 2-D pattern also provides a substantially richer set of unique features that can be used for analyte identification. Separation Power of GCGC. The level of the technique’s ability to resolve the components of a complex analyte was the main topic of my lecture. I outlined what I believed to be the first theory of GCGC optimization (with the goal of maximizing its separation power), presented a set of formulae for calculating the optimal parameters of major elements of a GCGC system (the length of the second-dimension column, modulation rate, etc.), and compared the performance of optimized GCGC with its optimized 1-D counterpart (1-D GC on the basis of the same column as the first-dimension column in GCGC). I described and/or clarified several relevant metrics, such as the peak capacity (4) representing the separation power in both GCGC and 1-D GC. Some conclusions follow. First, the peak capacity of GCGC can be more than an order of magnitude higher than the peak capacity of 1-D GC, which is an enormous potential advantage of GCGC. A 10-
fold increase in the peak capacity of 1-D GC could require a practically unacceptable 1000-fold increase in the analysis time. Second, the optimal conditions for GCGC are very challenging (optimal injection time into the second-dimension column should be several milliseconds; optimal data rate might be 1000 Hz or higher, etc.) and are not achieved in the currently known, actual implementations of GCGC. Because of the lack of specific quantitative data in the literature, it is hard to arrive at a quantitative conclusion regarding the separation power that is actually achieved by GCGC, but even its parity with the separation power currently available using 1-D GC can be questioned. Third, although GCGC, in its current state, might not have achieved superiority over 1-D GC in terms of separation power, the fact that the theory predicts superiority of GCGC is very encouraging. The theory says exactly how superior GCGC can be and under what specific conditions the superiority can be achieved. This was my main and, I believe, very encouraging message. Sensitivity of GCGC. Some researchers expected that, because GCGC produces sharper peaks than 1-D GC, GCGC should have a better detection sensitivity. This can be true for the minimum detectable amount (MDA), but not for the minimum detectable concentration (MDC), which is far more important in practice. The GCGC peaks are sharp because the second-dimension column is short. But the short second-dimension column also reduces the sample capacity of GCGC. A theoretical analysis shows that the favorable peak sharpness in GCGC and the unfavorable low peak capacity cancel each other. As a result, the MDC of GCGC should be generally the same as that of 1-D GC. I am not aware of any experiment that contradicts this conclusion. Leonid M. Blumberg Fast GC Consulting
[email protected] References (1) (2) (3) (4)
Blumberg, L. M. J. Chromatogr., A (in press). Harris, C. M. Anal. Chem. 2002, 74, 410 A. Liu, Z.; Phillips, J. B. J. Chromatogr. Sci. 1991, 29, 227. Giddings, J. C. Anal. Chem. 1967, 39, 1027.
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