LCGC The Peak - April 2008 - (Page 14)

14 THE PEAK APRIL 2008 For triglycerides, this has been well demonstrated by Dugo and colleagues (15). A field in which the concept of sample dimensionality can be applied elegantly is that of synthetic polymers. The dimensionality of a sample of a homopolymer with fixed end groups is one (molecular weight or degree of polymerization). If the end groups vary, the dimensionality can be seen as two (it could be higher, depending upon the variability between the different end groups). In random copolymers (with fixed end-groups), the sample dimension is two. In the latter case, highly structured chromatograms can be obtained if we can separate the sample according to chemical composition in one dimension and according to molecular weight in the other dimension. This is demonstrated in Figure 4, where gradientelution LC and size-exclusion chromatography (SEC) have been used as the separation mechanisms for the respective purposes. Orthogonality onal, as is demonstrated clearly by Figure 4. However, orthogonal separation mechanisms — while highly desirable — are not a prerequisite for performing successful two-dimensional separations. In many cases, the mechanisms might not be orthogonal, but well-structured two-dimensional separations can be obtained that cannot be separated in one dimension. Obstacles and Outlook In the specific case of Figure 4, the separation dimensions are truly orthogonal. The positioning of the peaks along the horizontal axis is only affected by the chemical composition; the positioning along the vertical axis is determined by the size of the molecules. Such orthogonal separations are the best one can hope for in LC LC. They allow the entire “separation plane” to be filed with peaks, implying that the entire peak capacity can — in principle — be used. Also, the interpretation of comprehensive two-dimensional chromatograms is much easier if the separation dimensions are well defined and orthog- One of the main limitations of LC LC is the availability of suitable detectors. To some extent, this problem is the same as that encountered in one-dimensional LC. Liquid chromatographers have learned to deal with this problem over the years. However, the problems are aggravated in LC LC, because the sample is diluted twice (in each column) (13) and because the peaks are eluted quickly. The sensitivity of UV detection is highly dependent upon the nature of the analytes, which makes it difficult to interpret twodimensional chromatograms of very complex samples. The response of an evaporative light scattering detector is more universal, but the inherent nonlinearity of this detector is a severe complicating factor. The chargedaerosol detector appears promising. The detection problem can be reduced if we are able to effectively focus the analytes between the first- and second-dimension separations. In GC GC, this is achieved easily by creating (and moving) a cold spot (“cryogenic modulation”). In LC LC, we have nothing with the same elegance, effectiveness, and universal applicability. However, in a number of cases effective focusing can be

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LCGC The Peak - April 2008 Contents LC3LC: Comprehensive Two-Dimensional Liquid Chromatography Split Injections in Gas Chromatography: How to Reduce Inlet Discrimination A Simple and Solvent-Free Method for Determining Tetracyclines in Prawns Industry News Events Education

LCGC The Peak - April 2008

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