Multidimensional liquid chromatographic techniques

In parallel with recent developments in GC, multidimensional HPLC (LC x LC) is now also finding application in environmental analysis.33 The combination of two sufficiently different separation dimensions (e.g., NP-HPLC x RP-HPLC or IC x RP-HPLC), however, remains difficult because of the solvent compatibility issues discussed above. Here, too, HILIC may bring about a significant improvement, since its mobile phase requirements are much closer to RP-HPLC than those of other liquid chromatographic techniques.34 In contrast to GC x GC, LC x LC cannot be implemented with a (thermal) modulator that collects the analytes after the first separation dimension and reinjects them into the second column it is most practically realized with a double-loop interface that alternately collects and transfers the analytes from the first to the second dimension (Figure 13.7). Even though the second dimension chromatogram is also very fast, detection is not normally a problem since the peak widths in the second dimension are usually still of the order of 1-2 s. 

A certain amount of qualitative information can be obtained by means of so-called multidimensional chromatography (1). Tlris is a combination of different chromatographic techniques in which fractions from a primary separation step are transferred online to a secondary separation step. Multidimensional gas chromatography (GC), for example, involves coupling of GC columns of different selectivities so that the primary column isolates the fraction of interest, and the secondary column takes care of the final separation of that fraction. Using multidimensional liquid chromatography (LC), determination of androgen hormone residues in cattle liver has been possible (2). 

The use of different modes of liquid chromatography facilitates the separation of complex samples, selectively, with respect to different properties like hydrodynamic volume, molar mass, chemical composition, or functionality. Using these techniques in combination, multidimensional information on different aspects of molecular heterogeneity can be obtained. If, for example, two different chromatographic techniques are combined in a cross-fractionation mode, information on chemical composition distribution and molar mass distribution can be obtained. Reviews on different techniques and applications involving the combination of GPC and various LC methods can be found in the literature [6-8]. 

Coupled systems include multidimensional and multimodal systems. Multidimensional chromatography involves two columns in series preferably two capillary columns, with different selectivity or sample capacity, to optimize the selectivity of some compounds of interest in complex profiles or to provide an enrichment of relevant fractions. In multimodal systems, two chromatographic methods or eventually a sample preparation unit and a chromatographic method are coupled in series. Coupled systems that received much interest in recent years are multidimensional CGC (MDCGC), the combination of high-performance liquid chromatography with CGC (HPLC-CGC) and the on- or off-line combination of supercritical fluid extraction with CGC (SFE-CGC). Multidimensional and multimodal techniques in chromatography arc described in detail in [65],... 

Chromatographic and electrophoretic separations are truly orthogonal, which makes them excellent techniques to couple in a multidimensional system. Capillary electrophoresis separates analytes based on differences in the electrophoretic mobilities of analytes, while chromatographic separations discriminate based on differences in partition function, adsorption, or other properties unrelated to charge (with some clear exceptions). Typically in multidimensional techniques, the more orthogonal two methods are, then the more difficult it is to interface them. Microscale liquid chromatography (p.LC) has been comparatively easy to couple to capillary electrophoresis due to the fact that both techniques involve narrow-bore columns and liquid-phase eluents. 

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