Chapter 8 - Introduction - Page 287

     The analysis of real-world samples, e.g., biological, food, and environmental, poses a high demand for “conventional” one-dimensional liquid chromatography (1D-LC) methods enabling compound identification and quantification. However, a single column does not often have sufficient separation power for the baseline separation of all components in complex samples and the extent of peak overlap is enhanced as the number of compounds increases. The development of comprehensive two-dimensional (2D) high-performance liquid chromatography (HPLC) (LC x LC) has been relatively slow until the last two decades, even though the advantages of the technique were already described in the late 1970s and early 1980s (Karger et al., 1973; Erni and Frei, 1978; Giddings, 1984). In the 1990s the application of LC
LC was limited to proteomic applications, and only afterward the utility of this technique in the separation of a variety of other complex mixtures has been demonstrated. The general motivation which lies ahead of this research area is linked to its potential for substantially more resolving (separating) power in comparison with the 1D-LC counterpart.
     The most general set-up of an LC x LC system consists of two pumps, two columns, injector, interface, and detector. The interface is, in general, a high-pressure 2-position switching valve, and this device is often referred to as modulator or sampling device. The suitability of any LC separation system for resolving complex samples can be expressed by theoretical peak capacity, nc, which determines the maximum number of peaks that can be separated side by side, into the separation space at a desired degree of resolution e.g., R_S = 1(n_c = t/w). The separation time can be specified as the retention time interval Dt_R = t_R, Z x t_R, 1, between the first (1) and the last (z) sample peaks, while w can be estimated from the average bandwidth, w_av.