ABSTRACT

Chromatography, by definition, is a separation methodology for multicomponent sample mixtures, based on the selective movement (or differential migration) of individual component zones of the sample. An essential feature of chromatographic separations is that the components of the sample are transported through a separating medium, often within the closed environment of a tubular column. The differences in interaction with the medium lead to a selective redistribution of the component zones from a commonly shared region or zone at the start after injection, toward largely individual regions inside or outside the separating medium. In the latter event, elution from the separation column takes place and the appearance of individual component zones in time can be recorded, with the aid of a detector, as the chromatogram. The goal of the separative operation is to obtain a more or less complete separation of the original mixture in its individual chemical constituents. This is achieved by subjecting solute molecules to regions with different velocity, thereby creating velocity differences which, for different components, lead to selective movement of the respective zones. In chromatography, solutes are distributed selectively between the stationary-and mobile-phase regions inside the separation column, and separation occurs because the mobile-phase region undergoes a flowing movement relative to the immobile stationary region. If the mobile-phase region were stopped in its movement from time to time, as happens, for example, in countercurrent distribution (CCD) in a train of discrete extraction vessels, full thermodynamic equilibrium partitioning of solute molecules between the two phases would be obtained within a finite time interval. This is caused by the fairly rapid movement of individual molecules (e.g., by diffusion), which creates the possibility for exchange of molecules between the phases. In chromatography, however, where the mobile phase is flowing continuously, mass exchange between the phases may be rapid, and equilibrium may be approached, but never reached. This is because the continuous flow disturbance of the mobile-phase region of the zone, possibly with the exception of the zone's center [1] (see Sec. II.C). Consequently, the peak center will move under equilibrium conditions, the very reason why the migration equations for retention times or volumes are validly written in terms of equilibrium concentrations c* and equilibrium partition coefficients K* (see Chap. 1).