The Mechanism of Chromatographic Retention - Chromatographic Interactions > Page 1

Introduction

A separation is achieved in a chromatographic system by moving the peaks apart and constraining their dispersion so that they are eluted discretely. To move the peaks apart each solute must be retained to a different extent, which (as shown by the plate theory, book 6) means that their distribution coefficients must differ or they must interact with different volumes of stationary phase. Retention is therefore controlled by regulating the magnitude of the distribution coefficient or modifying the quantity of stationary phase available to each solute. The former is employed in interaction chromatography and the latter in exclusion chromatography. In practice, it is rare that either procedure is exclusive in any given separation as both retentive processes are usually present to some extent, particularly in liquid chromatography (LC). Retention in gas chromatography (GC), using coated open tubes, is, perhaps, the exception, as, in this distribution system, exclusion processes (aside from chiral phases) are normally absent and, consequently, retention control is purely interactive. Interaction and exclusion processes, when they do occur together, act independently. Although, together they determine overall the retention of a solute, the exclusion properties of a stationary phase do not effect the magnitude of any of its interactive properties.

The mechanisms of retention have been discussed briefly in Book 1, but will now be considered in greater detail. Although both retention processes (interaction and exclusion) are usually active, because they contribute to retention independently, the two mechanisms will be considered separately.

Chromatographic Interactions

Solutes are retained in the stationary phase (irrespective of its volume) because the solute molecules interact more strongly with the stationary phase molecules than they do with the mobile phase molecules. This concept might be put in another way, the stationary phase holds the solute molecules more strongly than does the mobile phase. The situation in GC is unique, in that the retentive contribution from interactions of the solute molecules with the mobile phase molecules is very small indeed. This is partly because the interactive force between the small gas molecule and the large solute molecule is weak and partly because the probability of collision (and, thus, interaction) is also very small (relative to the probability of molecular interactions in a liquid). It should be noted, however, that solute interaction in the gas phase, although very small, is not zero and Desty et al. (1), demonstrated that the small effect of gas molecular weight on retention (hydrogen, nitrogen, argon etc.) could, in fact, be measured.