Liquid chromatography mobile-phase volume definition

In gas chromatography the analyte partitioning between mobile gas phase and stationary liquid phase is a real retention mechanism also, phase parameters, such as volume, thickness, internal diameter, and so on, are well known and easily determined. In liquid chromatography, however, the correct definition of the mobile-phase volume has been a subject of continuous debate in the last 30 years [13-16]. The assumption that the retardation factor, i /, which is a quantitative ratio, could be considered as the fraction of time that components spend in the mobile phase is not obvious either. 

By definition, the e]q>erlmentally determined average mobile phase velocity Is equal to the ratio of the column length to the retention time of an unretalned solute. The value obtained will depend on the ability of the unretalned solute to probe the pore volume. In liquid chromatography, a value for the Interstitial velocity can be obtained by using an unretalned solute that Is excluded from the pore volume for the measurement (section 4.4.4). The Interstitial velocity Is probably more fundamentally significant than the chromatographic velocity in liquid chromatography (39). 

Solvent interaction model for normal-phase liquid chromatography. The solvent-interaction model of Scott and co-workers (Scott and Kucera, 1979) assumes that the analyte partitions between the bulk mobile phase and a layer of solvent absorbed onto the stationary phase. The quantitative description of the relationship between retention and the composition of the mobile phase in the solvent-interaction model requires the definition of the void volume corrected retention volume (V), which is related to the retention volume (F ) and the void volume (Fq) by... 

If the mobile phase is incompressible, as in liquid chromatography (LC), the dead volume (as so far defined) will be the simple product of the exit flow rate and the dead time. However, in LC, where the stationary phase is a porous matrix, the dead volume can be a very ambiguous column property and requires closer inspection and a tighter definition. 

The definition of the hold-up time is simple in gas chromatography because the interactiorrs between mobile and stationary phases are practically negligible. This is not so in liquid chromatography [163-165]. The density of the mobile phase is not the same in the bulk and in the monolayer in contact with the surface of the adsorbent. The situation is more complex in RPLC because the bonded layer swells when the proportion of the organic modifier in the mobile phase increases [32,166,167]. The organic modifier dissolves in the bonded layer and, when its concentration in this layer is sufficient, some molecules of water may also penetrate it. The hold-up time of a column is a fxmction of the nature and concentration of the organic modifier in an aqueous solution [168,169]. In order to predict accurately the elution band profiles, it might be necessary to account for the dependence of the hold-up volume on the mobile phase composition, which requires the use of a different mass balance equation in which the phase ratio has been left in the differential elements [170]. 

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