Normal-phase chromatography retention equation

In NP systems where the retention is controlled by the three-parameter retention equation (Equation 5.11), the elution volumes in normal-phase gradient-elution chromatography can be calculated using [55-57]... 

With polar liquid-liquid adsorption chromatography, based on chemically bonded normal-phase systems, the distribution coefficient can be equated with the solubility parameter 8j of a solute such that retention is given by... 

Those QSRR equations, which comprise physically interpretable structural descriptors, can be discussed in terms of the molecular mechanism of the chromatographic process [90]. There is literature evidence that different structural parameters of analytes account for retention differences in gas chromatography on polar as compared to non-polar stationary phases. Also, the structural descriptors in QSRR equations, which are valid for normal-phase HPLC, are different from those valid for reversed-phase HPLC. In the case of apparently similar chromatographic systems the differences in retentive properties of stationaty pha.ses may be reflected by the magnitude of the regression coefficients for analogous descriptors [9I.92]. Comparative QSRR studies are especially valuable when new chromatographic phases are introduced. 

From the literature there is evidence that in GC on polar phases and in normal-phase (adsorption) liquid chromatography (HPLC and TLC) the chemically specific, molecular size-independent intermolecular interactions play the main retention-determining role. For example, the HPLC retention parameters determined for substituted benzenes on porous graphite are described by QSRR equations comprising polarity descriptors but containing no bulk descriptors [93-95]. Because, in general, it is difficult to quantify the polarity properties precisely, the QSRR for GC on polar phases and for normal-phase HPLC are usually of lower quality than in the case of GC on non-polar phases and in the case of reversed-phase liquid chromatography. 

Two very simple relationships have been derived from the general framework of the Snyder and Soczewinski model of adsorption chromatography these have proved useful for rapid prediction of solute retention in chromatographic systems employing binary mobile phases. One (known as the Soczewinski equation) proved successful for adsorption and normal-phase TLC the other (known as the Snyder equation) proved similarly successful in reversed-phase TLC. 

Equation (5) is regarded as a fundamental equation of column chromatography as it relates the retention volume of a solute to its distribution ratio. Planar separations (PC and TLC). Separations are normally halted before the mobile phase has travelled completely across the surface, and solutes are characterized by the distance they have migrated relative to the leading edge of the mobile phase (solvent front). A solute retardation factor, Rf, is defined as... 

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