Isocratic chromatography retention factor

A characteristic of small-molecule liquid chromatography is the reversibility of their contacts with the stationary phase. The distribution equilibrium constant determines the duration of the stationary periods and, thus, the retention of the solute. With polymers, isocratic retention factors of normal degree (i.e., 1 gk 10) generally do not occur. A fractional alteration of elution conditions may cause transition from zero retention to infinity. As a rule of thumb, polymers either pass without retention or remain in the column. This off or on behavior produces the impression of irreversible fixation under the conditions of retention. 

Eigure 17-17. Theoretical plates versus retention factor for series of parabens separated by isocratic chromatography. Extra-column effects result in decreased efficiency for early-eluting components (low values of k). 

Snyder s thorough model [1-5] of gradient elution provides an extremely convenient means to achieve the objectives outlined above. The model uses the general resolution equation for isocratic chromatography in terms adapted to gradient elution. This equation defines resolution between two closely resolved analytes in gradient RP-HPLC as a function of mean column efficiency N, mean selectivity a, and the effective retention factor Aavc experienced by the compounds during the elution process j 1-3,5). 

In RPC, as in all modes of chromatography, when a peptide is eluted under isocratic conditions, the retention can be expressed in either time, te, or volume, Ve, units. Expressions of the elution time or elution volume of a peptide, eluting with a peak width w (= 4ot=4ov), from a column packed with RPC particles of mean particle diameter dp, incorporate the physical aspects of the column (diameter dc, length L), the flow rate F (or linear flow velocity, v = LFIVm) and the phase ratio, mobile phase in the chromatographic column. Usually, retention dependencies for a peptide P, are represented in terms of a capacity factor k as follows ... 

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