Retention Volume and Time

Eor an analyte that is not retained at aU on the stationary phase = 0 (Equation [3.1]) and V q = V, i.e., in the simplest form of the Plate Theory an unretained compound will pass through the column as if it formed part of the mobile phase itself. In practice, however, the experimental retention volume Vg of an unretained peak (the void volume ) will not be simply the total volume of mobile phase in the column 

It is sometimes convenient to define an adjusted retention volume Vja  

It is useful to attribute different values of Vj to the two analytes (e.g., some analyte molecules might be too large to be able to penetrate deep within the particle pores and thus interact with aU of the stationary phase that might be accessible to smaller molecules). Thus V,a can differ from V g (i.e. A and B can be chromatographicaUy resolved) not only via differences between the adsorption coefficients (K and Kg), but also by differences between the effective values of for the two analytes. The latter corresponds to a different separation mechanism (size exclusion) from the adsorption mechanism that is the main focus of the Plate Theory. 

The bottom line conclusion of this Section can be stated thus If two analytes A and B are to be chromato-graphically separated, their adjusted retention volumes V, and V,g (Equation [3.15]), and thus the corresponding retention times tj, must be sufficiently different. To separate two solutes, either their distribution coefficients and K (Equation [3.1]) must be made to differ (choose appropriate phase systems) or the volumes of stationary phase with which they interact must be made to differ (choose a stationary phase with appropriate exclusion properties based on molecular size so that the effective values of are different for different analytes) or a shrewd combination of both. It is appropriate to mention that if mass spectrometric detection is used, clean chromatographic separation of solutes in the analytical extract is not as crucial as for less selective detectors in view of the additional selectivity provided by the m/z information. 

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