chapter  4
22 Pages

^#.^t4: Optimization of the Mobile Phase Composition

The main aim of the chromatographic analysis-the separation of analyzed mixture (RS)—is described by well-known Purnell’s equation [1]:

R N k

kS = ⋅

+4 1

1 α

α (4.1)

where N is the number of theoretical plates α is the chromatographic system selectivity k is the retention factor

Three parts of this equation show the efciency, selectivity, and retention, respectively. The last part of Equation 4.1 presents the fraction of separated substance in the stationary phase,

and depending on retention factor value (k), it can receive values from 0 (for the compound not adsorbed by stationary phase) to 1 (for strong retention-big values of k). The resolution factor RS = 0 when k = 0 and the analyzed compounds could not be separated, and for k > 10 the last segment of Equation 4.1 approaches the maximal value 1. However, the big values of retention factor (k) are not advantageous because of the peak diffusion and the long time of analysis:

V V kR M= +( )1 (4.2)

where VR is the retention volume VM is the dead retention volume

4.1 Introduction ............................................................................................................................ 41 4.2 Eluotropic Series ..................................................................................................................... 42 4.3 Quantitative Relationships of k Values versus Eluent Composition .......................................44