Effective retention factor

By comparing the measured ratio of each nuclide detected in the water to the T in the water with the calculated ratio for the Cambric source term (Table I), an effective retention factor, E, for each nuclide on the solid debris was estimated as follows. 

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 order to illustrate the critical process parameters of SMB process validation, we will consider the separation of the racemic drug as described in Process design. The study represents the effect of the influence of feed concentration, number of plates and retention factor on the second eluting enantiomer. The simulation of the process for different values of feed concentration is performed and the variations of the extract and raffinate purities are shown in Fig. 10.10. 

The effect on purity and the influence of retention factor by adjusting operating flowrates is illustrated in Fig. 10.12. 

Some advice can be formulated for the choice of organic modifier, (i) Acetonitrile as an aprotic solvent cannot interact with residual silanols, whereas the protic methanol can. Thus, when measuring retention factors, methanol is the cosolvent of choice, as it reduces the secondary interactions between the solutes and the free silanol groups, (ii) For the study of the performance of new stationary phases one should use acetonitrile, as the effects of free silanol groups are fuUy expressed [35]. (iri) Acetonitrile with its better elution capacity can be considered as the best organic modifier for Hpophilicity measurements of highly Hpophihc compounds with adequate stationary phases [36]. 

The retention factor, k, is the basic value in chromatography, and is related to the void volume (dead volume). The void volume is the space inside the column, where no retention of solutes has occurred and can be measured on a chromatogram, as shown in Figure 1.3. The void volume is about half the total volume of the column when it is packed with porous stationary phase materials. In practice, the effective void experienced by the analyte is smaller because the molecular mass of the analyte is usually much greater than that of the eluent molecule. In a model of porous stationary phase material, the pores can be represented as V-shape valleys (Figure 3.8), where region a is a support, such as... 

Figure 4.5 Effect of pH on the retention factors of 3-chloro- and 4-ethylbenzoic acids.

FIGURE 1.5 pH -effect on retention factors k and separation factors a. CSP 0-9- tert-butylcarbamoyl)quinine bonded to sihca column dimension, 150 x 4 mm ID eluent, methanol-ammonium acetate buffer (80 20, v/v) (adjusted with acetic acid) temperature, 25°C 1 mL min sample, N-benzoyl-leucine (Bz-Leu). (Reproduced from M. Lammerhofer et al., American Laboratory, 30 71 (1998). With permission.)... 

On the other hand, optionally added co-ions of the eluent may also interfere with the ion-exchange process through competitive ion-pairing equilibria in the mobile phase. The effect of various amines added as co-ions to the polar-organic mobile phase was systematically studied by Xiong et al. [47]. While retention factors of 9-fluorenylmethoxycarbonyl (FMOC)-amino acids were indeed affected by the type of co-ion, enantioselectivities a and resolution values Rs remained nearly constant. For example, retention factors k for FMOC-Met decreased from 17.4 to 9.8 in the order... 

FIGURE 1.6 Effect of organic modifier (methanol) percentage in the elnent on the retention factors (a) and observed enantioselectivities (b) of Af-(2,4-dinitrophenyl)-a-(2-chlorobenzyl)-proline employing an 0-9-[(2,6-diisopropylphenyl)carbamoyl]quinine-based CSP. Experimental conditions Elnent, ammonium acetate buffer-methanol (total ionic strength = 25 mM pHj, = 6.5), methanol content varied between 60 and 90%, while ionic strength and apparent pH were kept constant temperature, 40 C flow rate, 0.8 mLmin . (Reproduced from A. Peter et al., J. Sep. ScL, 26 1125 (2003). With permission.)... 

FIGURE 1.8 Effect of the mole fraction of polar modifier (ethyl acetate) in n-hexane on the reciprocal of the retention factor for the separation of 3-chloro-l-phenylpropanol enantiomers on a 0-9-(terf-butylcarbamoyl)quinidine CSP. Temperature, 22°C. (Reproduced from L. Asnin, and G. Guiochon, J. Chromatogr. A, 1091 11 (2005). With permission.)... 

FIGURE 4.1 Effect of the plate number (N), the separation factor (a ), and the retention factor (k) on resolution (Rs). (Adapted from Sandra, P.J. 1989. High Resolut. Chromatogr. 12 82-86. With permission.)... 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

FIGURE 4 The effect of organic modifier content on retention factor (Ink) in CEC. (Reproduced with permission from reference 13.)... 

Bakalyar ef al. (157) also examined the selectivity of different mobile phases for various aromatics and found it to be nuu-kedly different for the eluents methanol-water (50 50), acetonitrile-water (40 60), and TQF-water (37 63), compositions of which were chosen to give constant retention factor for benzene at 35X. The results given in Fig. 31 are in general agreement with those of Tanaka et al. (92) and serve to show the rather large effect of polair fbnctional groups in eluite molecule on the selectivity in RPC. 

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