Capacity factor stationary phases

With k capacity factor, (J) phase ration, K equilibrium constant. A stationary phase capacity (for monovalent salt counterions), c carrier salt concentration. 

The time taken for an analyte to elute from a chromatographic column with a particular mobile phase is termed its retention time, fan- Since this will vary with column length and mobile phase flow rate, it is more useful to use the capacity factor, k. This relates the retention time of an analyte to the time taken by an unretained compound, i.e. one which passes through the column without interacting with the stationary phase, to elute from the column under identical conditions (to). This is represented mathematically by the following equation ... 

Table 2.1 HPLC capacity factors for secbuto-barbitone and vinbarbitone with an octadecyl silyl stationary phase and mobile phases of methanoiyO.l M sodium dihydrogen phosphate (40 60) at (a) pH 3.5, and (b) pH 8.5. From Moffat, A.C. (Ed.), Clarke s Isolation and Identification of Drugs, 2nd Edn, The Pharmaceutical Press, London, 1986. Reproduced by permission of The Royal Pharmaceutical Society...

Temperature has an influence on the retention and consequently on the capacity factors of carotenoids in HPLC columns. Usually, as the column temperature increases, the retention decreases however, in a polymeric C30 column, after an initial decrease of the t values of cis isomers of carotenoids, the retention of cis isomers actually increases at temperatures above 35°C. This different behavior can be explained by the increased order and rigidity of the C30 stationary phase at lower temperatures that in turn induce preferential retention of long, narrow solutes as the trans isomer and partial exclusion of bent and bulky cis isomers. The greater chain mobihty and less rigid conformation of the C30 at higher temperatures may increase the contact area available for interaction with the cis isomers and also may lower... 

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]. 

For optimization of chromatographic separations the ratio of the time spent by the solute in the stationary phase to the time it spends in the mobile phase is more fundiwentally i tortant. This ratio is called the solute capacity factor and is given by equation (1.8)... 

Figure 1.4 Variation of the resistance to mass transfer in the mobile phase, C , and stationary phase, Cj, as a function of the capacity factor for open tubular columns of different internal diameter (cm) and film thickness. A, df 1 micrometer and D, 5 x 10 cm /s B, df 5 micrometers and D, 5 x 10 cm /s and C, df - 5 Micrometers and 0, 5 x 10 cm /s.

Many chromatographic systems show linear relationships between the logarithm of the capacity factor and the reciprocal of the column temperature (van t Hoff plots) [255,258-261]. In thermodynamic terms the interaction of the solute with the stationary phase can be described by... 

Retention in HIC can be described in terms of the solvophobic theory, in which the change in free energy on protein binding to the stationary phase with the salt concentration in the mobile phase is determined mainly by the contact surface area between the protein and stationary phase and the nature of the salt as measured by its propensity to increase the surface tension of aqueous solutions [331,333-338]. In simple terms the solvopbobic theory predicts that the log u ithn of the capacity factor should be linearly dependent on the surface tension of the mobile phase, which in turn, is a llne2u function of the salt concentration. At sufficiently high salt concentration the electrostatic contribution to retention can be considered constant, and in the absence of specific salt-protein interactions, log k should depend linearly on salt concentration as described by equation (4.21)... 

Normal alkanes or fatty acid methyl esters are generally used as the standard homologous compounds. The column separation number is dependent on the nature of the stationary phase, the column length, column temperature, and carrier gas flow rate [42-44]. Referring to Figure 1.2, at a sufficiently high capacity factor value either n, N, or SN provides a reasonable value for comparing... 

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

The capacity factor, k, of., a sanple zone is defined as the ratio of the tiee spent by the substance in the stationary phase cospared to the tiae it spends in the aobile phase, and is related to the Rf value by equation (7.2). 

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