Capacity factor mobile phase

A solute s capacity factor can be determined from a chromatogram by measuring the column s void time, f, and the solute s retention time, (see Figure 12.7). The mobile phase s average linear velocity, m, is equal to the length of the column, L, divided by the time required to elute a nonretained solute. 

A useful guide when using the polarity index is that a change in its value of 2 units corresponds to an approximate tenfold change in a solute s capacity factor. Thus, if k is 22 for the reverse-phase separation of a solute when using a mobile phase of water (P = 10.2), then switching to a 60 40 water-methanol mobile phase (P = 8.2) will decrease k to approximately 2.2. Note that the capacity factor decreases because we are switching from a more polar to a less polar mobile phase in a reverse-phase separation. 

Changing the mobile phase s polarity index, by changing the relative amounts of two solvents, provides a means of changing a solute s capacity factor. Such... 

Otto and Wegscheider report the following capacity factors for the reverse phase separation of 2-aminobenzoic acid on a C18 column when using 10% v/v methanol as a mobile phase. ... 

The chromatography literature contains a vast amount of dispersion data for all types of chromatography and, in particular, much of the data pertains directly to GC and LC. Unfortunately, almost all the data is unsuitable for validating one particular dispersion equation as opposed to another. There are a number of reasons for this firstly, the necessary supporting data (e.g., diffusivity data for the solutes in the solvents employed as the mobile phase, accurate distribution and/or capacity factor constants (k")) are not available secondly, the accuracy and precision of much of the data are inadequate, largely due to the use of inappropriate apparatus with high extracolumn dispersion. 

A computer program was compiled to work out the ray-tracing of UV detector of high performance capillary electrophoresis at the investigation of 5 and 6 (98MI59). The capacity factor of 5 at different temperature and at different mobile phase compositions was experimentally determined in bonded-phase chromatography with ion suppression (98MI15). 

For preparative or semipreparative-scale enantiomer separations, the enantiose-lectivity and column saturation capacity are the critical factors determining the throughput of pure enantiomer that can be achieved. The above-described MICSPs are stable, they can be reproducibly synthesized, and they exhibit high selectivities - all of which are attractive features for such applications. However, most MICSPs have only moderate saturation capacities, and isocratic elution leads to excessive peak tailing which precludes many preparative applications. Nevertheless, with the L-PA MICSP described above, mobile phases can be chosen leading to acceptable resolution, saturation capacities and relatively short elution times also in the isocratic mode (Fig. 6-6). 

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 ... 

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

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...

Classical PLC involves migration of a mobile phase by capillary action through a 0.5- to 2-mm layer of adsorbent for separating compounds in amounts of 10 to 1000 mg. This separation method requires a good knowledge of chromatography, the most basic equipment, and simple operational skills. The main aim of PLC is to obtain a maximum yield of separation, not a maximum peak (spot) capacity [3]. The principal factors that may influence a PLC separation [1 ] are shown in Figure 4.1. 

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.

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)... 

The terns k, and kj are the solute capacity factors in the Mobile phase, V, the volune of the nonolayer of adsorbed solvent per unit nass of adsorbent, w the weight of adsorbent (gn), V the colunn... 

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