Chromatography selectivity factor

As in chromatography, selectivity factor is the most important parameter in optimizing the separation. Since the micelle corresponds to the stationary phase in chromatography, the selection of the surfactant is of primary importance. SDS is a good initial choice and, if not successful, bile salts or CTAB/CTAC should be the second choice. If other surfactants are available, they may be tested. However, the number of different surfactants available for MEKC is rather limited and, hence, the selection of additives to modify selectivity should be considered when expected separation is not obtained by changing the surfactant. Some of the additives suggested above should be considered, as well as the use of mixed micelles. In MEKC, the efficiency is much higher than that of HPLC, the minimum a that provides successful separation may be as low as 1.02. 

Selectivity In chromatography, selectivity is defined as the ratio of the capacity factors for two solutes (equation 12.11). In capillary electrophoresis, the analogous expression for selectivity is... 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

Affinity chromatography using factor XII as ligand leads to purification of u-PAR rather selectively, with only trace quantities of cytokeratin 1 or gClqR present [K. Joseph and A. Kaplan, unpubl. observations]. It is of interest that none of these three proteins possesses a transmembrane domain but u-PAR has a phos-phatidylinositol linkage within the cell membrane. Nevertheless, each of them has been isolated from purified cell membranes and they have been demonstrated to exist within the cell membrane by immunoelectron microscopy [41] presumably... 

SRM 869a Column Selectivity Test Mixture for Liquid Chromatography [44] is composed of three shape-constrained PAHs (phenanthro[3,4-c]phenanthrene, PhPh l,2 3,4 5,6 7,8-tetrabenzonaphthalene, TBN and benzo[a]pyrene, BaP) and is routinely employed to evaluate the shape selectivity of stationary phases. The retention differences between the nonplanar TBN and planar BaP solutes (expressed as a selectivity factor axEN/BaP = provide a numerical assessment of... 

Equation 6 Calculation of optimum ratio of particle size and column length, with selectivity factor, a capacity factor of second component of critical pair under analytical chromatography conditions, fe 02 diffusion coefficient, (cm /s) (typical value for MW 1000 10 cm /s) viscosity, p (cP) specific permeability (1.2 X 10 for spherical particles), feo third term of the Knox equation, C and maximum safe operating pressure, Ap, (bar). 

The main conclusion from Fig. 1 is that the well-known ideas of linear chromatography cannot be used when the isotherm is not linear. For example, the position of the peak maximum should not be used for the determination of a retention factor k. As Fig. 1 shows, such a k value would depend on the sample concentration and thus it might not be used for characterizing the retention of the analyte independently from its concentration. The width of the peak also has little to do with the number of plates. It will also be shown later in this chapter that the separation of two analytes cannot be simply characterized by a selectivity factor a, which is calculated as the ratio of the corresponding k values. 

If there is no or little information on the method s performance characteristics, it is recommended that the method s suitability for its intended use in initial experiments be proven. These studies should include the approximate precision, working range, and detection limits. If the preliminary validation data appear to be inappropriate, the method itself, the equipment, the analysis technique, or the acceptance limits should be changed. In this way method development and validation is an iterative process. For example, in liquid chromatography selectivity is achieved through selection of mobile-phase composition. For quantitative measurements the resolution factor between two peaks should be 2.5 or higher. If this value is not achieved, the mobile phase composition needs further optimization. 

Many pharmaceutical preparations contain multiple components with a wide array of physico-chemical properties. Although CZE is a very effective means of separation for ionic species, an additional selectivity factor is required to discriminate neutral analytes in CE. Terabe first introduced the concept of micellar electrokinetic capillary chromatography (MEKC) in which ionic surfactants were included in the running buffer at a concentration above the critical micelle concentration (CMC) [17], Micelles, which have hydrophobic interiors and anionic exteriors, serve as a pseudostation-ary phase, which is pumped electrophoretically. Separations are based on the differential association of analytes with the micelle. Interactions between the analyte and micelles may be due to any one or a combination of the following electrostatic interactions, hydrogen bonding, and/or hydro-phobic interactions. The applicability of MEKC is limited in some cases to small molecules and peptides due to the physical size of macromolecules... 

Due to the central role of the ligand in affinity chromatography, some factors should be considered when selecting a ligand for the purification of a protein ... 

Selectivity Factors Affecting, in Supercritical Fluid Chromatography... 

In summary, resolution in liquid chromatography is dependent on three factors (1) the efficiency of the chromatographic system, measured by the theoretical plate value N (2) the selectivity of the chromatographic system, measured by the selectivity factor a and (3) the degree of retention of the components on... 

Selectivity factor, a In chromatography, a = KglK, where is the distriburion constant for a less strongly retained species and is the constant for a more strongly retained species. Self-absorption A process in which analyte molecules absorb radiation emitted by other analyte molecules. 

---