Selectivity in chromatography

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... 

Roger M. Smith, Retention and Selectivity in Chromatography, Elsevier, Amsterdam, ISBN 0-444-81539-2, 1995. 

Aires-de-Sousa, J. and Gasteiger, J. (2002) Prediction of enantiomeric selectivity in chromatography. Application of conformation-dependent and conformation-independent descriptors of molecular chirality. J. Mol. Graphics Mod. 20, 373-388. 

We have seen that the selectivity in chromatography can be related to the relative retention of two solutes. However, this parameter does not describe the actual separation between two chromatographic peaks. There are two factors which determine whether or not two peaks are completely resolved, as is illustrated in figure 1.3. The relevant parameters are the distance between the peaks and their width. The distance can be expressed as the difference in retention times (AtR), while the peak width at the peak base (usually determined by drawing tangent lines along the slopes of the peaks) can be denoted by w. 

It should be noted that since selectivity in chromatography depends not only on the stationary phase but on the mobile phase as well, orthogonal separations can be achieved through fine-tuning of the separation conditions, even if the principal separation mechanisms of both dimensions are similar. Such tuning removes the inaccessible area from the 2D retention plane and ensures that the remaining retention space is used efficiently.66... 

Schematic diagram of an orthogonal Q/TOF instrument. In this example, an ion beam is produced by electrospray ionization. The solution can be an effluent from a liquid chromatography column or simply a solution of an analyte. The sampling cone and the skimmer help to separate analyte ions from solvent, The RF hexapoles cannot separate ions according to m/z values and are instead used to help confine the ions into a narrow beam. The quadrupole can be made to operate in two modes. In one (wide band-pass mode), all of the ion beam passes through. In the other (narrow band-pass mode), only ions selected according to m/z value are allowed through. In narrow band-pass mode, the gas pressure in the middle hexapole is increased so that ions selected in the quadrupole are caused to fragment following collisions with gas molecules. In both modes, the TOF analyzer is used to produce the final mass spectrum.

Factors to be considered in maldng the selection of chromatography processing steps are cost, sample volume, protein concentration and sample viscosity, degree of purity of protein product, presence of nucleic acids, pyrogens, and proteolytic enzymes. Ease with which different types of adsorbents can be washed free from adsorbed contaminants and denatured proteins must also be considered. 

An interesting and practical example of the use of thermodynamic analysis is to explain and predict certain features that arise in the application of chromatography to chiral separations. The separation of enantiomers is achieved by making one or both phases chirally active so that different enantiomers will interact slightly differently with the one or both phases. In practice, it is usual to make the stationary phase comprise one specific isomer so that it offers specific selectivity to one enantiomer of the chiral solute pair. The basis of the selectivity is thought to be spatial, in that one enantiomer can approach the stationary phase closer than the other. If there is no chiral selectivity in the stationary phase, both enantiomers (being chemically identical) will coelute and will provide identical log(Vr ) against 1/T curve. If, however, one... 

R. E. Boehm and D. E. Martiie, A unified theory of retention and selectivity in liquid chromatography. 1. Eiquid-solid (adsorption) clrromatography , J. Phys. Chem. 84 3620-3630(1980). 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

Column chromatography is the most generally used method, and the use of several different solvents, or graded-concentration mixed solvents affords a considerable selectivity in many cases. The difficulty always remains, however, that an unidentified and unexpected species may accompany one of the carriers and give false results. This can be particularly misleading in cases where the various eluted fractions are not specifically identified, but are classified only by the polarity of the solvent. 

Rill, RL Van Winkle, DH Locke, BR, Templated Pores in Hydrogels for Improved Size Selectivity in Gel Permeation Chromatography, Analytical Chemistry 70, 2433, 1998. 

Human interleukin 2, a 133-residue protein, has been separated into multiple molecular forms by selective immunoaffinity chromatography and chromatofocusing. Most of the heterogeneity has been attributed to variations in glycosylation of the threonine residue in position 3 of the polypep-... 

Trone, M. D., Khaledi, M. G. Statistical evaluation of linear solvation energy relationship models used to characterize chemical selectivity in micellar electrokinetic chromatography. J. Chromatogr. A 2000, 886, 245-257. 

Figure 4.29 An example of the use of ternary solvents to control mobile phase strength and selectivity in reversed-phase liquid chromatography. A, methanol-water (50 50) B, tetrahydrofuran-water (32 68) C, methanol-tetrahydrofuran-water (35 10 55). Peak identification 1 - benzyl alcohol 2 phenol 3 3-phenylpropanol 4 2,4-dimethylphenol 5 benzene and 6 -diethylphthalate. (Reproduced with permission from ref. 522. Copyright Elsevier Scientific Publishing Co.)...

Eluent selection in ion chromatography is made more complex by the need to consider detector operating characteristics. 

Solvent optimization in reversed-phase liquid chromatography is commenced by selecting a binary mobile phase of the correct solvent strength to elute the seuaple with an acceptable range of capacity. factor values (1 < k <10 in general or 1 < k < 20 when a larger separation capacity is required). Transfer rules (section 4.6.1) are then used to calculate the composition of other isoeluotropic binary solvents with complementary selectivity. In practice, methanol, acetonitrile and tetrahydrofuran are chosen as the selectivity adjusting solvents blended in different... 

The rapid and reversible formation of complexes between some metal ions and organic compounds that can function as electron donors can be used to adjust retention and selectivity in gas and liquid chromatography. Such coordinative interactions are very sensitive to subtle differences in the composition or stereochemistry of the donor ligand, owing to the sensitivity of the chemical bond towards electronic, steric and strain effects. A number of difficult to separate mixtures of stereoisomers and isotopomers have been separated by complexation chromatography. 

Okamoto, M., Kakamu, H., Nobuhara, K. and Ishii, D., Effect of silver-modified silica on retention and selectivity in normal-phase liquid chromatography,. Chromatogr. A, 722, 81, 1996. 

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