Affinity techniques basic principles

The next four chapters discuss the basic principles underlying operation and method development of the most common electrodriven analytical techniques CE, capillary isoelectric focusing (cIEF), capillary gel electrophoresis (CGE), and affinity capillary electrophoresis (ACE). Weinberger presents a comprehensive approach for method development in CE with an emphasis on small-molecule applications. This is followed by Kilar s chapter describing the principles of and method development in cIEF, as well as recent innovations... 

Examples illustrating the basic principles of these techniques are described, and their advantages and drawbacks compared to those of affinity chromatography are discussed. 

Synthetic polymers [5, 51] as well as derivatives of natural macromolecules, such as hydroxypropylcellulose acetate succinate [52, 53] or chitosan [54, 55] have been used. Precipitation can be induced by pH shift, salinity modification or temperature. Certain polysaccharides, such as sodium alginate whose solubility is sensitive both to acidic conditions and to the presence of divalent cations (e.g. Ca ) or carrageenan which precipitates upon decreasing temperature, have not yet been exploited but are eligible candidates for a future use in affinity precipitation. Various works can be selected from the literature to illustrate the basic principles of this technique. 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

In general, basic compounds with high proton affinity can be detected with the highest limit of detection [30], and in some cases, a strong temperature dependence has been ob.served [31]. In principle. APCI is compatible with every separation technique which allows the effluent to be sprayed in an aerosol. Thus, not only HPLC but also SFC can be interfaced to this ion source. 

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