Mobile phase hydroorganic

For aromatic amino and hydrazino acids and several other structurally related compounds, the influence of MeOH content in both RP and POM was investigated on a teicoplanin CSP [90]. Using a hydroorganic mobile phase, complete enantiosep-arations of a-methylamino acids were not attained. However, this type of separation was suitable for the enantiomers of dopa. Further experiments performed by the same authors in POM allowed the complete enantioseparation of a-methyl-ooPA enantiomers [91]. 

Fio. 26. Methylene group selectivity, ocn,i of several hydroorganic mobile phases when octadecyl silica stationary phase is used. The selectivity is the ratio of the retention factor of a member of a homologous series to that of another member which differs in having one less methylene group. The solvents shown here are (A) acetone, (B) acetonitrile, and (C) methanol. The dau were taken at ambient temperature and the selectivity values are plotted on a logarithmic scale. Reprinted with permission ftom Kaiger et al. (/4S).. ... 

It was then recognized early in the development of the technique that there were possibilities for dramatic differences in the chromatographic performance of hydroorganic and micellar mobile phases. Since that review appeared there have in fact been several examples of micellar mobile phases providing solutions to inherent limitations of hydroorganic mobile phases allowing chromatographic capabilities that are not possible with traditional mobile phases. Yet in spite of these advances it was said in 1986 ( 3 ... 

A major drawback in the early reports of micellar chromatography was a serious loss of efficiency when compared to traditional hydroorganic mobile phases. If micellar mobile phases are ever to be widely accepted as a viable chromatographic technique, the efficiency achieved must at least approach that of conventional reversed-phase LC. 

Micellar mobile phases will never replace traditional hydroorganic mobile phases. They do, however, deserve serious consideration by practicing chromatographers as they can provide the solution to certain fundamental limitations of hydroorganic mobile phases. Hopefully the advantages will overcome the skepticism and resistance to change shown by many chromatographers and micellar mobile phases will soon assume a role of importance. 

The principle of solvophobicity as presented by Horvath et al. is based upon the tendency of the mobile phase to minimize the site of the cavity occupied by the solute molecules in the hydroorganic mobile phase. This can be viewed as a reversible association of the solute molecules with the hydrocarbonaceous stationary phase. The magnitude of the solvophobic effect for a given solute is due largely to the following four properties of the hydrooi anic solvent system (H23) ... 

The interaction of the solute with the mobile phase can bring about forces opposing those of the hydrophobic effect. In addition to van der Waals forces, which are dependent upon the size of the molecule involved, electrostatic interactions play a key role in solute retention. Solutes which have polar substituents can interact more strongly with the polar hydroorganic mobile phase, leading to a decrease in retention compared to similar compounds with no polar moiety. The ionization of a solute molecule under the appropriate mobile-phase conditions, results in an increase in electrostatic attraction between solute and eluent and ultimately to a decreased c q>acity for chromatographic retention. 

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