Dynamically coated phases

08% CTMA in methanol/water/0.2 M potassium phosphate buffer (pH 8.0) (60 35 5). Peaks 1, benzene 2, toluene 3, ethylbenzene 4, 2-methylnaphthaIene 5, phenanthrene. Reprinted from Ref. 32. 

As mentioned above ion-pair partition is automatically involved in chromatography of anionic solutes on dynamically coated phases using long chain quaternary ammonium ions. The possibility of ion-pair partition of cationic solutes, however, has not been investigated and problems might occur due to the apparent possibility of ion-pair formation between the counterion and surfactant ions, as well as the solute in question. A dynamic modification of the support surface (alumina), using 

Derivatization is used in a number of liquid chromatographic analyses of biochemical substances in order to obtain a sufficiently sensitive and selective detection. 

Derivatization may be performed before the sample is injected into the liquid chromatograph (pre-column), as in the derivatization reactions used with gas chromatography. In liquid chromatography the derivatization may, as an alternative, be performed post-column, i.e., the reagent is added to the effluent from the column. The advantages and disadvantages of these two derivatization strategies are discussed in section 4.3.3. 

An overview of some of the commonly used derivatization reactions is given in Table 4.4.3. 

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The following four types of chromatography are dealt with adsorption liquid-liquid partition chemically bonded phases and dynamically coated phases. It should be emphasized that the borders between the four modes are often rather fluid. 

FIGURE 14.9 Separation of Mn(II), Zn(II), Cd(II), and Pb(II) using (a) a dynamically coated and (b) a pre-coated MTB ODS reversed-phase column. (From Pauli, B. and Haddad, P.R., Trends Anal. Chem., 18, 107, 1999. Copyright 1999. With permission from Elsevier.)... 

A better alternative is to use nonpolar additives or ion-pairing reagents that have the ability to modify the RPC sorbent surface into dynamic liquid-liquid phases. Because of their properties, such dynamic coatings frequently provide improved solubility for highly hydro-phobic peptides in the mobile phase as well as providing enhanced resolution. One downside of employing such dynamic liquid-liquid phases is that long re-equilibration times are re-... 

In contrast to the various CSPs mentioned so far, but still based on covalently or at least very strongly adsorbed chiral selectors (from macromolecules to small molecules) to, usually, a silica surface, the principle of dynamically coating an achiral premodified silica to CSPs via chiral mobile phase additives (CMPA) has successfully been adapted for enantioseparation. The so-called reverse phase LC systems have predominantly been used, however, ion-pairing methods using nonaqueous mobile phases are also possible. 

Case II dynamically coated CSP via a chiral mobile phase additive = (S)-CMPA ... 

The application area of IPC and IC is overlapping, whereas IPC fills the gap between IC and reversed phase chromatography [ 3]. As one borderline case IPC can be described as IC with dynamically coated exchangers. The ion pairing reagent is simply... 

Kimura, K., Hayata, E., and Shono, T. (1984) Convenient, efficient crown ether-containing stationary phases for chromatographic separation of alkali metal ions dynamic coating of highly lipophilic crown ethers on octadecylsilanized silica, Chem. Commun., 271-272. 

M. Pawlowska, Separating anatiomers by HPLC on silica dynamically coated with a chiral stationary phase of permethylated-cyclo-dextrin, Chirality, 5 136 (1991). 

In this reversed phase high performance liquid chromatographic method for neutral and cationic metal chelates with azo dyes, tetraalkylammonium salts are added to an aqueous organic mobile phase. The tetraalkylammonium in salts are dynamically coated on the reversed stationary support. As a result of the addition of tetraalkylammonium salts, the retention of the chelates is remarkably reduced. Tetrabutylammonium bromide permits rapid separation and sensitive spectrophotometric detection of the vanadium(V) chelate with 2-(8-quinolylazo)-5-(dimethylamino)-phenol, making it possible to determine trace vanadium(V). 

Model makers named the technique solvent generated ion exchange [7] and hydrophobic chromatography with dynamically coated stationary phase [8], thereby emphasizing a dynamic ion exchange model. 

Ghaemi Y. and Wall R.A. Hydrophobic chromatography with dynamically coated stationary phases. J. Chmmatogr. 1979, 174, 51-59. 

J. Chromat., 1981, 209, 203-210). Hydrophobic quaternary ammonium ions are strongly adsorbed on silica giving a dynamically coated stationary phase. Retention may be controlled by varying the concentration or nature of the quaternary ammonium ion, changing the ionic strength or pH of the buffer, or changing the concentration or nature of the organic component. 

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