Ion exchange retention

A stoichiometric model can conveniently be invoked to explain the ion-exchange retention process [43 6]. As discussed in detail in these cited papers on ion-exchange theory, useful information about the involved ion-exchange process can be deduced from plots of log k vs. the log of the counterion concentration [X], which commonly show linear dependencies according to the stoichiometric displacement model (Equation 1.1)... 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

The data in the table summarized above show good agreement thus showing the relevance of different approaches as a necessary condition in obtaining information on the stability constant. The structure has been suggested for the complex based on experiments involving ion exchange retention on IR-45. 

Chiral cation exchangers are rarely reported. However, the dominance of an ion-exchange retention model could be established by Sellergren and Shea for an acidic-imprint type CSP that was prepared from methacrylic acid as functional monomer and basic phenylalanine-anilide as oppo.sitely charged template, (1). 380. Maxima in retention were observed at pH-values close to the (apparent) pKa-value of the solutes... 

The ion-exchange model assumes that the ion-pairing reagent is adsorbed onto the stationary phase surface where it behaves as a liquid ion-exchanger. Retention... 

V Here Z equals [(change on solute ion)/(charge on mobile>phase ion)] Z luistlresame significance in ion exchange as Z has for RPLC. The parameter ft die equilibrium distribution constant for ion-exchange retention. 

An elegant example of SPE method based on ion-exchange retention was used for inline preconcentration of inorganic anions. A single capillary containing a preconcentration zone (adsorbed layer of cationic latex particles) and a separation zone (fused-silica modified by adsorption of a cationic polymer) was used. Analytes were retained in the preconcentration zone and eluted isota-chophoretically into the separation zone by means of an eluotropic gradient. This approach was used to determine nitrate in Antarctic ice cores at the 2.2-11.6 p,g L" level. 

Longer spacer arms would of course reduce any influence the benzene ring might have on ion-exchange retention. 

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