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Anion-Exchange Mechanism in PO and NP Modes

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). [Pg.11]

A thorough study on the ion-exchange mechanism and the effect of distinct counterions in this PO mode was recently presented by Gyimesi-Forras et al. [41]. A large variety of distinct acid additives to methanol, acetonitrile, and tetrahydrofuran (Table 1.1) (without any base added) was investigated in view of the stoichiometric displacement model and their effect on the enantiomer separation of 2-methoxy-2-(l-naphthyl)propionic acid. The stoichiometric displacement model (Equation 1.1) was obeyed also in the PO mode, as revealed by linear plots of log k vs. acid concentration. The slopes and intercepts along with the concentration ranges used with the distinct competitor acids are summarized in Table 1.1. [Pg.11]

Influence of Acid Additives on Retention Characteristics of 2-Methoxy-2-(1-Naphthyl)Propionic Acid on a 0-9-(tert-ButylcarbamoyOQuinine CSP as Assessed by the Characteristic Parameters of the Stoichiometric Displacement Model (Slopes and Interc  [Pg.12]

Methoxyacetic acid A-Acetylglycine Glycolic acid Formic acid Acetic acid Malonic acid Succinic acid o-Phosphoric acid Citric acid ACN [Pg.12]

Methoxyacetic acid Formic acid Acetic acid THF [Pg.12]


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