Stationary phases weak cation-exchange

An unfavorable property of silica is its solubiHty at high pH, which, however, is rarely applied with the mobile phases used in NPLC. Silica gel has weak cation-exchange properties (the silanols on the silica gel surface may be ionized to SiO groups), whereas the bonded amine stationary phases behave as weak anion exchangers. This mixed retention mechanism may cause excessive retention and tailing or even split peaks of ionic or ionizable compounds. 

With HILIC, various polar stationary phases with differing selectivity are used. Basically, it must be distinguished between three different selective types weak anion exchanger (silica modified with aminopropyl groups) and amide columns, weak cation exchanger (usually unmodified (bare) silica) and neutral supports (diol or zwitterionic stationary phases (ZIC-HILIC)). With ionizable compounds, in addition to the distribution equilibrium between the mobile phase ( pseudo-stationary phase ) near to the polar surface and the less polar mobile phase, ionic interactions can also occur, resulting in differing separation characteristics on the different stationary phases. 

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

In the case of amorphous intermediate 4, the weakly acidic cation exchange resin DIAION WKIOO was selected as the CEC stationary phase. The resin comprises a methacrylate resin matrix and carboxylic acid, which functions as an exchange group with a high ion exchange rate. Figure 9.4 illustrates the concept behind the CEC procedure for 4 ... 

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