Anion-cation separations on a mixed bed alumina-silica column

Mixed bed ion-exchange (MBIE) columns containing alumina and silica were evaluated for the simultaneous separation of anion and cation analytes. At the mobile phase pH used alumina provides anion exchange sites while silica provides cation exchange sites. Since alumina and silica exhibit weak acid and base properties, their anion and/or cation exchange properties are pH dependent. Ion exchange capacities, rates of exchange and analyte ion exchange selectivities are also pH dependent. The major mobile phase parameters affecting analyte anion and cation resolution and elution order are pH and type and concentration of counter anion and counter cation, respectively. The weight ratio of the two exchangers and/or the exchange capacities of the two in the column can also be used to alter resolution and elution order. Several examples of the simultaneous separation of inorganic mono- and divalent anions and cations using a single sample injection, a single column and a single detector (conductivity) illustrate the parameters and scope of the alumina-silica MBIE column.