Anion Exchange

SIELC Technologies provides 4 types of different B-type columns with positively charged functional groups. Primesep B is a strong basic column. The recommended pH range is from 1.5 to 4 created by the addition of trifluoroacetic, phosphoric or perchloric, or formic acids to the mobile phase. Primesep B2 is a weak basic column. It also has carboxilic acidic functional groups. At pH of the mobile phase below 5 the acidic groups are not ionized and B2 column surface becomes positively charged. This dual chemistry offers extended pH range from 0.5 to 7, suitable for the ammonium acetate and ammonium formate buffered mobile phases. Primesep D column originally developed for direct plasma analysis, became very useful for other anion-exchange/reverse phase applications. It has extended pH range from 1.5 to 7 and offers similar properties as Primesep B2 column but the column has no carboxilic groups and remains positively charged throughout the working pH range. The fourth column, which offers anion-exchange properties is Primesep AB column. This column offers also cation-exchange properties and useful in separation of complex mixtures of polar anionic and cationic compounds.

All B-type columns provide at least two main interactions with analytes: the reverse-phase interaction and the anion-exchange interaction. Neutral analytes retained by reverse-phase interaction. The presence of the charged group in the alkyl chain provides additional selectivity uncommon for typical reverse-phase columns. Acidic analytes can be retained by both anion-exchange and reverse-phase mechanisms. To control retention and selectivity, there is a broad selection of the mobile phases with concentration of organic modifier in the 0-100% range. The concentration and the type of an acid will also significantly affect the retention of anionic compounds. Basic compounds can be retained only by the reverse-phase mechanism, but the presence of positively charged groups on both analyte and stationary phases produces the unique selectivity due to the ion-exclusion phenomena.

Primesep B columns retain acid residue in the stationary phase in equimolar amount. When a column switches to the mobile phase with another type of acid, it should be sufficiently equilibrated to replace all counter-ions from the previous mobile phase with new counter-ion.