In order for electrostatic retention to occur, both analyte and sorbent functional groups must be in their ionized form. This is done through strict pH control of the sample matrix. For basic analytes, the pH should be adjusted to at least 2 pH units below the molecule’s pKa. For acidic analytes, the pH should be adjusted to at least 2 pH units above the molecule’s pKa.
To elute, the opposite is true. By adjusting the pH of the eluant to at least two pH units above or below the analytes’ and/or sorbent’s pKa, one can effectively neutralize one or both functional groups disrupting the electrostatic interaction allowing for elution to occur.
Note: Because the kinetic exchange processes between sample and sorbent functional groups are considerably slower for ion-exchange than for normal and reversed-phase, flow rates should be drop wise (~1 drop/second). One may also need to increase elution and wash volumes, allowing sufficient residence time for the mobile phase and stationary phase to interact.
Counter ion selectivity is defined as the degree to which a counter ion is capable of competing with other counter ions for the functional group of an ion exchanger sorbent. Retention is facilitated by having a sorbent and/or sample matrix preequilibrated with a counter-ion that is less selective than the analyte functional group (minimum competition). Analyte elution is facilitated by using buffers with counter-ions more selective than analyte functional group.
For Cation Exchangers:
For Anion Exchangers:
To change to a higher selective ion, pass 2-5 bed volumes of 1N solution of the new counter ion through sorbent. To change to a lower selective ion, pass 5-65 bed volumes of 1N solution of the new counter ion through sorbent.
Note: Number of bed volumes dependent on how much less selective the new counter ion is than the present one on the sorbent.