vitamins, flavourings and colourings and the quality control of polymers, plastics and resins are further
examples of the very wide and growing scope of this technique. Figure 4.47 illustrates a number of
separations employing different modes of HPLC.
Ion Chromatography
For the separation of inorganic and some organic anions and cations a form of HPLC known as ion
chromatography (IC) has been developed. The technique involves an ion-exchange column and a
means of suppressing (removing) ionized species other than the sample ions in the eluting mobile phase
to facilitate detection of the sample ions by a conductivity monitor (Figure 4.45(a)). Separation of the
analytes is achieved using a pellicular (vide infra) cation- or anion-exchange resin (p. 162) and eluting
with sodium hydroxide or sodium hydrogen carbonate/sodium carbonate (for anionic analytes) or
methane sulphonic acid (for cationic analytes). The eluent from the column passes through a suppressor
where the eluting or 'background' electrolyte is effectively removed by converting it into water or water
and carbon dioxide, i.e. sodium ions are replaced with hydronium ions or methylsulphonate ions with
hydroxyl ions. This is a necessary step which leaves the analytes (all now in the same ionic form) as the
only ionized species and thereby easily detected down to very low (sub ppm) levels by a conductivity
monitor.
To separate anionic analytes the analytical column is packed with a pellicular strong-base anion-
exchange resin in the hydrogen carbonate (HCO 3 – ) form. The particles have an inert non-porous central
core surrounded by a surface layer of anionic resin micro-beads. Mass transfer in pellicular materials is
very fast and, although the columns are of limited capacity, they have very high efficiencies. The
suppressor is a cartridge containing a porous polymeric cation-exchange membrane in the H+ form,
which enables hydronium ions to replace sodium ions in the eluent and to form water by combination
with hydroxyl ions (Figure 4.45(b)). As the membrane becomes depleted of H+ ions it is continually
replenished from an external acidic solution whilst the Na+ ions passing into the external solution are
removed. For cationic analytes, a pellicular cationic resin in the H+ form is used, the suppressor
utilizing an anionic membrane.
A miniaturized 'self-regenerating' suppressor cartridge incorporating an electrolysis cell is also
available. Hydronium ions and oxygen are continually formed by the electrolysis of a stream of
deionized water passing through an anode compartment whilst hydroxyl ions and hydrogen are
similarly formed in a cathode compartment. Both compartments are separated from the eluent by either
cation- or anion-exchange membranes depending on whether anionic or cationic analytes are to be
separated. The process is shown diagrammatically in Figure 4.46. The oxygen and hydrogen generated
by the electrolysis are vented to the atmosphere while the hydronium or hydroxyl ions pass through the
membrane into the eluent in