Instant Notes: Analytical Chemistry

Ion-exchange chromatography is not that widely used. Inorganic ions and

some cations are better separated by a related mode known as ion chro-

matography (vide infra), whilst for organic ions, ion-pair chromatography

is generally preferred because of its superior efficiency, resolution and

selectivity.

● Ion chromatography (IC). This is a form of ion-exchange chromatography

for the separation of inorganic and some organic cations and anions with

conductometric detectionafter suppressing(removing) the mobile phase

electrolyte (Fig. 3(a)).

The stationary phase is a pellicularmaterial (porous-layer beads), the

particles consisting of an impervious central core surrounded by a thin

porous outer layer (~ 2 mm thick) incorporating cation- or anion-exchange

sites. The thin layer results in much faster rates of exchange (mass transfer)

than is normally the case with ion-exchange and therefore higher efficiencies.

Mobile phases are electrolytes such as NaOH, NaCO 3 or NaHCO 3 for the

separation of anions, and HCl or CH 3 SO 3 H for the separation of cations. The

detection of low levels of ionic solutes in the presence of high levels of an

eluting electrolyte is not feasible unless the latter can be removed. This is

accomplished by a suppressor cartridge that essentially converts the

electrolyte into water, leaving the solute ions as the only ionic species in

the mobile phase.

The following equations summarize the reactions for the separation of

inorganic anions on an anion-exchange column in the HCO 3 - form using a

sodium hydrogen carbonate mobile phase:

D7 – HPLC: modes, procedures and applications 169

Delivery mode

Separation mode

Detection mode Conductivity

cell

Suppressor

Separator

column

Sample

injector

Pump

Eluent

reservoir

Data mode Electronic
integrator

Recorder Computer

Inject

1

2

4

3

5

6

7

8

9

024

Time (min)

6810

Fig. 3. Ion chromatography. (a) Schematic diagram of an ion chromatograph. (b) Anions in water separated on an anion-
exchange column. Reproduced from Dionex UK Ltd with permission.

(a) (b)