Analytical Chemistry

a rate depending on their selectivity coefficients. Sorption isotherms are approximately linear in dilute
solutions so that elution peaks are symmetrical. Tailing can be expected at high concentrations as the
isotherms curve towards the mobile phase concentration axis (p. 82).

The mobile phase contains an ion of low resin affinity, and the separated components collected at the
bottom of the column are thus accompanied by a relatively high concentration of this ion. Procedures
often adopted in ion-exchange chromatography are gradient elution, involving continuous variation of
the composition of the eluting agent, stepwise elution, in which the composition is altered at specific
points during the separation, and complexing elution where a reagent which forms complexes of varying
stability with the sample components is included in the solution. Acids, bases and buffers are the most
widely used eluting agents.

Separated components emerging in the column effluent can be monitored by means of a physical
measurement, e.g. UV or visible absorbance, refractive index, conductivity or radioactivity.
Alternatively, separate fractions can be collected automatically and subjected to further analysis.

Applications of Ion-exchange Chromatography

The applications of ion-exchange chromatography are exemplified by the selection shown in Table 4.18.
Among the most notable are the separation of lanthanides and actinides using a citrate, lactate or EDTA
eluting agent;

Table 4.18 Some applications of ion-exchange chromatography

Separations Resin used Elution method

transition metals, e.g. Ni2+, Co2+, Mn2+, Cu2+,

Fe3+, Zn2+

anion stepwise elution 12 M to 0.005 M HCl

lanthanides cation stepwise or gradient elution with citrate buffers

Zr, Hf anion 3.5% H 2 SO 4

phosphate mixtures, e.g. ortho-, pyro-, tri-,

tetra-, etc.

anion 1 M to 0.005 M KCI

trace metals in industrial effluents chelating reins concentrated acids

amino acids cation stepwise or gradient elution with citrate buffers

aldehydes, ketones, alcohols anion ketones and aldehydes held as bisulphate addition

compounds. Eluted with hot water and NaCl

respectively

sugars anion as borate association complexes. Eluted by gradient

pH

carboxylic acids anion gradient pH elution

pharmaceuticals cation/anion acid or alkali buffer