The chloromethylation reaction can result in cross-linking additional to that produced by the DVB.
Thus for anionic resins, the DVB content is not a reliable guide to pore size.
Resins can also be prepared with chelating functional groups which show selective affinities for certain
metals. Iminodiacetic acid, vinylacetylacetone, glyoxal-thiophenol and 8-hydroxyquinoline have all
been used to produce these so-called 'chelating resins'. Their selectivities are similar to those of the free
reagent.
Non-resinous Ion-exchange Materials
Cellulose, modified by the introduction of ionic groups, is available in paper form or as a powder for
use in TLC and is particularly useful for the separation of macromolecules and biological materials.
Cation exchangers are produced by introducing acidic groups, e.g. —OCH 2 SO 3 H (sulphomethyl,
SM), —OCH 2 COOH (carboxymethyl, CM), which are bonded to the cellulose structure via ether or
ester linkages. Anion exchangers are formed by reacting the cellulose with epichlorhydrin and an
amine, e.g. —OCH 2 CH 2 N+(C 2 H 5 ) 3 (triethylaminoethyl, TEAE).
Liquid ion-exchangers have been discussed in the section on solvent extraction (p. 65). They can be
used in column form by coating them on to a solid support such as cellulose powder or Kel-F
(polytrifluorochloroethylene). Tris-n-octylamine (TNOA) and bis(2-ethylhexyl)phosphoric acid
(HDEHP) behave as strong-base and strong-acid exchangers for anions and cations respectively.
Inorganic ion-exchange materials with similar properties to resins are sometimes used to separate
mixtures at high temperatures or under conditions of high-energy radiation. These include micro-
crystalline heteropolyacid salts (e.g. ammonium molybdophosphate), hydrated zirconium oxide and
zirconium phosphate. Some of the materials are highly selective but tend to react with acids, bases and
complexing agents. For HPLC,