The main separation modes of liquid chromatography were described in previous posts entitled "Selection of the Liquid Chromatographic (LC) / HPLC Separation Mode" and "Which are the main liquid chromatography separation modes? / Liquid-Liquid Chromatography (LLC)".
The name ion chromatography refers to any modern and efficient method of separating and determining ions utilizing HPLC. Modern ion chromatography involves using conventional LC equipment with a variety of polymeric columns for determining the ions in question. Ion exchange chromatography is generally applicable to ionic compounds, ionizable compounds(i.e. acids, bases) and compounds which can interact with ionic groups (e.g. liquids, chelates…).
In ion-exchange chromatography the stationary phase is a cross-linked polymer resin, usually divinylbenzene cross-linked polystyrene, with covalently attached ionic functional groups (Fig. 1).
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| Fig. 1: A sulphonated polystyrene cation exchange resin |
The ion exchange process may be carried out in aqueous or nonaqeous solvents. The mobile phase usually contains a counter ion, opposite in charge to the surface ionic functional group, which is in equilibrium with the resin in the form of an ion pair. The mechanism of ion-exchange is shown in Fig. 2.
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| Fig. 2: A cation exchange resin |
Ion-exchange resins are divided into four categories: strong acid cation exchangers, weak acid cation exchangers, strong base anion exchangers, and weak base anion exchangers.
Table I.1 lists some typical ion exchange resins. Strong acid cation exchangers have attached
Type | Functional Group | Examples |
strong acid cation exchanger | sulfonic acid | -SO3-, -CH2CH2SO3- |
weak acid cation exchanger | carboxylic acid | -COO-, PO3- |
strong base anion exchanger | quaternary amine | -CH2N(CH3)3+ |
weak base anion exchanger | amine | -NH3+ |
Table I.1: Types of ion exchange resins
a sulfonic acid functional group that retains its anionic form and thus its capacity for ion-exchange, in strongly acidic solutions. The functional groups for a weak acid cation exchanger, however, are fully protonated at pH levels less than 4 and they lose their exchange capacity. The strong base anion exchangers are quaternary amines therefore retaining a positive charge even in strongly basic solutions. Weak base anion exchangers, however, remain protonated only at pH levels that are moderately basic. Under more basic conditions, a weak base anion exchanger loses its positive charge and its exchange capacity.
Ion exchange chromatography has found wide application in many diverse areas. Because in many biological systems exist water soluble, polar compounds, ion exchange has been widely used in this area for the analysis of nucleosides and carbohydrates. In the pharmaceutical industry have been used for the analysis of barbiturates, organic weak acids and sulphonic acids.
References
1) D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
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