Coupling of Ion Chromatography with ICP-OES

Prepared samples can be passed through an ion-exchange column attached to an HPLC system to selectively retain metals present in the samples while the remaining sample is eluted from the column. The separation of organometallic or inorganic ions is favoured by ion exchange columns and the operating conditions are different from the analysis of non-metallic compounds particularly for retention and separation of trace elements. 

The M + and Na+ refer to ions in the mobile phase with Na+ ions being the counter ions and an ionic group on the particle surface of the packing in the column being represented by S-. Retention of a sample containing ions X- in an anionic exchange column, e.g. SOJ as counter ion, is given by  

Considering these reactions, it is apparent that an increase in the mobile phase counter ion concentration (Na+ or SO3 ) will proportionately decrease the retention of the sample ion. The pH of the mobile phase is very important as it can affect the relative ionisation of acids or bases, i.e. a higher pH leads to increased ionisation of acids on anion exchangers while a lower pH favours increased ionisation and retention of bases on cation exchangers. 

The advantage of using HPLC-ICP-OES for metal analysis is when direct nebulisation of solutions of samples can cause matrix interference on ICP-OES. An important use of this technique would be the detection of variable oxidation states of elements and it can also preconcentrate trace elements on a column which can be eluted from the column and nebulised using ICP-OES friendly solvents. See schematic diagram 7.16 showing an anion and cation HPLC coupled with ICP-OES. 

Factors which affect selectivity, mobile phase, temperature and nature of solute can be shown by the following selective coefficient equation  

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