Ion Chromatography - The Electrical Conductivity Detector
The column employed in the separation was the IonPacCS12 (a proprietary ion exchange column) and the mobile phase consisted of a 20mM aqueous methane-sulphonic acid solution and the flow rate was 1ml/min. A 25ml volume of sample was used.
Figure 21. Determination of Alkali and Alkaline Earth Cations
The separation is also an interesting example of the use of the ion suppression technique. In this separation, methanesulphonic acid solution was used as the mobile phase (at 20nM concentration). As a consequence, this mobile phase mixture would, if passed directly from the column into the detector, give a large detector base current (as a result of the high electrical conductivity of the methanesulphonic acid) which would swamp any signal from the ions being monitored. Thus, immediately after the mobile phase left the column (and after the methane sulphonic acid had achieved its purpose in the separation) the reagent had to be removed so that the mobile phase entering the detector contained only those ions of interest and minimal background conductivity.
The methanesulphonic acid was removed by a short reverse phase column situated between the analytical column and the detector. The reverse phase removes all organic material by adsorption due to the strong dispersive forces between the hydrocarbon chains of the reverse phase and the methyl group of the methanesulphonic acid.
The technique of ion suppression is a common and very useful technique often employed in ion chromatography, particularly in the separation of cations. To maintain the integrity of the separation achieved by the column, great care must be taken to reduce the peak dispersion that takes place in the suppressor column to an absolute minimum, Thus, all extraneous volumes involved in the suppressor system must be minimized and the suppressor column must be packed with the same care as the analytical column to reduce extra column peak dispersion to a minimum. Again it must be emphasized that it is very important to ensure that the separation attained by the analytical column is not destroyed by peak dispersion in the suppressor column
Suppressor columns can be packed with reverse phase materials to remove mobile phase constituents such as organic buffers etc, or high capacity ion exchange material to remove inorganic buffers by ionic interactions. All ion suppression columns will eventually become saturated and require regeneration This can be achieved with ion exchange materials by washing with he appropriate acid or base solution or in the case of a reverse phase suppressor column by desorbing the methane sulphonic acid with a strong dispersive water miscible solvent such as acetonitrile.