Liquid Chromatography - The Electrical Conductivity Detectors 3

 

The column used was the IonPacCS12 (a proprietary ion exchange column) with a mobile phase consisting of a 20 nM methanesulphonic acid solution in water at a flow rate of 1 ml/min. The sample volume was 25 ml and the separation is also an interesting example of the use of the ion suppression technique. If the methanesulphonic acid solution was passed directly from the column through the detector would have a high electrical conductivity and give a large detector base current which would swamp the signal from the ions being monitored. Thus, subsequent to the mobile phase leaving the column (and after the methane sulphonic acid has achieved its purpose in producing the desired separation) the reagent must be removed to ensure that the mobile phase entering the detector only contains those ions of interest and minimal background conductivity. The methane sulphonic acid was removed by passing the mobile phase through a short reverse phase column before it entered the detector. The reverse phase will remove all organic material by adsorption due to the strong dispersive forces that will occur between the hydrocarbon chains of the reverse phase and the methyl group of the methanesulphonic acid. The ion suppression column eventually saturates and require regeneration by desorbing the methane sulphonic acid with a strong dispersive solvent that is miscible with water such as acetonitrile. This technique of ion suppression is frequently used in ion exchange chromatography when using the electrical conductivity detector. A wide variety of different types of ion suppression columns are available but it should be pointed out that, any suppresser system introduced between the column and the detector, will cause some degree band spreading and consequently reduce the resolving power of the system. It follows, that the connecting tubes and suppression column itself must be very carefully designed to eliminate or reduce this dispersion to an absolute minimum.