Ion Chromatography - Ion Suppression Techniques

Ion Suppression Techniques

Instruments and techniques used for ion suppression have already been discussed but some indication of when to use the techniques must be given. Normal ion chromatography employs a single column and to differentiate the technique from that which employs ion suppression it has been given the term Single Column Ion Chromatography with its own acronym SCIC. In this book the terms ion chromatography and ion chromatography with ion suppression will be used. Ion suppression is necessary when the mobile phase has a high conductivity and the contribution of the eluted ions are relatively so small that their signal is swamped by the background signal form the mobile phase. In addition, if, as an alternative, the mobile phase signal is electronically backed-off, the signal from the conductivity of the analyte ions will still be small compared with the noise from mobile phase conductivity signal. As a consequence, to detect the change in electrical conductivity by the eluted ions, the ions from the mobile phase must be removed. Ion chromatography is practiced mostly without the use of suppression techniques and it is only resorted to when the phase system must contain a high concentration of ions and, thus, have high conductivity in order to achieve the required separation.

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Figure 29. Separation of the Alkaline Earth Cations without Ion Suppression

The alternative separation to that shown in figure 21 is given in figure 29. The separation was carried out on Altech Universal Cation Column 10cm long 4.6mm in diameter using a mobile phase consisting of 3mM methanesulphonic acid at a flow rate of 1ml per minute. In this separation only 3mM methanesulphonic acid was used as opposed to the other separation where a suppressor column was necessary because the concentration of methanesulphonic acid was 20mM. It is seen that the two separations compare quite favourably. Again it should be emphasized that additional techniques should only be resorted to when essential, particularly when any device that is used could impair the resolution obtained from the column. The technique of ion suppression should not be allowed to unnecessarily complicate an already fairly complex system. As the example shows, the separation of the alkaline earth shown figure in 21 can also be achieved without a suppressing system by a simple change in the phase system