Ion Chromatography - Selection of Stationary Phase
Selection of Stationary Phase
Each specific ion analysis will require a unique stationary phase, mobile phase and column to effect the separation in the most efficient manner. The most efficient separation will be achieved when all the solutes are separated according to a defined standard and in the minimum time. In practice, this will usually mean that that the separation is achieved in a practical separation time the minimum time not being necessary. Most separations involving less than ten components can be separated (if really necessary) on a fully optimized system in a few seconds. The practical separation time will depend somewhat on the operation of the laboratory. If the complete separation is automated and repetitive then an analysis time of 3 or 4 minutes is usually as fast as is necessary (the throughput of the analysis will probably be determined more by the time involved in collecting the sample, sample preparation and in the reporting procedures than by the actual analysis). If the analysis is needed to control the operation of a plant then the practical analysis time will depend on the time constant of the plant (i.e a function of the time taken for the plant to respond to a definite change in an operational variable). It follows that, in practice, a fully optimized system is not usually needed and this is just as well as the conditions for a fully optimized analysis can take several weeks to identify.
There are some general rules to follow to identify a suitable column and a practical phase system for a specific analysis but inevitably some experimentation by the analyst will be necessary. Manufacturers of columns and stationary phases have a wealth of experience in ion separations and they should be consulted in the first instance. Most manufactures also have a large number of application examples available that they are usually ready to share with a potential buyer and some of the applications may be similar to the problem involved and give some leads to the choice of phase system. Some companies will actually develop a method for a potential buyer. However, this can be costly, and it is always worth-while to try to develop the procedure if possible as the process will provide useful experience for coping with future problems. The following points will need consideration but it must be noted that the recommendations can only be general and a specific analytical problem will demand more detailed evaluation, which must remain the responsibility of the analyst.