The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 82

At a relatively high concentration of ammonium nitrate (0.1 M) the separation is much more rapid but much of the chiral selectivity is lost. At the intermediate concentration of 0.025 M, elution is still fairly rapid, the selectivity is maintained, and the enantiomers are well resolved. Changing the organic solvent to acetonitrile, has very little effect on the resolution.

It is seen that, as with the other types of LC stationary phases, some experimental work is essential if the optimum conditions to achieve the separation is to be identified. The best stationary phase, solvent, solvent concentration, buffer, buffer concentration and pH must be ascertained, and this is best achieved by starting from experimentally confirmed conditions for the separation of similar types of solutes. If the sample is unique to the technique, then a rather laborious set of experiments may need to be carried. In practice the guidelines given by the supplier of the stationary phases should be followed with reference to the basic mechanisms of retention described in this book. It should also be emphasized that the recommended procedure will vary between stationary phase types, and between suppliers. If either is changed the conditions may well need to be adjusted for optimum performance.