Optimisation of selectivity in the separation of metallo-cyanide complexes by ion-interaction liquid chromatography

A study into the optimisation and selectivity of a reversed-phase ion-interaction liquid chromatographic method for the separation of metallo-cyanide complexes is described. A stable ion-interaction system was developed in which a C₁₈ stationary phase was equilibrated with a 60 mM solution of tetrabutylammonium hydroxide ion-interaction reagent in order to saturate the stationary phase and to minimise retention changes caused by adsorption and desorption of this material. The effects on retention of the metallo-cyanide complexes caused by changes in pH and ionic strength were minimised through the addition of a high concentration of a phosphate buffer (150 mM)to the mobile phase. Perchlorate (0.32–5.62 mM) was then added to the mobile phase as a competing anion and its effect upon the capacity factors of each complex determined. A linear relationship between the logarithm of capacity factor and the logarithm of the concentration of perchlorate was observed, although the slopes of these plots were not accordance with those predicted by a simple ion-exchange model. However, the linearity of the data allowed a simple optimisation procedure to be applied and the concentration of perchlorate could be used to manipulate the separation selectivity of the system. Three differing elution orders of metallo-cyanide complexes were achieved by varying the concentration of perchlorate in the mobile phase within the range 0.94–5.62 mM.