On-column titration and investigation of metal complex formation for aminopolycarboxylate functionalised monoliths using scanning contactless conductivity detection

The application of scanning capacitively coupled contactless conductivity detection (SC4D) for the determination of pH dependant behaviour of two aminopolycarboxylates immobilised onto the surface of a monolithic capillary column is described. The use of SC4D to visualise changes in conductivity of the discrete zones of functional groups within monolithic capillary columns allows for the effects of immobilisation on the physicochemical properties of zones to be compared to that of the functional group in solution. The perturbation of the pKa values of the functional groups can be attributed to the change in chemical environment experienced by the functional group through the presence of local hydrogen bonding and surface induced effects. These bonds, both between adjacent functional groups and with the monolithic polymethacrylate scaffold, result in a modification of the electron density on the functional group and therefore a change in pKa. Changes in the pKa of N-(2-acetamido)iminodiacetic acid (ADA) from 2.48 to 5.2 for one of the acidic protons, with little change in the pKa of the amine group, were observed, which correlates to similar changes in aggregated systems of aminopolycarboxylates. Similar results were obtained for the iminodiacetic acid (IDA) once immobilised onto the surface of the monolith. Furthermore, the ability to measure changes in the charge of such discrete zones of functional groups allows for the visualisation of complexation events occurring directly on-column, where such complexes result in a change in charge of the functional group. This potentially useful technique is illustrated within for the formation of aminopolycarboxylate complexes with a selection of metal ions.