Detection limit of polymeric membrane potentiometric ion sensors: how can we go down to trace levels?

Chemical reasons that may dictate the lower detection limit of polymeric me mbrane ion-selective electrode are discussed in terms of a possible leachin g process of measuring ions from the membrane into the sample. As such elec trode membranes are measured under zero current conditions, the charge of s uch released ions must be compensated for. Three distinct mechanisms are di scussed: co-diffusion of measuring ions and counteranions from the backfill ing solution into the sample, counter-diffusion of measuring ions and inter fering ions of the same charge, and simple partitioning of measuring ions w ith the lipophilic ion-exchangers. A number of experiments are performed to illustrate these different mechanisms. Silver-selective electrode membrane s are studied where the decrease in backfilling electrolyte concentration l eads to detection limit variations from 10(-5.3) to 10(-8) M. The leaching behavior of sodium tetraphenylborate as anionic membrane additives is shown to be difficult to quantify in membranes containing no ionophore. Interest ingly, much of the additive seems to leave the membrane in just a few minut es after water exposure. The presence of a neutral ionophore in the membran e is expected to slow this process. (C) 1999 Elsevier Science B.V. All righ ts reserved.