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 membrane ion-selective electrode are discussed in terms of a possible leaching process of measuring ions from the membrane into the sample. As such electrode membranes are measured under zero current conditions, the charge of such released ions must be compensated for. Three distinct mechanisms are discussed: co-diffusion of measuring ions and counteranions from the backfilling solution into the sample, counter-diffusion of measuring ions and interfering ions of the same charge, and simple partitioning of measuring ions with the lipophilic ion-exchangers. A number of experiments are performed to illustrate these different mechanisms. Silver-selective electrode membranes are studied where the decrease in backfilling electrolyte concentration leads 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. Interestingly, much of the additive seems to leave the membrane in just a few minutes after water exposure. The presence of a neutral ionophore in the membrane is expected to slow this process. (C) 1999 Elsevier Science B.V. All rights reserved.