Potentiometric polymeric membrane electrodes for measurement of environmental samples at trace levels: New requirements for selectivities and measuring protocols, and comparison with ICPMS

It is here established that potentiometric polymeric membrane electrodes ba sed on electrically neutral ionophores are useful analytical tools for heav y metal ion determinations in drinking water at nanomolar total concentrati ons. This means that they can compete with the most sophisticated technique s of instrumental analysis. With optimized ion-selective membranes based on the lead-selective ionophore 4-tert-butylcalix[4]arenetetrakis(thioacetic acid dimethylamide) as model example, a number of native and spiked drinkin g water samples are potentiometrically assessed for lead, and the results c ompared with ICPMS measurements. The goal of this work is to demonstrate th at detection limits in real world samples are routinely achieved that are, with 1.5 ppb, at least 10-fold lower than the lead action limit imposed by the U.S. Environmental Protection Agency (15 ppb), In contrast to earlier r eports, different conditioning and measuring protocols are followed, and me mbranes and inner tilling solution of optimized composition are used. The s ensors are shown to be useful for the speciation analysis of lead in water as well. Typical water samples are acidified to pH 4 to assess total lead r ather than free, uncomplexed lead. For lead concentrations above 2 ppb, the values compare very well with ICPMS. Main interferences are found to be Hf and Cu2+, although Cu2+ only shows significant interference at levels arou nd or above its own action limit (1.3 ppm), in which case the water sample would anyway show quality problems. An explicit, simplified nux model targe ted to the practical use of these sensors explains the extent of expected i nterference. Sensors are shown to require a higher selectivity than predict ed by models not considering ion fluxes, since in dilute samples, the count erdiffusion nux of lead from the membrane into the sample becomes potential determining. The model and experiments shown here are a foundation for fut ure trace level applications of potentiometric polymeric membrane electrode s.