Guidelines for improving the lower detection limit of ion-selective electrodes: A systematic approach

Zero-current membrane fluxes are the principal source of bias that has prohibited researchers from obtaining true, thermodynamic selectivity coefficients for membrane-based ion-selective electrodes (ISEs). They are also responsible for the mediocre detection limits historically seen with these types of potentiometric sensors. By choosing an experimental protocol that suppresses these fluxes, it becomes possible to obtain unbiased thermodynamic selectivity coefficients that are needed to produce ISEs with greatly improved detection limits. In this work, a Cs+-selective electrode based on calix[6]arene-hexaacetic acid hexaethyl ester (Cs 1) is used to systematically demonstrate how unbiased selectivity coefficients can be obtained, and how they can be used to optimize inner filling solutions for low detection limit measurements. A comparison of biased selectivity methods (e.g., classical separate solution method (SSM), fixed interference method (FIM), matched potential method (MPM)) with the unbiased modified separate solution method (MSSM) found that selectivity coefficients were underestimated in several cases by more than 4 orders of magnitude (logK(CsMg)(pot,MSSM) = - 8.7; log K-Cs,Mg(pot,SSM) = -4.5). The importance of key experimental parameters, including diffusion coefficients and diffusion layer thicknesses in the aqueous and organic phases, on the minimization of ion fluxes and the improvement of lower detection limits is also described. A dramatic reduction of membrane fluxes by the covalent attachment of a Ca2+-selective ionophore to a methyl methacrylate-decyl methacrylate (MMA-DMA) copolymer matrix is also demonstrated. The ionophore-immobilized ISE exhibited no super-Nernstian response and yielded a detection limit of 40 ppt (109 a(Ca) = - 9-0) with an inner filling solution of 1 x 10(-3) M KCl. Finally, a set of guidelines for experimental protocols leading to obtaining unbiased selectivity coefficients and producing ISEs for trace level analyses is given.