Insights into Primary Ion Exchange between Ion-Selective Membranes and Solution. From Altering Natural Isotope Ratios to Isotope Dilution Inductively Coupled Plasma Mass Spectrometry Studies

Although ion-selective electrodes have been routinely used fordecades now, there are still gaps in experimental evidence regarding howthese sensors operate. This especially applies to the exchange of primary ionsoccurring for systems already containing analyte ions from the pretreatmentstep. Herein, for thefirst time, we present an insight into this process lookingat the effect of altered ratios of naturally occurring analyte isotopes andachieving isotopic equilibrium. Benefiting from the same chemical propertiesof all isotopes of analyte ions and spatial resolution offered by laser ablationand inductively coupled plasma mass spectrometry, obtaining insights intoprimary ion diffusion in the preconditioned membrane is possible. Forsystems that have reached isotopic equilibrium in the membrane through ion exchange and between the membrane phase and thesample, quantification of primary ions in the membrane is possible using an isotope dilution approach for a heterogeneous system(membrane−liquid sample). Experimental results obtained for silver-selective membrane show that the primary ion diffusioncoefficient in the preconditioned membrane is close to (6±1)×10−9cm2/s, being somewhat lower compared to the previouslyreported values for other cations. Diffusion of ions in the membrane is the rate limiting step in achieving isotopic exchangeequilibrium between the ion-selective membrane phase and sample solution. On the contrary to previous reports, quantification ofsilver present in the membrane clearly shows that contact of the membrane with silver nitrate solution of concentration 10−3M leadsto pronounced accumulation of silver ions in the membrane, reaching almost 150% of ion exchanger amount. The magnitude of thiseffect increases for higher concentration of the electrolyte in the solution