Monolithic capillary-based ion-selective electrodes

Poly(styrene-co-divinylbenzene)-based monolithic capillaries of an inner diameter of 200 mu m and a length of 2 - 5 mm have been used to construct Ca(2+)-, Ag(+)-, and Na(+)- selective electrodes. The membranes consist of a solution of ionophore and ion exchanger in bis(2-ethylhexyl) sebacate or 2-nitrophenyl octyl ether, which are used as plasticizers in conventional PVC-based membranes. With capillaries of low porosity, the potentiometric responses down to 10(-8)-10(-9) M solutions do not depend on the composition of the internal solution, which indicates a strong suppression of transmembrane ion fluxes. Thus, no tedious optimization of the inner solution is required with monolith ISEs. The lower detection limits of Ag(+)- and Ca(2+)-ISEs are comparable to the best ones obtained earlier with optimized inner solutions. Additionally, a monolithic Na(+)-selective ISE has been obtained exhibiting a lower detection limit of 3 x 10(-8) M Na(+). With monolithic capillaries of higher porosity and fused-silica GC capillaries, the transmembrane flux effects are noticeable but still significantly smaller than with conventional PVC membranes.