Lithium Ion-Selective Electrode with Improved Lifetime. -A polymer membrane-based lithium-selective electrode using a polyurethane/poly(vinyl chloride) mixed matrix with 8:2 mixing ratio exhibits a lifetime of over 90 days. Since polyurethane has stronger adhesion to the solid-state substrate and improved biocompatibility, the mixed matrix system can be used to prepare a miniaturized lithium ion-selective electrode with increased lifetime.

The potentiometric response characteristics of lithium ion-selective membrane electrodes employing three different THF-type 16-crown-4 derivatives as ionophores were investigated. Of the THF-based crown compounds tested, a cis-type isomer of the derivative with only methyl groups (H/CH 3 ) at four bridged positions (cis-THF-I) was found to exhibit the best selectivity for Li + over other cations. The composition of the cis-THF-I-based PVC-matrix membrane was optimized and its performance was compared with those based on lithium ionophores reported previously. The new cis-THF-I-based membrane exhibits remarkably enhanced selectivities for lithium over other cations: logK pot Li,Na = 22.8, logK pot Li,K = 24.6, logK pot Li,NH 4 = 25.4, logK pot Li,Ca = 25.7 and logK pot Li,Mg = 25.4. Furthermore, it was found that serum components do not interfere significantly with the lithium measurements when the cis-THF-I-based electrode is used. The response of the electrode was found to be linear in the clinical range of interest.