Tweezer-type neutral carrier-based calcium-selective membrane electrode with drastically reduced anionic interference

A new calcium-selective ionophore N,N-dioctyl-3alpha,12alpha-bis(N-heptyl-N-methylcarbamoyl-methoxyacetamidoace- toxy)-5beta-cholan-24-amide (denoted as BACA), was synthesized, and its potentiometric performance has been evaluated in comparison with that of the best known calcium-selective neutral carriers, ETH 129 and ETH 1001. The H-1 NMR spectra of BACA tibrated with Ca(SCN)(2) suggest that BACA form a 1: 1 complex, tweezing a calcium ion between the two parallel I diamide groups substituted on a rigid bile acid frame. The calciumselective membrane based on BACA was less selective to calcium (log K-Ca2+j(POT) = -4.2, -4.2, -4.6, and -4.8, respectively, for j = Mg2+, Li+, Na+,'. and K+) than those based on EM 129 (log K-Ca2+j(POT) = -4.4, -4.3, -5.4, and -5.4, respectively, in the same order) and ETH 100 1 (log K-Ca2+j(POT) = -4.4, -4.4, -5.4, and -5-.4), implying that BACA forms a weaker calcium complex from the other two ETH compounds. In our experimental conditions, potentiometrically determined complex formation constants of calcium-selective neutral carriers (logbeta(Ca2+L)) were 15.2, 14.0, and 8.6 for ETH 129, ETH 100 1, and BACA, respectively. A slightly reduced calcium selectivity of BACA, however, affects the anionic interference-free calcium-selective membrane; the BACA-based membrane exhibits a Nemstian response up to 10(-1) M Ca2+ in the presence of lipophilic anions (e.g., SCN-, ClO4-, salicylate, and I-) and anionic surfactant,. whereas the EM 129- and ETH 1001-based ones suffer from serious anionic interference showing a curvature or leveled off response over about 10(-4) M It was demonstrated that such a trade off does not affect the analytical performance of BACA-based electrode in most applications, including clinical analysis.