Potentiometric response of magnesium-selective membrane electrode in the presence of nonionic surfactants

Although not normally present in biological samples, nonionic surfactants c an be present in calibrating, rinse, and quality control solutions used wit hin modern automated electrolyte analyzers. This paper describes the effect of nonionic surfactants containing poly(ethylene oxide) units (Brij 35 and Triton X-100) within these structures and alkyl-N-methylglucamide-based (M EGA) surfactants on the potentiometic response of neutral carrier-based mag nesium-selective electrodes. Results are presented for plasticized PVC memb ranes doped with magnesium carrier ETH 7025 along with lipophilic anionic a dditives. Experiments were carried out in model electrolyte solutions with a composition similar to that of the blood serum. It is shown that the ion selectivities of magnesium-selective membrane electrodes can be affected gr eatly by the presence of Brij 35 or Triton X-100 in the sample. This phenom enon can be explained by the partitioning of nonionic surfactants into the membrane phase and the concomitant enhanced extraction of cations (mainly p otassium and sodium) present in the sample phase. Experimental results show that the dramatic loss in selectivity for magnesium membrane makes it esse ntially impossible to measure magnesium ions by direct potentiometry when B rij 35 or Triton X-100 are present. The poisoning of the magnesium ISE in t he presence of Brij 35 or Triton X-100 can be almost completely eliminated by changing the type of nonionic surfactant to one void of poly(ethylene ox ide)-groups. It will be shown that the N-methylglucamide-based surfactants can be employed for this purpose. (C) 1999 Elsevier Science B.V. All rights reserved.