Influence of natural, electrically neutral lipids on the potentiometric responses of cation-selective polymeric membrane electrodes

Ionophore-free ion exchanger electrodes were found to exhibit quite a high selectivity for the creatininium ion; however, measurements in diluted urin e samples revealed large emf drifts. Potentiometric, chromatographic, NMR, and mass spectrometric evidence did not reveal any major cationic interferi ng agents, and anionic interfering agents cannot trivially explain the cons istently positive emf drifts; Ultrafiltration of urine samples showed that the interfering agents have molecular weights below 1000 u, The drifts are apparently caused by electrically neutral lipophilic compounds of low molec ular weight that are easily extracted into organic phases. Follow-up experi ments showed that p-cresol and cholesterol cause no significant emf respons es but that coproporphyrin, phosphatidylserine, taurocholic acid, cholic ac id, phosphatidylethanolamine, and octanoic acid cause positive emf drifts o f the type that was observed with the urine samples. The extent of the resp onses and the response time depend not only on the specific compound but al so on the cation in the sample solution. These results suggest that the emf drifts are due to extraction of such natural lipids into the organic membr ane phase where they interact in an ionophore-like fashion with the analyte and interfering ions. Changes in the potentiometric selectivities after co ntact with natural lipids support this interpretation. The same effect of n atural lipids is also expected for ionophore-based electrodes. Indeed, expo sure of a valinomycin-based electrode to a methylene chloride extract of ur ine resulted in a significant reduction of the Na+ discrimination, increasi ng log K-K,Na(pot) from -3.9 to -3.1.