Neutral lonophore-Based Selective Electrode for Assaying the Activity of Magnesium in Undiluted Blood

The concentration of free, ionized magnesium in undiluted blood serum can be determined potentiometncally with a new magnesium-selective carrier in polymeric membrane elec-trodes (by weight 66 % plasticizer,-33 % polyvinyl chloride, and-.1 % ionophore). However, discrimination of calcium by the new ionophore-added membrane is not sufficient to keep calcium interference to <1%. To correct for the interference by calcium, we determine the calcium activity of the serum sample with a calcium-selective electrode and calibrate the magnesium-selective electrode with standard solutions con-taining calcium in the concentrations found. In this way we determined the free ionized magnesium in five plasma sam-ples from five differentdpersons and found it to be within the range expected for magnesium in undiluted serum. Additional Keyphrases: ionized magnesium. tris(malondiamide