ENDOGENOUS LITHIUM DETERMINATION IN BLOOD-PLASMA AND URINE BY ISOTOPE-DILUTION MASS-SPECTROMETRY AND PRELIMINARY ISOLATION OF LITHIUM FRACTION USING PAPER-CHROMATOGRAPHY

A version of isotope dilution mass spectrometric technique elaborated for measuring endogenous lithium concentrations in human blood plasma and urine (10(-7) M region) and applicable in clinical practice is described. A tracer solution of lithium (LiCl) enriched in Li-6 (with abundances Li-6 92%, Li-7 8%) is added to a certain volume of human plasma (0.2-0.4 ml) or urine (0.05-0.1 ml) and dried under an infrared lamp. Thereupon a soluble part extracted from dried plasma with the aid of 0.1 N HCl is dried as well. Dry extracts from plasma and dried samples of urine are processed by 30% H2O2 and subsequently subjected to paper chromatography (with ethanol as a solvent). Such processing of samples is simple and short (about an hour, 3 min of chromatographic proc inclusive), achieving a good separation from organic matrix and interfering macroelements. Contamination of sample in processing is about 2-5 pmol; routine control of contamination and account of their influence are accomplished by measuring two different volumes of each sample. Measurements are made with high precision: cyclic repeated scanning of Li-7 and Li-6 peaks have a standard deviation of Li-7/Li-6 ratio no more than 0.7%. The method described was used to determine endogenous lithium clearance of hypertensive patients and patients with transplanted kidney.