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 6Li (with abundances 6Li 92%, 7Li 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 process 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 7Li and 6Li peaks have a standard deviation of 7Li/6Li ratio no more than 0.7%. The method described was used to determine endogenous lithium clearance of hypertensive patients and patients with transplanted kidney.