Lithium determination in artificial serum using flow injection systems with a selective solid-state tubular electrode based on NASICON membranes

The main objective of this work is the determination of lithium in the serum in the concentrations found in the plasma and the urine of patients being treated with lithium salts. Results of measurements under quiescent conditions and with flow injection analysis are compared. The solid-state tubular lithium sensor evaluated is based on conductive ceramics. A flow injection analysis (FIA) system using Li+-NASICON as the sensitive membrane and a BaCl2 0.1 M solution as carrier shows several advantages such as a good sampling frequency (20 samples h-1), a low detection limit (logarithmic range from 5x10-4 M) and a near-Nernstian sensitivity (57 mV/decade). Lithium measurements in artificial serum, using 0.1 M BaCl2 as a carrier solution, provided better results. Lithium determination in urine is possible because the usual concentration (10-30 mM) is in the logarithmic response range of the sensor. Concerning blood analysis (from 7x10-4 to 2x10-3 M in Li), the non-Nernstian response of the system in the lithium therapy concentration range does not seem to be a major problem because of the systematic calibration possibilities when using FIA instrumentation.