Determination of lithium in microlitre amounts of human body fluids at therapeutic and normal levels by stable isotope dilution and field desorption mass spectrometry

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The use of field desorption mass spectrometry for the determination of lithium in body fluids at therapeutic levels--ppm region--as well as at the normal level--ppb region--has been developed. The use of a stable isotope enriched internal standard, together with the outstanding sensitivity of field desorption for alkali cations and the high specificity of mass spectrometry, allows a quantitative determination of lithium in microlitre amounts of body fluids, such as plasma, saliva and urine. The assay allows a determination of lithium even at ultratrace concentrations where routine spectroscopic procedures cannot be applied. Analysis of plasma required a simple protein precipitation, whereas saliva and urine could be analysed without treatment. The precision of the data obtained ranged from 2--10%. The time consumption for one analysis in routine work is about 20--30 min.