Determination of lithium in blood serum — a comparison between molecular absorption spectrophotometry and emission flame spectrometry

Abstract

Emission flame spectrometry in acetylene-nitrous oxide and acetylene-air flames enables rapid, sensitive and precise determination of lithium in blood serum at 670.8 nm [l]. No deproteination of serum is necessary. Calibration plots are prepared for serum samples diluted 10-l 000 times with twice distilled water or sodium chloride solution to resulting 6-150 mmol/l concentration. Sample viscosity does not interfere with the measurements for samples diluted to 40% of serum and more. Insufficiently diluted serum samples (1: 2-l : 5) may cause partial blocking of the nebulizer capillary and the emission values decrease. A procedure for 100 ~1 serum containing 10 pmol/l-50 mmol/l Li and diluted with 10 ml of physiologic solution (1: 10) is recommended. The emission value of the diluted physiologic solution represents the blank measurement. One to ten microlitres of serum may also be sampled and diluted 1: 1000 for lithium concentrations higher than 100 nmol/l. If a highly efficient double grating monochromator producing monochromatic bandpasses of l-10 pm (picometer) is used with a low-noise photomultiplier of high quantum efficiency giving linear response in a five orders interval, 0.01-500 pmol/l of Li may be determined with a precision of f1.5% rel. The sensitivity falls by several orders and the precision of the method considerably decreases if current flame photometers containing interference filters or simple monochromator and phototube or selenium barriere cell as detector are used. We have, however, shown that the spectrophotometric determination of lithium with the common analytical reagent, thoron (2-arsonobenzene)-1-azo-1’-(2’-hydroxynaphthalene-3’,6’-disulphonic acid), [2,3] gives similar precision and accuracy in a concentration range of 0.25-1.5 mmol/l lithium in serum as the precise variant of the flame emission spectrometry. One hundred microlitres of serum containing 2.0 mmol/l Li is sampled into a 1.5 ml test tube of the Eppendorf type, 100 yl of 1.0 mol/l trichloroacetic acid is added, stirred and allowed to stand for 10 min at room temperature. The plugged test tube is then centrifugated for 2 min at 12000 rpm. A 100~~1 portion of the supernatant is