INCREASED PERFORMANCE IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY OF LITHIUM IN RENAL TUBULAR FLUID BY USE OF TANTALUM FOIL

A method for the determination of lithium by electrothermal atomic absorption spectrometry in nanolitre samples of tubular fluid of rat kidneys was developed. The signal-to-noise ratio was improved by use of tantalum foil in the atomization step. Peak height absorbance increased ten times and integrated absorbance four times. The atomization temperature could be reduced by 300-degrees-C. The sensitivity increased from 1.7 to 0.45 pmol, which permits the determination of lithium in 10 nl samples. Addition of lithium-free tubular fluid to lithium standards showed that no matrix effects were present and that background correction was not required. At a level of 0.25 mmol l-1, the within-day and between-day relative standard deviations (RSDs) were 5 and 4%, respectively. For plasma, the within-day and between-day RSDs were also 5 and 4%, respectively, at a level of 0.25 mmol l-1. The recovery of lithium standards added to plasma was 99 +/- 4%. In untreated anaesthetized rats, the tubular fluid-to-plasma lithium concentration ratio for the late proximal part of the renal tubule was 1.10 +/- 0.10 (mean +/- SD; n = 180) and for the early distal part of the tubule 1.08 +/- 0.13 (n = 102). Since tubular fluid-to-plasma concentration ratios can be assessed in both proximal and distal tubular fluid, the proposed method provides a tool to test whether lithium is a good marker of proximal tubular sodium reabsorption.