Electrothermal atomic absorption spectrometry (ETAAS) is considered the most common and advanced technique to determine trace lithium in biological fluids. However, chloride existing in samples has been reported to create serious interferences. Nitric acid was verified as a chemical modifier to eliminate the interference of chloride in determining trace lithium in urine samples and the possible mechanism was also elucidated. The influence of chloride was completely eliminated by using 0.5% (v/v) HNO3 as a chemical modifier. Confidence interval analysis on the difference for the slopes of linear regression curves indicated no significant difference between the slopes of aqueous and of urine-matched standard curves with and without 30 mmol/L NaCl in the presence of 0.5% (v/v) HNO3 (P = 0.146). Thus the direct standardization with an aqueous calibration curve could be used instead of the standard-addition method. We conclude that the developed method is accurate and easily applicable for both routine use and research investigations.

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