Determination of Ba, Cd, Cu, Pb and Zn in saliva by isotope dilution direct injection inductively coupled plasma mass spectrometry

Abstract

Trace elements in small sample volumes of saliva were determined by coupling a high efficiency direct injection nebulizer to inductively coupled plasma mass spectrometry and employing quantification by isotope dilution. Aliquots of 0.4 ml of human saliva were mixed with 0.1 ml of concentrated nitric acid and diluted to 2 ml with water. Sample solutions were spiked with an isotopic solution enriched in ¹³⁵Ba, ¹¹²Cd, ⁶⁵Cu, ²⁰⁶Pb and ⁶⁶Zn. The amount of each isotope added to the samples and the measurement procedure were adjusted to attain precise analytical results calculated from the isotope ratios ¹³⁵Ba/¹³⁸Ba, ¹¹²Cd/¹¹⁴Cd, ⁶⁵Cu/⁶³Cu, ²⁰⁶Pb/²⁰⁸Pb and ⁶⁶Zn/⁶⁸Zn. Data acquisition for Ba, Cu and Zn isotopes was performed for a single sample injection of 50 μl and in another sample injection the Cd and Pb isotopes were measured. Concentrations ranging from 5.0 to 16 μg l⁻¹ for Ba, from 0.50 to 1.1 μg l⁻¹ for Cd, from 6.0 to 50 μg l⁻¹ for Cu, from 0.8 to 18.8 μg l⁻¹ for Pb and from 46.0 to 230 μg l⁻¹ for Zn were found in saliva samples. Detection limits of 0.11, 0.03, 0.40, 0.05 and 0.59 μg l⁻¹ were determined for Ba, Cd, Cu, Pb and Zn, respectively. The concentrations found by isotope dilution were in agreement with those of the completely digested samples quantified by external calibration. The direct analysis of 30 samples per hour was attained with the proposed procedure, avoiding time-consuming digestion steps, contamination risks and matrix effects.