Determination of ultra-trace elements in human plasma or serum by ICP-MS using sodium in the presence of carbon as a single calibration matrix-match component

Abstract

A sensitive and high-throughput method was developed for ultra-trace analyses of Mn, As, Cd, W, Hg, Pb and U in plasma or serum specimens using inductively coupled plasma mass spectrometry (ICP-MS) equipped with a He mode collision cell. Calibration standards were prepared in basic solution with NaCl and n-butanol (present in the diluent) as Na and C sources, respectively. The remaining components of the diluent were NH₄OH, H₄EDTA, Triton X-100 and internal standards (Ga, Rh, Re and Ir). Both calibration standards and plasma specimens were diluted 1 : 10 and directly injected, using an integrated sample introduction system (ISIS), to the ICP-MS bypassing the time consuming and contamination prone chemical/heat digestion steps used elsewhere. The addition of 2% n-butanol to the calibration standards caused a substantial signal enhancement – as much as 450% for As and 120–130% for the other elements – due to the charge transfer from carbon ions (C⁺) to analytes in the instrument plasma. Further increase in n-butanol concentration steadily decreased elemental signal intensities in a very similar way as the addition of NaCl. Both C⁺ and Na⁺ signal intensities in the instrument plasma were at a similar level and their suppressing effect on analytes and internal standards seemed to be interchangeable. Therefore, a thermodynamic approach where one or more ions at higher concentration can influence ionization of other elements present in the instrument plasma at much lower concentrations, which would describe the observed phenomena. These findings were helpful to determine the optimal concentration of 1% NaCl in the intermediate calibration standards and 4% of n-butanol in the diluent to matrix-match the suppressing effect of inorganic and bio-organic components of plasma specimens.