Examination on matrix-dependent mass-discrimination effect for inductively coupled plasma mass spectrometry (ICP-MS): difference between shielded ICP and unshielded ICP

Abstract

Different matrix-dependent mass-discrimination effects on ⁵²Cr/⁵³Cr isotope-ratio measurements were observed between shielded ICP-QMS and unshielded ICP-QMS conditions. Chromium standard solutions prepared in different matrices such as 1% HNO₃ and 0.1%–10% H₂SO₄ were examined; consequently, different Cr isotope ratios were observed among different solutions due to matrix-dependent mass-discrimination effects under shielded ICP-QMS conditions. In contrast, such effect was not observed under unshielded ICP-QMS conditions. Accordingly, ion lens settings in ICP-QMS were investigated; it was found that omega-lens voltage strongly influenced the matrix-dependent mass-discrimination effect under shielded ICP-QMS conditions. After optimization of omega-lens voltage, similar Cr isotope ratios were obtained for different solutions even under shielded ICP-QMS conditions: the matrix-dependent mass-discrimination effect was eliminated. It was concluded that the space charge effect attributed to different matrices occurring at omega-lens influenced on the matrix-dependent mass-discrimination effect, particularly for lighter elements such as Cr.