Lead isotopic analyses of NIST Standard Reference Materials using multiple collector inductively coupled plasma mass spectrometry coupled with a modified external correction method for mass discrimination effect

Abstract

A correction method for the mass discrimination effect was developed for isotopic analyses using multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). For Pb isotopic analysis using MC-ICP-MS, the correction factor for the mass discrimination effect on Pb is based on the addition of TI to the sample solution and measurement of TI isotopic ratios; the correction factor obtained using Tl is directly applied to the Pb isotopes (conventional external correction). However, the series of measurements of discrimination factors for several elements, including Rb, Sr, Ru, Nd, Hf, Re, Os, Tl and Pb (mass range 80–210 u), clearly reveal that the mass discrimination factors observed using MC-ICP-MS were a linear function of mass, suggesting that the correction factors observed using Tl isotopes were not exactly identical with those for Pb isotopes. Therefore, the correction factors obtained with Tl isotopes should be corrected for mass, and then applied to the Pb isotopes. The resultant Pb isotopic ratios for NIST Standard Reference Materials show excellent agreement (within 0.03% for ²⁰⁶Pb/²⁰⁴Pb and 20 ppm for ²⁰⁷Pb/²⁰⁶Pb) with the data obtained by the thermal ionization mass spectrometry. The correction method presented clearly demonstrates the wide versatility of the external correction technique for the precise isotopic analysis using MC-ICP-MS. The possible cause of the ‘exceptionally large’ mass discrimination effect observed for Ru and Os is discussed.