Multiple linear regression and on-line ion exchange chromatography for alternative Rb–Sr and Nd–Sm MC-ICP-MS isotopic measurements

Abstract

A mathematical procedure of multiple linear regression in combination with on-line liquid chromatography was applied for the measurement of Sr and Nd isotope ratios by multicollector ICP-MS in the presence of isobaric isotopes. The separation of Rb and Sr and the separation of Nd and Sm were accomplished by ion exchange chromatography with large volume injection. This mode of injection allowed wide flat-topped chromatographic peaks to be obtained while maintaining chromatographic resolution and avoiding sample dilution. The application of the multiple linear regression procedure to the raw chromatographic data allowed the determination of the isotope ratios of Sr or Nd without interferences from isobaric isotopes. For Sr measurements, the raw ⁸⁷Sr/⁸⁶Sr isotope ratio precision (0.006 to 0.010%) was similar to that of continuous sample introduction (ca. 0.003%) at the same concentration level (50 ng g⁻¹). The mathematical procedure was first evaluated for the cation exchange separation of Rb and Sr. Strontium isotope ratios, obtained using NIST SRM 987, were in agreement with the certified values (bias ca. 0.01%). For the case of Nd and Sm, their chromatographic separation was accomplished by anion exchange using EDTA as mobile phase. Neodymium eluted at 12.8 minutes while samarium eluted at 17.5 minutes, showing flat-topped chromatographic peaks and maintaining baseline neodymium–samarium separation. Precise neodymium and samarium isotope ratios suitable for Nd–Sm dating could be measured by MC-ICP-MS using the proposed multiple linear regression procedure using only one cup configuration.