Calculation methods for direct internal mass fractionation correction of spiked isotopic ratios from multi-collector mass spectrometric measurements

Abstract

It is difficult to do internal mass fractionation corrections for isotope dilution analysis by thermal ionization mass spectrometry (TIMS) or multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), especially for MC-ICP-MS. In this study, calculation methods for direct internal fractionation correction of spiked isotope analysis by TIMS or MC-ICP-MS cycle by cycle for elements having at least two internal reference isotopic ratios are presented. For TIMS, direct internal mass fractionation correction calculation methods, based on both power and exponential laws, are derived; whereas for MC-ICP-MS, due to larger mass fractionation effects, only exponential law is considered. These calculation strategies can be applied for both static and multi-dynamic measurements. For multi-dynamic measurements, the isotope fractionation effect, gain and cup efficiency effects of different collectors, as well as ion beam fluctuation effects are all simultaneously eliminated. The calculation methods were verified by Sr isotopic analyses of spiked NBS987 standard solutions by TIMS and Hf isotopic analyses of spiked geological reference materials by MC-ICP-MS. In addition, precise and accurate calibrations of isotopic ratios of the spikes, based on the calculation methods, are discussed.