Factors influencing the precision and accuracy of Nd isotope measurements by thermal ionization mass spectrometry

Taking the example of Nd, we present a method based on a 4-mass-step acquisition scheme to measure all isotope ratios dynamically by thermal ionization mass spectrometry (TIMS); the aim being to minimize the dependency of all mass fractionation-corrected ratios on collector efficiencies and amplifier gains. The performance of the method was evaluated from unprocessed JNdi-1 Nd standards analyzed in multiple sessions on three different instruments over a period of similar to 1.5 years (n = 61), as well as from standards (18 JNdi-1 and 19 BHVO-2) processed through different chemical purification procedures. The Nd isotopic compositions of standards processed through fine-grained (25-50 mu m) Ln-spec resin show a subtle but clear fractionation caused by the nuclear field shift effect. This effect contributes to the inaccuracy of Nd isotope measurements at the ppm level of precision.