Determination of absolute isotope ratios of vanadium by internal standardisation

Abstract

Due to the prohibitively high cost of materials enriched in the ⁵⁰V isotope, internal standardisation is the only viable method for the determination of absolute vanadium isotope ratios by mass spectrometry. This work describes methodology that applies, for the first time, internal standardisation to the determination of n(⁵¹V)/n(⁵⁰V) isotope ratios by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). To achieve this, solutions of vanadium standard NIST SRM 3165 were doped with iron standard IRMM-014 with certified Fe isotope amount ratios, which was used as a calibrant in the measurements. A comparison of the performance of the established models to correct for instrumental mass fractionation showed that although the mean n(⁵¹V)/n(⁵⁰V) values obtained by the regression model and the exponential model were statistically indistinguishable (399.8 ± 12.6 and 397.3 ± 1.7, U_expanded, k = 2, respectively), their uncertainties were remarkably different. High uncertainty of the n(⁵¹V)/n(⁵⁰V) value by the regression model was attributed to bias arising due to heterogeneity of variance of measured data on the log scale in a situation when isotope ratios of an analyte and an internal standard are significantly different. By its nature, the regression model offered an important advantage of assumption-free calibration, and, following its unsatisfactory performance, a new calibration method that does not rely on a priori assumed functional form of instrumental isotope fractionation was developed in this study. The new method is based on extracting a calibration factor for measured ⁵¹V/⁵⁰V ratios from the relationship constructed between calibration factors obtained for measured isotope ratios of the internal standard and ratios of relative atomic masses making up these ratios (⁵⁶Fe/⁵⁴Fe, ⁵⁷Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁶Fe). The data obtained allowed us to re-determine the atomic weight of vanadium in NIST SRM 3165, to get A_r(V) = 50.94146 ± 0.00003 (U_expanded, k = 2), with the uncertainties improved by a factor of four as compared to the last estimate made in 1977.