Investigation of the mass discrimination of multiple collector ICP-MS using neodymium isotopes and the generalised power law

Abstract

Neodymium isotopes have been used to show that the large instrumental mass discrimination of plasma source mass spectrometers cannot be accurately corrected with the common mass fractionation laws. The present study investigates the application of the generalised power law (GPL) for mass bias correction. To this end, the Nd isotope ratio data acquired with two Nu Plasma multiple collector ICP-MS instruments were compared with published Nd reference values determined by thermal ionisation mass spectrometry. The GPL applies a variable, n, such that it can be used to describe different mass-dependencies. In particular, the GPL is equivalent to the power law for n = 1 and approaches the exponential law for n → 0. The exponential and power law corrected data displayed significant (>100 ppm) deviations from the reference values for some Nd isotope ratios, whereas the instrumental mass discrimination was accurately described by the GPL. For three datasets acquired under optimal operating conditions this was achieved with n ≈ −0.23, but during routine operation of the instrument the n-values were observed to vary between −0.2 and −0.4. This shows that the GPL is a valuable tool for data evaluation in isotope ratio mass spectrometry and for the characterisation of instrumental mass-dependencies. The observed mass-dependencies are in accord with isotope fractionation by kinetic transport processes, if some of the mass discrimination is governed by reduced masses.