Oxide Formation and Instrumental Mass Bias in MC-ICP-MS: An Isotopic Case Study of Neodymium

High rates of oxide formation affect the magnitude and behaviour of instrumental mass bias for Nd isotopic measurements performed with multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) instruments, causing the traditional correction methods (e.g., internal and external normalisation) to fail. Here, we investigate the instrumental conditions that govern oxide formation and provide an extensive data set describing how different oxide formation rates affect the measurement error of Nd isotopic ratios. Results are reported for several instrumental set-ups including wet and dry plasma, different introduction methods, the addition of N-2, and various sampler and skimmer cone geometries. The differences in the behaviour of Nd isotopic ratios observed for dry and wet plasma require several reaction mechanisms to explain why oxide formation is associated with a non-linear mass bias for some rare earth elements. We developed a simple mathematical model to describe the behaviour of Nd isotopic ratios for a range of oxide formation rates and different instrumental settings and present a qualitative model that predicts the isotopic offsets of Nd ratios based on the cumulative contributions of the major sources of mass bias. A series of analytical recommendations for the determination of accurate and precise Nd ratios by MC-ICP-MS is presented.