Correcting nonlinearity and mass-bias in measurements by Inductively coupled plasma quadrupole mass spectrometry

Highlights: • Mass bias was shown to not be a smooth function of isotope ratio reference mass. • Nonlinearity was shown to vary between analyses. • Run-by-run nonlinearity and mass bias corrections can result in substantial improvements to precision. -- Abstract: A two-parameter modeling formula is proposed for quadrupole ICP-MS (ICP-QMS) data, which corrects for nonlinearity with a "dead-time" function and makes subsequent weighted mass-bias corrections to normalize isotope ratio data to pertinent isotopic reference materials. The methodology improves the short-term and long-term precision as well as accuracy of the measurements. Applying the model to isotopes spanning a wide mass range revealed that values for the parameters describing nonlinearity and mass bias do not conform to expectations based on conventional interpretation. Within a single analysis run for a specific element, nonlinearity and mass-bias corrections can be evaluated to very tight tolerances and the model fits the data to within a few parts per thousand. However, in a different run or with a different element, the values of the parameters might change. A comprehensive strategy for conducting measurements of isotope ratios or trace-element concentrations is described, which takes advantage of the model's strengths and accommodates the observed run-to-run variations. Supporting data is presented to demonstrate improved precision and accuracy with the widely available ICP-QMS system.