Nitrogen sustained Microwave Inductively Coupled Atmospheric Pressure Plasma (MICAP) is emerging as a cost-effective alternative to traditional argon-based plasma systems for mass spectrometry because of its use of cheaper nitrogen and the reduced complexity of instrumentation. Recent studies have demonstrated the robustness of nitrogen sustained MICAP in elemental concentration analysis, producing results comparable to those obtained by conventional Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) using argon as a plasma gas. However, the application of nitrogen sustained MICAP in isotope ratio measurements has not yet been studied well, particularly in comparison to argon-based plasma systems. In this study we explore the performance of a nitrogen sustained MICAP quadrupole Mass spectrometer for isotope ratio measurements of Li, Sr, Cd, and Pb using certified isotope reference materials.

The study examines the influence of instrument parameters such as RF power, sample gas flow and skimmer cone voltage on the measured isotope ratios and in particular on instrumental mass fractionation and the precision of the measured isotope ratio. Through systematic optimization of plasma conditions and data acquisition settings, we were able to significantly enhance the precision, obtaining fit for purpose isotope ratios. The study also examines instrumental isotope fractionation under these varying conditions. Conventional mass fractionation correction models using fractionation laws were found to produce inaccurate results, necessitating correction using standard-sample bracketing. Additionally, for lithium isotope measurements, we employed cold plasma conditions to reduce interferences from nitrogen and carbon.

Our results highlight the potential of MICAP-MS as a robust tool for isotope ratio analysis, offering advantages in cost effectiveness over conventional argon Plasma based ICP-MS systems.