Metal Ion Isotope Ratio Using ESI-Orbitrap HRMS: Proof of Concept and Initial Performance Evaluation for Lead Isotopic Ratios

This study introduces a novel approach using an electrospray source coupled to an Orbitrap MS instrument to determine metal isotope ratios. The procedure involves forming a complex between the ion of interest and an appropriate ligand, generating gas-phase ions via electrospray ionization, selecting the complex mass by quadrupole filtering, and performing collisional fragmentation to yield free metal ions. The isotopic pattern of the free ion is then analyzed by high-resolution MS. The approach ensures high selectivity and interference-free spectra. A proof-of-concept study was conducted to determine Pb isotope ratios, focusing on identifying the factors that influence the accuracy and precision of the procedure. At this early stage, optimal accuracy was achieved even in the presence of matrix components by applying mass bias correction methods originally developed for other isotope ratio techniques; precision is comparable to that achieved by single-collector ICP-MS instrumentation. This approach may complement conventional methods that suffer from limited mass resolution and usually require extensive sample preparation.