ESI-Orbitrap MS enables reliable lead isotopic measurements in lake sediment cores

High resolution mass spectrometry, and particularly Orbitrap based instrumentations, are emerging as powerful technologies for speciation analysis, mainly to collect reliable information on isotope ratios (IRs). During recent years, traditional, stable isotopic systems, namely H, C, N, O and S have been investigated, but we only recently demonstrated that a dedicated complexation procedure followed by collisional dissociation may enable the investigation of metal ion IRs, and particularly lead, employing a standard electrospray ionization source. The procedure has not yet been demonstrated to be applicable to real-world samples.

A total of 31 sediment samples from Lake Como were analyzed for lead IRs by both ESI-Orbitrap and Multi collector ICP-MS. Minor modifications to the original ESI-Orbitrap method, including increased ligand concentration, were required due to sample complexity; the simple Standard-Sample Bracketing (SSB) procedure was demonstrated to be effective in correcting the mass bias. Bland-Altman analysis showed no statistically significant differences between the two methods (95% confidence level) for the two isotope ratios ²⁰⁴Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb, while a small deviation (<2‰) was observed for the ²⁰⁸Pb/²⁰⁶Pb, remaining acceptable for environmental applications. The isotopic data trend identified leaded gasoline as the main contributor to lead contamination in Lake Como during the period 1950-2000.

This work presents the first reported application of ESI-Orbitrap-based metal isotopic analysis in complex environmental matrices. Factors influencing accuracy and precision were systematically evaluated, demonstrating robust analytical performance. The results establish the potential of the ESI-Orbitrap approach for determining isotope ratios of metal ions in real-world samples.