Interferences monitoring in Sr isotopes analysis for carbonate matrices by LA-ICP-MC-MS

Advances in analytical instrumentation have prompted a growing interest in strontium isotopes within carbonate using LA-MC-ICP-MS, due to the potential for sub-100 µm surface resolution, and high analytical precision. However, the mass bias deviation due the complexity of the matrix together with the interferences of isobaric elements and compounds generated in plasma by ionization may compromise data quality in laser ablation techniques. Within the plasma, interferences including ionization potential differences, oxide formation, polyatomic ion generation, double-charged element formation, and space-charge effects must be considered. These phenomena can affect strontium ratios, necessitating adjustments to analytical conditions or interference monitoring for mathematical corrections.

This study aims to assess how interference monitoring enhance the accuracy and precision of Sr isotope analysis in carbonate matrices using LA-MC-ICP-MS at the LAMIR Institute, Federal University of Paraná. Twenty carbonate samples representing Aptian laminate facies from the Campos Basin were analyzed, focusing regular patterns in carbonate precipitation with different chemical properties, including Sr concentrations, due the cyclicity of carbonates succession. The equipment setup was composed by CETAC Teledyne Analyte Excimer Laser (193 nm) coupled to the MC-ICP-MS model Neptune Plus Thermo Fischer Scientific. The sample were previous imaged by SEM JEOL 6010LA in backscattering mode. The laser ablation configuration was used in line mode, with spot size of 40-50 µm and 700 µm length, scanning speed of 5 µm.s-1, and He gas flows (MFC1=0.55 L.min^-1, MFC2=0.35 L.min^-1), balancing signal intensity, memory effect, and standard deviation. During the configuration was observed that the increase of helium flow reduced memory effects, but relatively diluted the analyte concentration in the plasma, leading low signals and plasma instabilities. For the carbonates analysis we considerate the polyatomic interferents e.g., ⁴⁰Ar-⁴⁴Ca ions and Ca dimers, and adjustments in instrumental conditions were fundamental for minimizing the interferences. The monitoring of strontium signal intensities allowed the evaluation of the environmental conditions influencing the maintenance of signal ratios ⁸⁵Rb/⁸⁸Sr, with tests in the NIST 610 synthetic standard, focusing adjustments of inlet conditions and best results to avoid mixing zones with Si-rich matrix. The interference of ⁸⁷Rb with ⁸⁷Sr is further complicated by the fractionation of the Rb/Sr ratio. Combining sample and standard analyses to determine laser-induced total fractionation of Rb, along with correcting for instrument mass discrimination, emerges as an effective strategy to manage substantial Rb corrections. Isotope measurements conducted MC-ICP-MS face challenges due to the interference by krypton at atomic masses 84 and 86, being necessary the monitoring of 83Kr for corrections of ^84,86Kr on the 84 and 86 masses. The cup configuration with 10ˆ13 Ω amplifiers significantly enhances the accuracy to monitoring double-charged REE, given their low signal intensities (<1 mV). Considering the natural abundance of each element, addressing the interference/correction of ^166,168,170Er²⁺ in masses 83, 84, and 85; ^{168,170,172,174,176}Yb²⁺ in masses 84, 85, 86, 87, and 88, significantly improves confidence, especially given the uncertainties regarding the potential presence of REEs in carbonates. The JCp-1 coral (Geological Survey of Japan) was analyzed as a reference standard, monitoring both ⁸⁸Sr intensity and the ⁸⁷Sr/⁸⁶Sr rations at ~0,7092. The ⁸⁶Sr/⁸⁸Sr ratio was normalized to 0,1194, considering the natural abundance of the stable isotopes for mass bias correction. For samples with strontium concentrations below 300 ppm, the signal intensity was significantly reduced, been more exposed to the background noisiness and leading to higher errors in isobaric interference corrections.