Do we have a choice of reaction gas for measuring Ca? Investigations using the Thermo Scientific Neoma MS/MS MC-ICP-MS

Importance of the topic

Precise measurement of calcium isotopes is essential in geochemistry, environmental tracing and biogeochemical studies. However, isobaric overlaps of 40Ca+ with 40Ar+ and 88Sr++ have historically hindered accurate determination, limiting applications in mineralogy, paleoclimatology and life‐science investigations.

Aims and overview of the study

This work evaluates SF6 as a reaction gas in a Thermo Scientific Neoma MS/MS MC-ICP-MS to form CaF+ ions, thereby eliminating major interferences. Both solution standards and laser-ablated solid materials were analyzed and compared to conventional H2/He cell chemistry.

Methodology and experimental design

Solution experiments used 1000 ng/g Ca in 0.5 M HCl for SRM915b and four sample matrices (including one with 400 000 ng/g Sr). Ten repeats of 75 × 4 s integrations were made for pure samples; four replicates for the high‐Sr matrix. The cell prefilter was tuned to transmit Ca nuclides while excluding m/z ≥ 59 species. Reaction cell flows were set to ~1.3 mL/min He and ~0.07 mL/min SF6. Laser-ablation studies employed an ESL NWR imageGEO 193 nm excimer laser with a TV3 ablation cell; 60 s spot ablations at 10 Hz and 5 J/cm2, using 100 µm spots for glasses and 50 µm for apatites. Data reduction was performed in Iolite 4 with a custom CaF Neoma scheme.

Instrumentation

• Thermo Scientific Neoma MS/MS MC-ICP-MS with collision/reaction cell, three 10^13 Ω Faraday amplifiers and SEM detector
• ESI Apex Omega Q desolvating nebulizer system
• ESL NWR imageGEO 193 nm excimer laser ablation system with TV3 ablation cell
• Iolite 4 software for data processing

Main results and discussion

Introducing SF6+He reduced sensitivity by ~5× versus H2/He, necessitating higher Ca concentrations. Formation of CaF+ successfully removed 40Ar+ and 88Sr++ interferences. Solution measurements yielded δ42Ca and δ43Ca precisions of 0.11–0.45‰ (2 RSD) and δ40Ca uncertainties up to ~0.41‰. Laser-ablation of reference glasses and apatites produced consistent 40Ca/44Ca, 42Ca/44Ca and 43Ca/44Ca ratios, though low-Ca materials showed greater scatter. Surprisingly, reproducibility for 40Ca/44Ca was poorer than for heavier isotope ratios, indicating additional variability sources.

Benefits and practical applications

• SF6 reaction gas enables interference-free measurement of calcium isotopes, particularly advantageous for in situ solid sampling.
• Eliminates extensive chemical separation for laser-ablation studies of minerals and glasses.
• Supports on-site or field-deployable geochemical isotope analyses.

Future trends and potential applications

Further optimization of reaction cell gases and hardware could improve sensitivity and precision. High-repetition-rate lasers, advanced cell designs and refined data models will expand micro-scale isotopic mapping. Application to other alkaline earth elements and complex natural matrices is anticipated.

Conclusion

SF6 as a reaction gas in MC-ICP-MS effectively neutralizes key interferences on calcium isotopes, enabling robust measurements in both solution and laser-ablation modes. While H2/He remains superior for solution work due to higher sensitivity, SF6 shows strong promise for direct solid analyses and in situ applications.

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