Measurement of 87Sr/86Sr Isotope Ratios in Rocks by ICP-QQQ in Mass-Shift Mode

Importance of the Topic

Strontium isotope ratios ((87)Sr/(86)Sr) are critical in geochronology and environmental tracing because they reflect radiogenic decay of (87)Rb to (87)Sr. Accurate measurement guides rock dating, sediment provenance studies, and pollutant source identification. Traditional methods like TIMS and MC-ICP-MS demand laborious Sr/Rb separation to avoid isobaric overlap, limiting throughput.

Objectives and Study Overview

This study aimed to develop a rapid, high-throughput analytical workflow for (87)Sr/(86)Sr ratios with precision better than 0.1% RSD using Agilent 8900 ICP-QQQ in O₂ mass-shift mode. The protocol eliminates chromatographic Sr/Rb separation, streamlining sample handling.

Methodology

Reactions in the collision/reaction cell exploit differential reactivity: Sr⁺ forms SrO⁺ with O₂, while Rb⁺ remains unreactive. Q1 isolates m/z 87 ions, and Q2 monitors m/z 103 (⁸⁷Sr¹⁶O⁺) and 102 (⁸⁶Sr¹⁶O⁺). Optimization of O₂ flow yielded 10–14% conversion efficiency. Method validation included monitoring potential ⁸⁵Rb¹⁶O⁺ formation, confirming negligible interference.

Instrumentation

Analysis was performed on an Agilent 8900 ICP-QQQ with ORS 4 reaction cell, quartz torch (2.5 mm i.d.), concentric nebulizer, and peristaltic sample introduction. Key settings: RF power 1550 W, nebulizer gas 1.15 L/min, cell gas 0.45 mL/min (O₂), Q1/Q2 masses 86→102 and 87→103.

Main Results and Discussion

Measurements of USGS rock reference materials (BCR-2, BHVO-2) with and without Sr purification yielded (87)Sr/(86)Sr ratios consistent with literature values and showed no significant bias. Seven replicates across days achieved 0.02–0.04% RSD (2 SD). Analysis of 23 geological samples (basalt, dolerite, andesite, rhyolite) without Sr purification covered ratios 0.705–0.748, matching MC-ICP-MS within Δ0.0016. Sr concentrations (60–350 ng/mL) did not affect accuracy.

Benefits and Practical Applications

• Eliminates time-consuming chromatographic separation.
• Increases sample throughput for large-scale surveys.
• Reduces labor and operational costs.
• Accessible to laboratories without specialized TIMS/MC-ICP-MS systems.

Future Trends and Potential Applications

Advances may include coupling with laser ablation for in situ assays, exploring alternative reaction gases for higher sensitivity, and extending mass-shift strategies to other isotope systems such as Hf and Pb. Portable triple-quadrupole platforms could further democratize isotope analysis.

Conclusion

The O₂ mass-shift ICP-QQQ protocol provides a robust, efficient alternative for (87)Sr/(86)Sr isotope ratio determinations in rocks, achieving precision suitable for geochronology and tracer studies without prior Sr/Rb separation.

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