Effective removal of isobaric interferences on strontium and lead using triple quadrupole ICP-MS

Importance of Topic

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is widely used for trace element quantification and isotope ratio measurements. Unresolved isobaric overlaps—where two different elements share the same mass number—can bias results and increase uncertainty in applications ranging from environmental monitoring to geochronology.

Objectives and Study Overview

This study evaluates the elimination of two common isobaric interferences—^87Rb on ^87Sr and ^204Hg on ^204Pb—using single-quadrupole and triple-quadrupole ICP-MS modes with reactive gases. The goal is to compare conventional kinetic energy discrimination (SQ-KED) against oxygen (O_2) and ammonia (NH_3) reaction chemistries in both single quadrupole (SQ) and triple quadrupole (TQ) configurations.

Methodology and Instrumentation

The Thermo Scientific iCAP TQ ICP-MS was operated in five modes: SQ-KED (He collision gas), SQ-O2, SQ-NH3, TQ-O2 and TQ-NH3. Key parameters included:

• MicroMist quartz nebulizer at 0.4 mL·min⁻¹, quartz cyclonic spray chamber at 2.7 °C.
• RF power 1550 W, interface insert with Ni cones, injector 2.5 mm quartz.
• CRC gas flows: 4.5 mL·min⁻¹ He for SQ-KED; 0.35 mL·min⁻¹ O2 or 0.33 mL·min⁻¹ NH3 for reactive modes.
• Daily autotune, gravimetric standards in 2% HNO3; precision from ten runs (30 sweeps each).

Main Results and Discussion

• O2 reaction in TQ-O2 shifts Sr to ^87Sr^16O+ and ^88Sr^16O+, removing Rb overlap. Sensitivity was ca. 30% below SQ-KED, but detection limits remained at 0.001 µg·L⁻¹ for ^88Sr.
• TQ-O2 measured ^88Sr/^87Sr ratios within 12.05–12.13 across Rb concentrations up to 10 mg·L⁻¹, matching theoretical value of 11.7971. SQ-KED and SQ-O2 showed strong bias with increasing Rb.
• NH3 reaction in SQ-NH3 removed ^204Hg interference on ^204Pb only in simple solutions but produced Yb(NH3)_n cluster interferences in mixed matrices.
• TQ-NH3 filtered out lower-mass ions (e.g., ^170Yb) before the reaction cell, delivering consistent ^204Pb/^208Pb ratios (~0.0259) even with up to 20 µg·L⁻¹ Hg and 1 mg·L⁻¹ Yb.

Benefits and Practical Applications

• Triple quadrupole ICP-MS with reactive gases enables accurate isotope ratio analysis in complex matrices without reliance on mathematical correction.
• Enhanced confidence for geological dating, environmental tracing and quality control laboratories.
• Minimizes method development time via software-assisted reaction gas selection and mass filtering.

Future Trends and Possibilities

• Extension of TQ-ICP-MS approach to other isobaric pairs and multi-element speciation workflows.
• Automated optimization of reaction gas parameters and dynamic mass filtering for complex sample streams.
• Integration with hyphenated techniques for simultaneous isotopic and molecular analyses.

Conclusion

Triple quadrupole ICP-MS using O2 and NH3 reaction gases with a dedicated Q1 mass filter provides robust, interference-free isotope ratio measurements of ^87Sr/^87Rb and ^204Pb/^204Hg. This strategy overcomes the limitations of single quadrupole modes and reduces uncertainty in trace isotope analysis.

References

No literature list was provided in the source document.