Lead isotope analysis: Removal of 204 Hg isobaric interference from 204 Pb using ICP-QQQ in MS/MS mode

Importance of the Topic

Lead isotopic analysis is a cornerstone in geosciences, environmental monitoring, archaeology and industrial QA/QC. Accurate measurement of 204Pb is critical because it serves as the non-radiogenic reference isotope for calculating primordial and radiogenic isotope ratios used in age dating, provenance studies and isotope dilution techniques. However, 204Pb quantification by ICP-MS is compromised by isobaric overlap from 204Hg, which can bias results unless effectively removed.

Objectives and Study Overview

This application note evaluates the capacity of the Agilent 8800 Triple Quadrupole ICP-MS (ICP-QQQ) operated in MS/MS mode with ammonia reaction gas to resolve the 204Hg isobaric interference on 204Pb. The study compares three measurement modes—no gas, single-quadrupole bandpass with NH3, and MS/MS with NH3—using spiked standards and mixed rare earth element (REE) matrices to assess interference removal, accuracy and applicability for complex geological and environmental samples.

Methodology and Instrumentation Used

Instrument: Agilent 8800 ICP-QQQ with MS/MS configuration and ammonia cell gas.
Sample introduction: Micromist nebulizer, quartz spray chamber, 2.5 mm injector, free aspiration to reduce noise.
Reagents: NIST 981 and 982 Pb isotope standards spiked with 10 ppb Hg and 50 ppb REEs; Tl and U spikes to test secondary interferences.
Operating modes:

• No gas (standard quadrupole)
• Single-quad bandpass (NH3 reaction gas)
• MS/MS mode (Q1 set to target mass, NH3 reaction gas, Q2 scanning or peak-jump)

Key experiments included Product Ion Scans to confirm that Pb does not react with NH3, and isotope ratio measurements for 204/206Pb under each mode.

Main Results and Discussion

• No-gas mode showed severe 204Hg overlap on 204Pb, with up to hundreds-fold bias in isotope ratios when Hg and REE spikes were present.
• Bandpass NH3 mode removed Hg but introduced extensive REE–NH3 cluster interferences that obscured Pb isotopes.
• MS/MS with NH3 reaction gas achieved complete removal of 204Hg and all REE cluster interferences by rejecting non-target ions in Q1, yielding accurate 204/206 ratios within 1–3% of certified values even in mixed matrices.
• Concurrent on-mass and mass-shift detection of U demonstrated the ability to perform Pb/U ratio measurements for geochronology under identical conditions.
• Application to laser ablation (dry plasma) confirmed the method’s suitability for in situ solid sampling without chemical preparation.

Benefits and Practical Applications

This MS/MS chemical resolution strategy eliminates time-consuming sample separation and the need for high-resolution sector-field ICP-MS. It delivers robust, interference-free Pb isotope analyses in complex matrices, supports direct solid sampling (e.g., LA-ICP-MS) for archaeological and geological dating, and enhances throughput and reliability in routine laboratory workflows.

Future Trends and Potential Applications

Expanding this approach to other challenging isobaric overlaps and reaction gases could further extend ICP-QQQ capabilities. Integration with time-resolved analysis and automated MS/MS methods will facilitate high-throughput in situ isotope mapping. Emerging fields such as environmental forensics, food authenticity and biogeochemical tracer studies stand to benefit from precise, matrix-tolerant isotope ratio measurements.

Conclusion

The Agilent 8800 ICP-QQQ in MS/MS mode with ammonia reaction gas provides a practical and reliable solution for removing the 204Hg interference on 204Pb. By rejecting non-target ions in Q1 and harnessing efficient charge-transfer chemistry, this method achieves accurate lead isotopic analyses in complex matrices without extensive sample preparation or mathematical corrections.

References

• Agilent Technologies. Lead isotope analysis: Removal of 204Hg isobaric interference from 204Pb using ICP-QQQ in MS/MS mode. Application note 5991-5270EN, 2014.