The ultratrace determination of iodine 129 in aqueous samples using the 7700x ICP-MS with oxygen reaction mode

Significance of the topic

Iodine-129 is a persistent radionuclide generated by nuclear activities and weapon tests, with a half-life of 15.7 million years. Its ultratrace measurement in environmental and aqueous samples is critical for monitoring nuclear releases, evaluating radioactive contamination, and ensuring long-term safety in nuclear waste management.

Objectives and Study Overview

This study demonstrates a rapid, direct method for quantifying iodine-129 at picogram per liter levels using a quadrupole ICP-MS (Agilent 7700x) equipped with a third-generation Octopole Reaction System (ORS3) in oxygen reaction mode. Key goals included eliminating isobaric and polyatomic interferences, achieving sub-ppt detection limits without elaborate sample preparation, and validating accuracy using NIST SRM iodine standards.

Methodology and Instrumentation

Samples were introduced via a glass concentric nebulizer and double-pass quartz spray chamber cooled to 2 °C. A basic diluent containing 0.5% tetramethylammonium hydroxide (TMAH) stabilized iodine speciation. The ORS3 cell was operated in reaction mode with oxygen (1.01 mL/min) and helium (4 mL/min) gases, optimized by monitoring background equivalent concentration (BEC) at m/z 129 while measuring a 127I standard. Plasma conditions were autotuned to "Robust Plasma" (CeO+/Ce+ ≤1%) for high matrix tolerance. The optional low-flow third-cell gas controller allowed independent gas adjustment for optimal charge-transfer reactions that convert Xe+ to neutral Xe.

Used Instrumentation

• Agilent 7700x ICP-MS with Octopole Reaction System (ORS3)
• Glass concentric nebulizer and double-pass quartz spray chamber (2 °C)
• Low-flow third cell gas mass flow controller for O2 and He
• Agilent MassHunter software with autotune for plasma and ion lens optimization

Main Results and Discussion

• Oxygen reaction gas at 90% flow removed xenon background and 127IH2+ overlap, yielding a 129I BEC of 0.6 ppt.
• Calibration for 127I and 129I was linear (R2 > 0.9998) over several orders of magnitude; detection limits were 0.14 µg/L for 127I and 1.1 ng/L for 129I (3σ, n=10).
• NIST SRM 3231 Level I and II standards and an intermediate 10-7 ratio standard matched certified 129I/127I values with 7–22% RSD at low count rates, confirming accuracy down to 10-8 ratios.
• Overlaid spectra of high-concentration 127I and low-level 129I showed no high-mass tail overlap, validating abundance sensitivity under real-world conditions.

Benefits and Practical Applications

• No radiochemical separation or lengthy sample preparation is required, enabling high sample throughput.
• Sub-ppt sensitivity allows routine environmental screening, nuclear facility monitoring, and QA/QC of drinking water and effluents.
• The approach is cost-effective and faster compared with neutron activation analysis and accelerator mass spectrometry.

Future Trends and Potential Applications

• Expansion to measure short-lived isotopes (e.g., 131I) using liberated m/z 131 in oxygen mode.
• Integration with on-line separation or speciation systems for chemical form analysis.
• Development of portable or field-deployable reaction-cell ICP-MS for in situ radiological monitoring.

Conclusion

The Agilent 7700x ICP-MS with ORS3 in oxygen reaction mode provides a robust, interference-free platform for ultratrace 129I analysis in aqueous matrices. Achieving detection limits of 1.1 ng/L and accurate isotope ratio measurement without sample pre-treatment supports rapid, routine screening in environmental and nuclear applications.

References

• Enhanced Helium Mode Cell Performance for Improved Interference Removal in ICP-MS. Agilent publication 5990-7573EN, February 2011.
• Bienvenue J., et al. Canadian Journal of Analytical Sciences and Spectroscopy, 49(6):423 (2004).