Direct Analysis of Ultratrace Rare Earth Elements in Environmental Waters by ICP-QQQ

Significance of the Topic

Rare earth elements (REEs) are critical components in modern technologies, including glass, magnets, lasers, clean energy systems, defense, and batteries. Their widespread use raises environmental concerns as mining, processing, and disposal can release ultratrace amounts of REEs into water bodies. Accurate, rapid, and sensitive methods are essential to monitor these emerging pollutants at background concentrations and support environmental research, regulatory decisions, and routine water quality surveillance.

Objectives and Study Overview

This study demonstrates a direct analytical approach for ultratrace REE determination in environmental waters using the Agilent 8900 ICP-QQQ operating in MS/MS mass-shift mode with nitrous oxide (N₂O) as the reaction cell gas. The goals were to (1) optimize cell gas selection for interference removal, (2) establish method detection limits (MDLs), (3) validate accuracy with a certified river water reference material (SLRS-6), and (4) analyze real river water samples from four locations along the Tama River in Japan.

Methodology and Used Instrumentation

Instrumentation and conditions were as follows:

• Agilent 8900 ICP-QQQ with Q1/Q2 quadrupoles around the CRC
• Quartz double-pass spray chamber, quartz torch (2.5 mm i.d. injector), Ni cones
• PFA microflow nebulizer with peristaltic pump
• Low-matrix plasma preset (total dissolved solids < 0.1 %)
• Reaction cell gas: N₂O (20 % of full-scale flow), chosen over O₂ for improved sensitivity via the REE⁺ + N₂O → REEO⁺ reaction
• MS/MS mass-shift mode with a +16 u shift to oxide ions
• Method detection limits calculated as 3× standard deviation of nine replicates of 0.3 ppt standard (1 % HNO₃)

River water samples were collected at four sites (A–D), including two wastewater treatment plant (WWTP) discharge points. Each sample was filtered (0.45 μm), acidified to 1 % HNO₃, and analyzed in triplicate.

Main Results and Discussion

The MS/MS mass-shift approach with N₂O achieved sub-ppt MDLs for all 16 REEs, demonstrating high sensitivity and interference removal. Validation with SLRS-6 CRM showed close agreement for all elements except Sc, where the certified value (333 ppt) likely reflected SiO⁺/SiOH⁺ interference. Direct MS/MS filtering by Q1 eliminated Si-based interferences, yielding accurate Sc values.
In the Tama River samples, concentrations of most REEs ranged below 50 ppt. A pronounced gadolinium (Gd) spike was observed downstream of WWTP outlets, indicating persistence of Gd-based MRI contrast agents. PAAS-normalized profiles revealed a smooth REE pattern except for elevated Gd and slight enrichment of heavy REEs in effluents.

Benefits and Practical Applications

The direct ICP-QQQ MS/MS method offers:

• Minimal sample preparation without resin preconcentration
• Sub-ppt detection limits suitable for background environmental levels
• Robust, predictable interference removal via controlled reaction chemistry
• Rapid, routine analysis for research, regulatory monitoring, and QA/QC in industrial or environmental laboratories

Future Trends and Applications

Advancements may include:

• Expansion to diverse matrices (e.g., industrial effluents, biological fluids)
• Exploration of alternative reaction gases and kinetic energy discrimination modes
• Coupling with speciation techniques to assess REE bioavailability and toxicity
• Automation and high-throughput workflows for large-scale monitoring networks
• Integration into regulatory frameworks for water quality standards

Conclusion

The Agilent 8900 ICP-QQQ in MS/MS mass-shift mode with N₂O enables direct, sensitive, and interference-free quantification of ultratrace REEs in environmental waters. Sub-ppt MDLs and accurate matrix validation illustrate its suitability for monitoring emerging pollutants, as exemplified by the detection of Gd contamination in river samples.

Reference

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