Resolving REE2+ Overlaps on Arsenic and Selenium With Hydrogen Cell Gas

Significance of the Topic

Inductively coupled plasma mass spectrometry (ICP-MS) is widely adopted for rapid, multi-element trace analysis across environmental, biological, geological, and industrial matrices. Critical elements such as arsenic (As) and selenium (Se) demand low detection limits and robust interference control. As Rare Earth Elements (REEs) become increasingly prevalent in technology and waste streams, their doubly charged ions (REE2+) can overlap As and Se signals, challenging conventional collision cell approaches.

Study Objectives and Overview

This overview evaluates a method using Agilent 7900 single quadrupole ICP-MS with hydrogen (H2) cell gas and half-mass correction to resolve REE2+ overlaps on m/z 75 and 78. It compares no-gas, helium (He) KED, and H2 KED modes to ensure accurate As and Se determination in samples containing up to ppm-level REEs.

Methodology and Instrumentation

Experimental work employed an Agilent 7900 ICP-MS equipped with ORS4 collision cell, ShieldTorch interface, and optional H2 gas line. Operating modes included:

• No cell gas (baseline overlap and background assessment)
• He KED with half-mass correction (optimized ORS4, He at ~4.3 mL/min, KED bias 5 V)
• H2 KED with half-mass correction (H2 at ~3.0 mL/min, lower octopole and quadrupole biases)

Mechanistic studies highlighted that H2 induces dipole interactions with doubly charged ions, increasing their collision cross-sections and preferential energy loss, enabling effective KED discrimination.

Key Findings and Discussion

Background equivalent concentrations (BECs) for As and Se in a 1 ppm REE mix were compared: He KED reduced BECs but left residual REE2+ contributions, whereas H2 KED achieved As BEC of 0.07 ppb and Se BEC of 0.02 ppb. Spike recovery tests (1 ppb As and Se in 1 ppm REE matrix) yielded 100 ± 2 % accuracy, confirming interference removal. Reaction of H2 with Ar2 dimers further improved Se analysis by proton transfer.

Benefits and Practical Applications

H2 KED on single quadrupole ICP-MS extends low-level As and Se analysis to challenging REE-rich samples without requiring triple-quadrupole instrumentation. The method integrates with existing He-mode workflows, offering:

• Sub-ppb detection limits in complex matrices
• Automated half-mass correction for REE2+ overlap mitigation
• Compatibility with routine QA/QC and environmental monitoring

Future Trends and Applications

Ongoing developments may include exploration of mixed cell gases, adaptive bias control, and expansion to other M2+-prone analytes. Integration of method selection algorithms in instrument software and broader adoption in industrial and regulatory laboratories are anticipated. Advanced collision-reaction cell designs may further enhance selectivity and throughput.

Conclusion

The Agilent 7900 ICP-MS with H2 cell gas and half-mass correction offers a practical, high-performance solution for accurate As and Se determination in ppm-level REE matrices. The approach reduces REE2+ interferences by over two orders of magnitude, achieving sub-0.1 ppb detection and reliable spike recoveries, thereby expanding single-quad ICP-MS capabilities for demanding analytical applications.

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