Accurate determination of arsenic and selenium in environmental samples using triple quadrupole ICP-MS

Significance of the Topic

Accurate measurement of arsenic and selenium in soils, sediments and rocks is crucial for environmental monitoring, human and animal health assessment, and geochemical studies. Arsenic, in its inorganic forms, is a known carcinogen even at trace levels, while selenium is an essential micronutrient with antioxidant properties. Both elements can be difficult to quantify reliably in samples containing high concentrations of rare earth elements (REEs) that generate doubly charged interferences in conventional ICP-MS analysis.

Objectives and Study Overview

This application note evaluates the performance of a triple quadrupole ICP-MS (iCAP TQe) equipped with a high matrix interface for the determination of arsenic and selenium in challenging matrices. The study compares single quadrupole modes (no cell gas, hydrogen reaction, helium KED) with a triple quadrupole mass-shift approach using oxygen reaction gas (TQ-O₂). Two sample types—a marine sediment and a USGS andesite reference (AGV-1)—were used to assess accuracy, detection limits and interference removal in the presence of elevated REE levels.

Methodology and Instrumentation

Samples were digested in closed PTFE vessels using HNO₃, HF and HClO₄, then evaporated, re-dissolved in 1 N HNO₃ and diluted 1:10 with 1% HNO₃/2% methanol. Calibration standards (0.2–5 µg/L) contained 2% HNO₃, 2% MeOH and 1 µg/L lutetium as an internal standard. A REE interference challenge solution (5 mg/L total) was prepared to evaluate background equivalent concentrations (BECs).

Instrumentation

• iCAP TQe ICP-MS with high matrix interface (3.5 mm insert) and Ni cones
• MicroMist quartz nebulizer (0.2 mL/min), quartz cyclonic spray chamber at 2.7 °C
• 2.5 mm quartz injector, RF power 1 550 W, nebulizer gas 1.04 L/min
• Measurement modes: SQ-STD, SQ-H₂, SQ-KED (He) and TQ-O₂ mass shift
• Autotune via Qtegra ISDS Software for consistent plasma/interface settings

Main Results and Discussion

External calibration in TQ-O₂ mode showed excellent linearity and low LODs (0.17 ng/L for 75As → 91[AsO]⁺, 2.02 ng/L for 78Se → 94[SeO]⁺). In a 5 mg/L REE matrix, TQ-O₂ achieved BECs of 30 ng/L for arsenic and 23–60 ng/L for selenium isotopes, outperforming all single quadrupole modes, which exhibited BECs in the µg/L range. Spike recoveries in the REE solution and in digested samples were 99–102% for both elements. Analysis of the AGV-1 standard returned 0.892 µg/g As (101% of certified) and consistent selenium quantification in the marine sediment (0.109 µg/g 78Se).

Benefits and Practical Applications

The triple quadrupole TQ-O₂ mass shift approach delivers:

• Superior removal of doubly charged REE interferences
• Low detection limits in complex matrices
• High accuracy and precision for environmental monitoring
• Reduced need for daily interference correction equations

These advantages support reliable risk assessment of arsenic in food and water, mapping of selenium deficiencies in soils and studies of elemental cycling in geological samples.

Future Trends and Potential Applications

Emerging trends include further automation of method selection tools, extension of triple quadrupole chemistry to additional elements, integration with chromatographic separation for speciation analysis, and development of compact field-deployable TQ-ICP-MS systems. Enhanced software intelligence will streamline tuning and monitoring, enabling routine high-throughput analysis in environmental, bio-monitoring and industrial QA/QC laboratories.

Conclusion

The Thermo Scientific iCAP TQe ICP-MS operated in TQ-O₂ mass shift mode provides robust removal of REE-related interferences, achieving trace-level detection and accurate quantification of arsenic and selenium in sediments and rock reference materials. Its performance in challenging matrices makes it an ideal tool for environmental and geochemical analyses.

References

• USGS AGV-1 andesite reference composition: http://crustal.usgs.gov/geochemical_reference_standards/andesite1.html