Speciation Analysis of Chromium by LC-ICP-MS Based on ISO 24384

Importance of the Topic

This application note addresses the critical need for accurate speciation of chromium in water samples, distinguishing between the essential nutrient form Cr(III) and the highly toxic form Cr(VI). Environmental monitoring, regulatory compliance, and public health assessments depend on reliable differentiation and quantification of these oxidation states.

Objectives and Study Overview

The study demonstrates a streamlined LC-ICP-MS method in compliance with ISO 24384 for simultaneous determination of Cr(III) and Cr(VI) in various water matrices. Key goals include minimizing sample pretreatment complexity, reducing matrix interferences, and achieving high sensitivity and reproducibility.

Methodology and Instrumentation

Sample Types and Pretreatment:

• Water matrices: mineral water (soft and hard), tap water, river water, wastewater
• Filtration through 0.45 µm syringe filter, pH adjustment to 6.9 with nitric acid
• Chelation: addition of EDTA solution (0.025 mol/L at pH 6.9), volume to 20 mL
• Heating at 70 ± 3 °C for 60 min, cooling, and final filtration at 0.2 µm

Instrumentation Used:

• Liquid Chromatograph: Shimadzu Prominence Inert system with Shodex SI-35-4D and SI-95(G) anion exchange columns, mobile phase 100 mmol/L HNO3 (pH 7.0), flow rate 0.45 mL/min, column temperature 25 °C
• Mass Spectrometer: Shimadzu ICPMS-2050 with nebulizer DC04, mini torch, cyclone chamber, nickel sampling and skimmer cones, collision cell operated with helium (3.5 mL/min)
• Operating Conditions: RF power 1.2 kW, plasma gas 9.0 L/min, auxiliary gas 1.1 L/min, carrier gas 0.45 L/min, dilution gas 0.40 L/min, cell voltage –15 V, energy filter 7 V, monitoring at m/z 52 for chromium

Main Results and Discussion

Retention and Separation:

• EDTA chelation converts Cr(III) to anionic complex, facilitating retention on the anion exchange column
• Cr(III) and Cr(VI) fully separated with resolution of 4.4

Calibration and Linearity:

• Linear response over 0.05 to 20 µg/L with correlation coefficients R2 = 0.99999 for Cr(III) and R2 = 0.99993 for Cr(VI)

Detection Limits and Precision:

• Detection limits (3σ) in measurement solution: 0.015 µg/L for Cr(III), 0.006 µg/L for Cr(VI)
• Undiluted detection limits: 0.031 µg/L Cr(III), 0.011 µg/L Cr(VI)
• Repeatability tested by five consecutive measurements at 0.05 µg/L standard, demonstrating stable peak areas

Sample Analysis and Recovery:

• Spike recovery between 97 % and 109 % across all matrices, within the ISO 24384 acceptance range (80 – 120 %)
• Total chromium by LC-ICP-MS comparable to total Cr by direct ICP-MS, with interference below the 30 % threshold

Benefits and Practical Applications

• Simultaneous quantification of Cr(III) and Cr(VI) reduces analysis time and labor
• Simple pretreatment minimizes sample handling and potential contamination
• Robust method less affected by coexisting ions compared to conventional diphenylcarbazide or coprecipitation approaches
• Easy integration: LC column outlet directly connects to ICP-MS nebulizer

Future Trends and Potential Applications

• Extension to other metal speciation studies using similar chelation and chromatographic approaches
• Automation and on-line sample preparation to increase throughput
• Expansion to field-deployable systems for real-time environmental monitoring
• Adoption of advanced collision/reaction cell technologies to further reduce spectral interferences

Conclusion

This LC-ICP-MS method, aligned with ISO 24384, offers a reliable, sensitive, and user-friendly approach for chromium speciation in diverse water samples. It achieves clear separation, low detection limits, and high recovery rates with minimal pretreatment. The ease of instrument configuration and robust performance make it suitable for routine environmental and regulatory analyses.

References

• ISO 24384:2024 Water quality – Determination of chromium(VI) and chromium(III) in water – Method using liquid chromatography with inductively coupled plasma mass spectrometry (LC-ICP-MS) after chelating pretreatment, Edition 1, 2024