Speciation of Cr(III) and Cr(VI) by IC-ICP-MS and Empower 3

Importance of the Topic

Speciation of chromium in drinking water has become a critical regulatory requirement due to the toxicity of hexavalent chromium at trace levels. In regions like California, maximum allowable Cr(VI) concentrations are set as low as 20 ng/L, demanding highly sensitive and accurate analytical techniques. Ion chromatography coupled with inductively coupled plasma mass spectrometry (IC-ICP-MS) offers the necessary sensitivity, low background, and metal-free flow paths to accurately differentiate and quantify Cr(III) and Cr(VI) in complex water matrices.

Objectives and Study Overview

• Develop and optimize an EDTA-based complexation protocol to ensure complete retention of Cr(III) and Cr(VI) on an anion-exchange column.
• Evaluate the influence of EDTA concentration on complex formation and recovery of both species.
• Demonstrate full control of the hyphenated Metrohm IC and PerkinElmer ICP-MS system under Empower 3 software.

Instrumental Setup

The speciation system comprised a Metrohm 940 Professional IC Vario One with 858 Sample Processor, coupled to a PerkinElmer NexION 350D ICP-MS. Key features included a metal-free flow path, a Metrosep Carb 2 column at 20 °C, and an ammonium nitrate eluent at pH 9. Two EDTA solutions (40 μmol/L and 500 μmol/L) were tested to form stable chromium-EDTA complexes. Samples and standards were heated at 60 °C for 1 h to accelerate complexation before injection. Detection employed DRC mode with ammonia for interference removal. Empower 3 software managed acquisition, calibration, and reporting.

Key Results and Discussion

The impact of EDTA concentration on complexation and recovery was assessed by spiking bottled water with 0.5 μg/L of each chromium species:

• At 40 μmol/L EDTA, Cr(III) recovery was only 30%, whereas Cr(VI) reached 111% recovery, indicating incomplete Cr(III) complexation and potential underestimation.
• At 500 μmol/L EDTA, both Cr(III) and Cr(VI) recoveries improved to 95% and 93%, respectively, confirming quantitative complexation and stable chromatographic separation.
• Calibration curves at both EDTA levels exhibited excellent linearity across the 0.01–10 μg/L range, though blank contamination of Cr(III) increased at higher EDTA concentrations.

Benefits and Practical Applications

• High sensitivity and low detection limits suitable for trace Cr(VI) monitoring in drinking water.
• Robust separation of Cr(III) and Cr(VI) using a single anion-exchange column.
• Single-software operation streamlines data acquisition, processing, and reporting under Empower 3.
• Method adaptability for regulatory compliance and routine QA/QC laboratories.

Future Trends and Opportunities

• Matrix-dependent optimization of EDTA concentrations to balance complexation efficiency and blank contributions.
• Automation and high-throughput screening to support large monitoring programs.
• Extension of the approach to other redox-sensitive metals and coupling with preconcentration modules for ultra-trace analysis.
• Integration of advanced data analytics and real-time quality control within chromatography data systems.

Conclusion

This work demonstrates that a Metrohm IC system coupled to a NexION ICP-MS and operated under Empower 3 can effectively speciate Cr(III) and Cr(VI) in water at low ng/L levels. Optimal EDTA concentration depends on sample matrix, with 40 μmol/L sufficient for pure standards and higher levels required for complex waters. The method offers high sensitivity, reliable recoveries, and streamlined data handling for environmental and regulatory applications.

Reference

Korstian S, Kolb T, Ernstberger H Speciation of Cr(III) and Cr(VI) by IC-ICP-MS and Empower 3 Metrohm and PerkinElmer Poster 2023