Chromium Speciation of Drinking Waters by IC-ICP-MS

Significance of the Topic

The differentiation between trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) in drinking water is critical due to their vastly different toxicological profiles. Cr(III) is an essential micronutrient, whereas Cr(VI) is a known carcinogen and reproductive toxicant. Reliable speciation methods help ensure compliance with regulatory limits, protect public health, and support environmental monitoring.

Objectives and Study Overview

This study aims to develop a rapid, robust, and high-throughput procedure for simultaneous speciation of Cr(III) and Cr(VI) in drinking water without lengthy sample pretreatment. By coupling ion chromatography (IC) to inductively coupled plasma mass spectrometry (ICP-MS), the method seeks to achieve high sensitivity, precision, and accuracy while eliminating complexing reagents and minimizing contamination risk.

Methodology

The approach employs a mixed-mode column capable of separating cationic Cr(III) aqua-hydroxo complexes and anionic chromate species in a single run. Key parameters:

• Mobile phase: 9 mM HNO₃ at pH 2, isocratic elution
• Flow rate: 0.3 mL/min
• Column temperature: 50 °C
• Injection volume: 20 µL
• Calibration range: 2–10 µg/L for both species

Used Instrumentation

Shimadzu Prominence IC system with inert flow path and PEEK plumbing, interfaced directly to a Shimadzu ICPMS-2030 equipped with a helium collision cell. Operating conditions included:

• RF power: 1.2 kW
• Plasma gas: 8.0 L/min, auxiliary gas: 1.1 L/min, carrier gas: 0.7 L/min
• Sampling depth: 5.0 mm
• Cell gas (He): 6.0 mL/min, cell voltage: –21 V
• Energy filter: 7 V, chamber temperature: 5 °C

Main Results and Discussion

Chromatographic separation achieved baseline resolution of Cr(III) and Cr(VI) in under 10 minutes. Calibration curves exhibited excellent linearity (R² > 0.9999). Detection limits were 0.20 µg/L for Cr(III) and 0.35 µg/L for Cr(VI), two orders of magnitude below the EPA maximum for total chromium. Method precision over 7.2 hours yielded RSD < 2.6% for peak area and < 1.4% for retention time. Analysis of tap and well water samples demonstrated accurate quantification with spike recoveries within ±5%. A multielement standard confirmed no significant matrix interferences.

Benefits and Practical Applications

• Rapid, direct analysis without complexation or lengthy sample preparation
• High sensitivity and low detection limits suitable for regulatory monitoring
• Elimination of salt-based mobile phases reduces background and interferences
• High sample throughput for environmental laboratories and QA/QC workflows

Future Trends and Applications

Opportunities for further development include:

• Lowering detection limits via larger injection volumes or high-capacity columns
• Automation and inline sample pretreatment for extended matrices
• Integration with speciation software for real-time data processing
• Extension to complex matrices such as wastewater and industrial effluents
• Exploration of alternative collision/reaction gases for enhanced interference removal

Conclusion

The combination of a mixed-mode IC column with a collision-cell ICP-MS offers a fast, accurate, and interference-free method for chromium speciation in drinking water. This workflow delivers industry-leading sensitivity, precision, and throughput without the need for complexing agents, making it a valuable tool for routine monitoring and regulatory compliance.

References

• Thomas R. Practical Guide to ICP-MS. Marcel Dekker, Inc.