Sensitive Determination of Hexavalent Chromium in Drinking Water

Significance of the Topic

Hexavalent chromium (Cr(VI)) is a potent oxidant and carcinogen regulated in drinking water. Recent proposals in California set a public health goal of 0.02 µg/L, requiring analytical methods capable of sub-ppb sensitivity. This application update addresses the need for routine, high-sensitivity determination of Cr(VI) as chromate in drinking water.

Objectives and Study Overview

The work aimed to modify EPA Method 218.6 to achieve detection and quantitation limits below the proposed California PHG of 0.02 µg/L. Key goals included lowering the method detection limit (MDL), validating performance in complex matrices, and demonstrating compatibility across multiple Thermo Fisher Dionex ICS platforms.

Methodology and Instrumentation

The optimized method employs anion-exchange chromatography using a Dionex IonPac AG7 guard column (2×50 mm) and IonPac AS7 analytical column (2×250 mm). The eluent comprises 250 mM ammonium sulfate and 100 mM ammonium hydroxide at a flow rate of 0.36 mL/min. A 1,000 µL sample injection is followed by postcolumn derivatization with 2 mM diphenylcarbazide in 10 % methanol/1 N H₂SO₄ delivered via a 125 µL reaction coil at 0.12 mL/min. Detection is by visible absorbance at 530 nm using a Dionex VWD UV-Vis detector with a PEEK flow cell. Postcolumn reagent delivery can be configured with DP, PC10, or AXP pump modules. Systems evaluated include Thermo Scientific Dionex ICS-2100, ‑1600, ‑1100, ‑3000, and ‑5000.

Key Results and Discussion

The method achieved an MDL of 0.001 µg/L and a minimum quantitation limit of 0.003 µg/L for Cr(VI) as CrO₄²⁻. Recoveries in high-ionic-strength water (HIW) spiked at 0.005–1 µg/L ranged from 89 to 103 %. A slight retention time shift (≈0.05 min) was observed in HIW without affecting peak shape or quantitation. The lowest concentration minimum reporting limit (LCMRL) was calculated as 0.019 µg/L, confirming reliable measurement near regulatory thresholds. Comparable performance was demonstrated on both ICS-3000 and ICS-2100 platforms. Tests with 125 µL and 375 µL reaction coils showed negligible differences in MDL at sub-0.02 µg/L levels.

Benefits and Practical Applications of the Method

• Enables ultratrace quantitation of Cr(VI) to meet or exceed stringent regulatory goals.
• Demonstrates high precision and accuracy in complex drinking water matrices.
• Offers flexible postcolumn reagent delivery options for diverse lab setups.
• Supports automation and high sample throughput in environmental monitoring laboratories.

Future Trends and Opportunities

Potential developments include coupling IC with mass spectrometry for confirmatory analysis, implementing microfluidic reaction coils to reduce reagent consumption, deploying portable IC systems for in-field monitoring, and integrating real-time data streams for continuous water quality surveillance.

Conclusion

The modified ion chromatographic method reliably quantifies hexavalent chromium at sub-ppb levels, supporting compliance with proposed California drinking water goals. Its robust performance across multiple Dionex platforms and adaptable reagent delivery configurations make it suitable for routine environmental analysis.

References

1. EPA Method 218.6, U.S. Environmental Protection Agency, 1991.
2. Dionex Technical Note 26 (LPN 034398-02), Thermo Scientific, 2007.
3. OEHHA Public Health Goal for Cr(VI) in Drinking Water, California EPA, 2010.
4. EPA Method 300.1 for Inorganic Anions, U.S. EPA, 1997.
5. Toxicological Review of Hexavalent Chromium, U.S. EPA, 2010.