Analysis Method of Dissolved Hexavalent Chromium According to EPA 218.6

Importance of the topic

Hexavalent chromium (Cr(VI)) is a known environmental and health hazard subject to stringent regulatory limits in drinking water, groundwater and industrial effluents. Reliable, sensitive and selective analytical methods are essential for monitoring trace levels of Cr(VI) to ensure compliance with environmental standards and to protect public health.

Objectives and study overview

This study applies EPA Method 218.6 using the Shimadzu Prominence Inert System to separate, derivatize and detect dissolved Cr(VI) in aqueous samples. The goals are to demonstrate method linearity, precision, detection limit and recovery in both standard solutions and simulated wastewater matrices.

Methodology and Instrumentation

The method employs anion‐exchange chromatography on a Shim-pack IC-SA2 column with a guard column and optimized mobile phase (25 mmol/L ammonium sulfate, 10 mmol/L ammonium hydroxide) at 1.5 mL/min. Post‐column derivatization with diphenylcarbazide in methanol/sulfuric acid generates a colored complex, detected at 530 nm in an inert UV‐VIS cell. Sample pH is adjusted to 9.0–9.5 prior to dilution to prevent reduction of Cr(VI) to Cr(III). Calibration standards (5–50 µg/L) are prepared by dilution of sodium chromate tetrahydrate in ASTM Type I water.

Used Instrumentation

• Shim-pack IC-SA2 analytical column (250×4.0 mm I.D., 9 µm) with Shim-pack IC-SA2(G) guard column
• Degassing unit
• Dual LC-20Ai pumps
• SIL-20AC autosampler with inert kit
• Column oven set to 40 °C
• Post-column reaction coil (PEEK, 10 m×0.5 mm I.D.)
• Inert UV-VIS detector at 530 nm
• Shimadzu 4 mL polypropylene vials

Main results and discussion

Calibration exhibited excellent linearity with r²>0.999 over 5–50 µg/L. Repeatability tests (n=7) yielded retention time RSD ≤0.04% and area RSD ≤2.16%. The method detection limit (MDL) calculated per EPA 218.6 was 0.1 µg/L. In simulated seawater‐based wastewater, percent recovery of a 1 µg/L spike was 106.7%, demonstrating robustness against matrix effects.

Benefits and practical applications

• Selective separation and detection of Cr(VI) minimizes interference.
• Sensitivity to sub-µg/L levels supports trace monitoring.
• Wide applicability across drinking water, groundwater and industrial effluents.
• Compliance with EPA regulatory requirements.

Future trends and potential applications

• Integration with on-site portable ion chromatography systems for real-time field monitoring.
• Automation and high-throughput analysis using multiplexed sample introduction.
• Coupling with mass spectrometry for speciation studies and confirmation of oxidation states.
• Adaptation of post-column derivatization schemes for other metal analytes.

Conclusion

The Shimadzu Prominence Inert System combined with EPA Method 218.6 provides a robust, sensitive and precise approach for dissolved Cr(VI) analysis in diverse water matrices. Its low detection limit, high linearity and excellent recovery demonstrate its suitability for routine environmental monitoring and regulatory compliance.

References

• EPA Method 218.6: Determination of Dissolved Hexavalent Chromium in Drinking Water, Groundwater, and Industrial Wastewater Effluents by Ion Chromatography, Rev. 3.3