Analysis of Hexavalent Chromium in Drinking Water According to EPA Method 218.7

Significance of the Topic

The accurate measurement of hexavalent chromium in drinking water is vital for public health and regulatory compliance. Hexavalent chromium is a known carcinogen and is subject to increasingly strict guidelines, such as California’s public health goal of 0.02 µg/L. Reliable, rapid and sensitive analytical methods are required to ensure safe drinking water and to meet both EPA and state-level requirements.

Objectives and Study Overview

This study demonstrates the application of EPA Method 218.7 using the Shimadzu Nexera™ lite inert system for quantifying hexavalent chromium in water samples. The main goals were to:

• Develop a method capable of separating hexavalent chromium from potential interferents.
• Achieve low detection limits suitable for regulatory thresholds.
• Validate precision, accuracy and robustness across real water matrices.

Methodology and Instrumentation

The analysis employed ion chromatography with post-column derivatization:

• Instrument: Shimadzu Nexera lite inert HPLC system equipped with DGU-403 degassing unit, SIL-20AC inert autosampler and LC-40i pumps.
• Column: Shodex SI-90 (250 mm×4.0 mm, 9 µm) with SI-90(G) guard (10 mm×4.6 mm, 9 µm).
• Mobile phase: 50 mmol/L ammonium sulfate and 20 mmol/L ammonium hydroxide at 0.8 mL/min.
• Post-column: 2 mmol/L 1,5-diphenylcarbazide mixed with 10% methanol and 1 N sulfuric acid at 0.3 mL/min, reacting in a 250 µL coil at 45 °C.
• Detection: UV-Vis at 530 nm using an inert cell.

Main Results and Discussion

Calibration was linear from 0.02 to 1 µg/L with r²=0.99997. The method detection limit (MDL) was calculated at 0.003 µg/L, yielding a limit of quantitation (LOQ) of 0.009 µg/L. Precision and accuracy tests at low, mid and high calibration points (0.02, 0.1, 1 µg/L) produced relative standard deviations below 6.4% and recoveries of 101–103%, comfortably within EPA criteria. Matrix spike experiments in mineral and tap water confirmed recoveries between 97% and 102% and RSDs below 5%. Chromatograms showed clear resolution of Cr(VI) from baseline and no sample carry-over.

Practical Benefits and Applications

This method enables rapid (<6 min) analysis of hexavalent chromium at sub-µg/L levels, facilitating routine monitoring of drinking water. Its robustness against matrix effects and its high sensitivity make it suitable for environmental laboratories, water utilities and regulatory agencies requiring reliable compliance testing.

Future Trends and Opportunities

Advances may include integrating automated sample preparation, coupling with mass spectrometric detection for speciation, and further miniaturization for field-deployable systems. Ongoing improvements in column materials and derivatization reagents could further reduce analysis time and detection limits.

Conclusion

The Shimadzu Nexera lite inert system, operated under EPA Method 218.7, offers a fast, sensitive and reliable approach for the quantification of hexavalent chromium in drinking water. Its demonstrated performance meets stringent regulatory requirements and supports effective water quality monitoring.

Reference

1. EPA Method 218.7: Determination of Hexavalent Chromium in Drinking Water by Ion Chromatography with Post-Column Derivatization and UV-Visible Spectroscopic Detection, Version 1.0.
2. Hexavalent Chromium. California State Water Resources Control Board. Accessed Feb. 2024.