Determination of hexavalent chromium in drinking water according to EPA Method 218.7

Importance of the Topic

Chromium(VI) is a highly soluble, toxic, and carcinogenic species regulated at ultra-trace levels in drinking water.
California’s Public Health Goal (PHG) of 0.02 µg/L demands analytical methods with exceptional sensitivity and precision.
Reliable quantification of hexavalent chromium is crucial for safeguarding public health and complying with stringent regulations.

Objectives and Study Overview

The study aimed to refine EPA Method 218.7 to achieve sub-PHG detection limits for Cr(VI).
Key goals included optimizing chromatographic separation and post-column derivatization parameters.
The modified protocol was evaluated against California’s PHG and compared to established EPA performance metrics.

Methodology and Instrumentation

Various experimental parameters were adjusted to enhance sensitivity and throughput.

• Anion-exchange chromatography on Metrosep A Supp 5 column (250×4.0 mm) at 45 °C.
• Eluent: 12.8 mmol/L Na2CO3, 4.0 mmol/L NaHCO3 and 2.5 g/L (NH4)2SO4.
• Flow rates: 0.8 mL/min for separation, 0.2 mL/min for post-column reagent.
• Post-column reactor: flow-through design, 0.39 cm3 volume, 0.5 mm i.d., at 45 °C.
• Reagent: 2 mmol/L 1,5-diphenylcarbazide acidified with H2SO4, detection at 530 nm.

Key Results and Discussion

Method detection limits (MDLs) of 0.008 µg/L and 0.01 µg/L were obtained for spiked levels of 0.025 and 0.05 µg/L, respectively.
These MDLs are below California’s PHG and demonstrate enhanced sensitivity over standard EPA requirements.
Precision assessed via seven replicate injections of a 0.027 µg/L standard yielded acceptable repeatability within regulatory criteria.
The revised protocol completes analysis in under six minutes per sample, improving laboratory throughput.
The Lowest Concentration Minimum Reporting Level (LCMRL) was established at 0.032 µg/L using EPA’s LCMRL calculator and recovery criteria of 50–150%.

Benefits and Practical Applications

• Sub-PHG detection supports regulatory compliance in regions with stringent limits.
• Rapid analysis enhances sample throughput for routine monitoring.
• High precision and accuracy facilitate reliable decision-making in water quality laboratories.
• Method flexibility allows variation in columns, eluents, and injection volumes while maintaining performance.

Future Trends and Potential Applications

Ongoing software tools for LCMRL determination will streamline method validation across laboratories.
Exploration of alternative derivatization reagents and inline mixing designs could further boost sensitivity.
Integration with mass spectrometric detectors and miniaturized chromatographic systems may enable field-deployable monitoring.

Conclusion

The optimized EPA Method 218.7 protocol achieves MDLs well below 0.02 µg/L for Cr(VI) in drinking water, ensuring compliance with the strictest health goals.
Enhanced precision, reduced analysis time, and methodological flexibility make it a robust tool for routine and specialized analyses.

References

• United States Environmental Protection Agency. Technical Basis for the Lowest Concentration Minimum Reporting Level (LCMRL) Calculator, EPA 815-R-11-001, 2010.
• United States Environmental Protection Agency. Determination of Hexavalent Chromium in Drinking Water by Ion Chromatography with Post-Column Derivatization and UV-Visible Spectroscopic Detection, EPA Method 218.7, Version 1.0, 2011.