Unregulated Contaminant Monitoring Rule 3 for USEPA Method 524.3 and 524.4

Significance of the Topic

Monitoring unregulated volatile organic contaminants in public water supplies is essential to ensure safe drinking water and to comply with the Safe Drinking Water Act requirements. The UCMR 3 initiative identifies low-level contaminants that may pose emerging health risks and mandates periodic assessment of water quality across U.S. water systems.

Objectives and Study Overview

This study evaluates USEPA Method 524.3 and the draft Method 524.4 for measuring seven VOCs listed under UCMR 3 assessment monitoring. The primary aim is to compare helium and nitrogen as purge gases in a purge-and-trap GC/MS workflow, assessing their impact on calibration linearity, precision, accuracy, and compound response.

Methodology and Instrumentation Used

The analytical setup comprised an EST Analytical Encon Evolution purge and trap concentrator with a Vocarb 3000(K) trap, an EST Centurion WS autosampler with chilled vial capability (<10 °C), and an Agilent 7890/5975 GC/MS system. Key parameters included:

• Purge times of 6.5 and 11 min at flows of 40 and 60 mL/min
• Desorb temperature of 260 °C and pressure control at 5 psi
• GC column Rxi-624 MS with a temperature program from 45 °C to 230 °C
• SIM acquisition targeting specific ions for each compound
• Calibration range from 25 ppt to 5 ppb with an internal standard at 1 ppb

Main Results and Discussion

Both purge gases achieved excellent quadratic regression fit (R² ≥0.999) across all analytes and purge volumes. Precision and recovery at 25 ppt averaged better than 8% RSD and 100% recovery, while 1 ppb tests yielded RSD <5% and recovery within 95–107%. Nitrogen produced slightly lower response factors compared to helium but still met EPA Method 524.3 performance criteria.

Benefits and Practical Applications

Use of SIM detection in conjunction with the optimized purge-and-trap parameters ensures sensitivity to meet UCMR 3 low-level requirements. The flexibility to employ either helium or nitrogen expands method applicability, cost control, and availability of purge gas options in routine drinking water laboratories.

Future Trends and Potential Applications

Advances may include enhanced trap materials for broader analyte range, integration of automated sample handling for increased throughput, adaptation to alternative inert gases or gas recycling systems, and application of these methods to emerging contaminants and non-target screening in water quality monitoring.

Conclusion

The Encon Evolution concentrator, Centurion WS autosampler, and Agilent GC/MS system deliver robust performance for UCMR 3 VOC analysis using both helium and nitrogen purge gases. Helium offers marginally higher response, but nitrogen is a viable, cost-effective alternative, maintaining method accuracy, precision, and regulatory compliance.

References

• Method 524.3, Measurement of Purgeable Organic Compounds in Water by Capillary Column Gas Chromatography/Mass Spectrometry, Version 1.0, June 2009
• Method 524.4 (Draft), Measurement of Purgeable Organic Compounds in Water by Capillary Column Gas Chromatography/Mass Spectrometry (Using Nitrogen Purge Gas), Version 1, September 2011