Drinking Water Analysis Conditions for USEPA Method 524.3 and the Newly Proposed Method 524.4

Significance of the Topic

The accurate measurement of purgeable organic compounds in drinking water is essential for regulatory compliance, ensuring public health protection and quality control in water treatment processes. Advances in analytical instrumentation and method flexibility enhance laboratory throughput and reliability while addressing supply challenges related to purge gas availability.

Study Objectives and Overview

This application note examines the performance of USEPA Method 524.3 (helium purge gas requirement) and the draft Method 524.4 (allowing nitrogen purge gas) using two purge volumes (440 mL and 390 mL). The study aims to compare linearity, precision, accuracy and compound response factors under both purge gas conditions to determine if nitrogen can serve as a viable alternative to helium.

Methodology and Instrumentation Used

The experimental setup combined an EST Analytical Encon Evolution purge-and-trap concentrator and Centurion WS autosampler with an Agilent 7890/5975 GC/MS system. Key purge-and-trap parameters included:

• Trap: Vocarb 3000 K; trap temperature 35 °C; valve oven and transfer line at 150 °C.
• Purge gas: high-purity helium or nitrogen; purge times 11 min and 6.5 min; flows 40 mL/min and 60 mL/min.
• Desorption: 1 min at 260 °C under 5 psi; dry purge and bake steps controlled moisture using a foam sensor and Moisture Reduction Trap (MoRT).

The GC/MS conditions employed a split injection (30:1) at 220 °C, an Rtx-VMS column (30 m x 0.25 mm, 1.4 µm), and a temperature program from 45 °C to 230 °C over a 15.58 min runtime. Mass spectrometry scanned m/z ranges 47–265 and 35–265 at 3.1 scans/sec.

Main Results and Discussion

• Linearity: All analytes showed excellent quadratic regression fits (R2 ≥ 0.995) with both gases and volumes.
• Compound Response: Average response factors were marginally lower when using nitrogen (0.560–0.568) versus helium (0.681–0.704).
• Precision and Accuracy: At 0.5 ppb and 20 ppb levels, both gases met USEPA criteria. Low-level precision ranged 6.8 %–7.2 % with nitrogen and 6.3 %–7.2 % with helium; accuracy remained within 90 %–110 % for most compounds.
• Purge Volume Comparison: The 440 mL and 390 mL volumes delivered comparable performance, indicating flexibility in cycle optimization.

Benefits and Practical Applications

• Helium and nitrogen purge gases both provide reliable quantitation of volatile organic compounds, mitigating helium supply concerns.
• Method flexibility supports shortened desorb times and optimized cycle throughput without sacrificing method compliance.
• Enhanced moisture control features (foam sensor, MoRT exclusion) improve system robustness and reduce sequence interruptions.

Future Trends and Applications

Innovations in purge-and-trap technology may include alternative trap media, integration with automated sample cooling modules, and real-time monitoring of moisture and foam. The acceptance of nitrogen as a purge gas paves the way for cost-effective method adaptations in environmental and industrial water analysis. Ongoing developments in gas chromatography detectors and mass analyzers will further enhance sensitivity and expand the target analyte list.

Conclusion

The EST Encon Evolution concentrator and Centurion WS autosampler achieved excellent performance using both helium and nitrogen purge gases at two purge volumes. Nitrogen proved to be a suitable alternative to helium, meeting USEPA Method 524.3 criteria with only a slight reduction in compound response. Laboratories can adopt nitrogen purge gas to reduce helium dependence while maintaining analytical quality and 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.