Using Alternative Carrier Gases for US EPA VOC Drinking Water Methods

Importance of the Topic

Helium has long been the preferred carrier gas in gas chromatography and GC/MS analysis due to its chemical inertness, efficiency, and compatibility with detectors. Recent supply constraints and rising costs of helium have driven laboratories to seek viable alternatives. Implementing hydrogen or nitrogen as carrier or purge gases can reduce operational expenses, enable onsite gas generation, and alleviate reliance on a finite resource. Evaluating these substitutes within established US EPA methods ensures analytical performance is maintained while improving sustainability.

Objectives and Study Overview

This application note aims to assess the feasibility of replacing helium with hydrogen as the carrier gas and using nitrogen as the purge gas in two purge and trap methods: EPA 524.2 for volatile organic compounds (VOCs) in drinking water and EPA 8260C for a broader VOC suite. Key goals include:

• Comparing calibration linearity and method detection limits (MDLs) with alternative gases
• Evaluating precision through relative standard deviation (%RSD)
• Assessing chromatographic performance and instrument tuning criteria
• Determining carryover and overall method robustness

Methodology and Instrumentation

Sample Preparation and Automation

• Teledyne Tekmar Atomx multimatrix autosampler with integrated purge and trap concentrator enabled full automation for liquid, soil, and extract analyses.
• Sample volumes of 5 or 25 milliliters were purged using nitrogen onto a Vocarb 3000 sorbent trap. Analytes were thermally desorbed to the GC/MS.

GC/MS Configuration

• Gas chromatograph: Agilent 7890A with J&W DB-624 column (20 m × 0.18 mm × 1.0 µm).
• Mass spectrometer: Agilent 5975C inert XL with triple-axis detector.
• Carrier gas: hydrogen at 0.3309 mL/min; purge gas: nitrogen at 40 mL/min.
• Oven program: 35 °C hold, ramp at 15 °C/min to 240 °C.
• MS scan range: 35–300 m/z at ~5 scans per second.

Calibration and Data Processing

• Calibration standards spanned 0.2–50 ppb for EPA 524.2 and 1–200 ppb for EPA 8260C.
• Internal standards/surrogates introduced to achieve final concentrations of 5 ppb (524.2) and 50 ppb (8260C).
• Agilent ChemStation software calculated response factors, %RSD, coefficient of determination (r2), MDLs, and carryover.

Main Results and Discussion

Calibration Performance

• Most compounds exhibited linear response with r2 > 0.995; a subset required quadratic regression when using hydrogen.
• %RSD values generally fell below 10%, indicating acceptable precision.
• MDLs ranged from 0.03 to 0.32 ppb (524.2) and 0.07 to 2.31 ppb (8260C), comparable to helium-based methods.

Chromatography and Tuning

• Hydrogen carrier gas yielded chromatograms similar in resolution to helium, as evidenced by representative 10 ppb standards.
• Achieving consistent BFB tune criteria, particularly the 95/96 ion ratio, was challenging with hydrogen; reducing column flow and controlling residual methanol improved tune success rates.
• Occasional failing tunes highlight the need for method optimization of ion ratio thresholds under hydrogen conditions.

Carryover and Robustness

• Blank injections following high-level standards demonstrated negligible carryover for most analytes.
• Automation and controlled purge/desorb parameters ensured reproducible sample handling.

Benefits and Practical Applications

Implementing hydrogen and nitrogen in VOC purge and trap analyses offers:

• Significant cost reduction by eliminating helium purchases.
• Onsite generation capability, reducing supply chain dependency.
• Comparable analytical figures of merit (linearity, precision, sensitivity) to traditional helium methods.

This approach can be applied to drinking water monitoring, soil remediation analysis, and routine industrial QA/QC, where cost and sustainability are critical.

Future Trends and Opportunities

Further work is needed to refine method parameters for hydrogen use:

• Optimize BFB tuning protocols and revisit ion ratio criteria specific to hydrogen carrier gas.
• Explore matrix-matched calibrations to enhance linearity of compounds showing quadratic behavior.
• Extend validation to additional EPA and ISO VOC methods across diverse matrices.
• Investigate the impact of alternative trap chemistries and newer column phases on separation efficiency with hydrogen.

Broad adoption of alternative gases can drive greener laboratory practices and secure analysis continuity amid helium shortages.

Conclusion

Hydrogen and nitrogen effectively replace helium in EPA 524.2 and 8260C purge and trap VOC methods without sacrificing sensitivity or precision. Despite tuning challenges, performance metrics align closely with established helium protocols. Adopting these alternatives addresses helium scarcity, reduces costs, and supports sustainable lab operations. Ongoing optimization of tuning parameters and calibration strategies will further solidify their role in routine environmental analysis.

References

1. USEPA Method 524.2, Measurement of Purgeable Organic Compounds in Water by Capillary Column Gas Chromatography/Mass Spectrometry, Revision 4.1, 1995
2. USEPA Method 8260C, Volatile Organic Compounds by Gas Chromatography/Mass Spectrometry, Revision 3, August 2006