Using Alternative Carrier Gases for US EPA VOC Drinking Water Methods

Importance of the Topic

Rising helium costs and supply constraints threaten routine gas chromatography workflows. Alternative gases like nitrogen and hydrogen promise cost savings and onsite generation but require validation in established EPA VOC methods. This study evaluates their feasibility in purge and trap analysis of drinking water with Methods 524.2 and 8260C.

Study Objectives and Overview

The application aimed to assess the performance of hydrogen and nitrogen as carrier and purge gases respectively, fully replacing helium. Using a multimatrix autosampler coupled with GC/MS, comparisons were made to conventional helium-based protocols. Key metrics included calibration linearity, detection limits, precision, carryover, and tuning stability.

Methodology and Instrumentation

Sample preparation was automated with a Teledyne Tekmar Atomx purge and trap system handling 5 to 25 mL samples. Nitrogen served as the purge gas, desorbing analytes onto a sorbent trap before transfer to gas chromatography–mass spectrometry.

Instrumentation Used

• Autosampler: Teledyne Tekmar Atomx multimatrix with purge and trap concentrator
• GC/MS: Agilent 7890A gas chromatograph with 5975C inert XL mass selective detector and triple axis detector
• Sorbent Trap: Vocarb 3000
• Column: J&W DB-624, 20 m x 0.18 mm x 1.0 µm

Key Results and Discussion

• Calibration: Linear response achieved for most compounds over 0.2 to 200 ppb; some analytes required quadratic fitting with hydrogen.
• Precision: Percent relative standard deviations generally below 15 percent for both methods.
• Method Detection Limits: MDLs ranged from 0.02 to 2.31 ppb, comparable to helium methods.
• Tuning Challenges: Hydrogen carrier gas altered ion ratios in BFB tuning, requiring flow adjustments and further optimization.

Practical Benefits and Applications

Replacing helium entirely eliminates supply concerns and reduces operating costs. The proven analytical performance supports routine implementation for drinking water monitoring under EPA 524.2 and 8260C, after method adjustments for tuning stability.

Future Trends and Potential Applications

• Optimization of GC/MS tuning parameters for hydrogen to ensure consistent ion ratios.
• Investigation of matrix matched calibrations to improve linearity for challenging compounds.
• Expansion to soil and vapor analysis using alternative gases.
• Integration of onsite gas generation systems to further reduce laboratory logistics.

Conclusion

Hydrogen and nitrogen demonstrate equivalent analytical capability to helium in EPA purge and trap VOC methods, with MDLs and precision meeting regulatory requirements. Addressing tuning nuances will enable full adoption, offering a sustainable and economical path for environmental laboratories.

References

• USEPA Method 524.2, Measurement of Purgeable Organic Compounds in Water by Capillary Column GC/MS, Rev. 4.1, 1995
• USEPA Method 8260C, Volatile Organic Compounds by GC/MS, Rev. 3, 2006