Comparison of Helium and Nitrogen Purge Gases for the Analysis of Volatile Organic Compounds (VOCs) by Purge and Trap

Importance of the Topic

The analysis of volatile organic compounds by purge and trap coupled with GC/MS is widely used in environmental and industrial laboratories. Helium purge gas is an established standard but its high cost and limited availability drive the search for cost-effective alternatives such as nitrogen.

Objectives and Study Overview

This study evaluates the performance of helium versus nitrogen purge gases under conditions based on USEPA Method 8260D. The goal is to determine whether nitrogen can deliver acceptable accuracy and precision while reducing laboratory operating expenses.

Methodology and Instrumentation

The experimental setup included an OI Analytical Eclipse 4760 Purge and Trap concentrator, an OI Analytical 4100 Water/Soil Sample Processor and an Agilent 7890A/5975C GC/MS system. Key methodological adjustments to accommodate nitrogen’s higher molecular weight and heat capacity were:

• Reducing purge flow to 38 mL/min
• Raising purge temperature to 50 °C
• Increasing desorb temperature to 200 °C

A nine-point calibration from 1–200 ppb was performed for both liquid and solid matrices. Internal standards were used for response factor calculation and initial demonstrations of proficiency were conducted at 50 ppb.

Main Results and Discussion

Both helium and nitrogen purge methods met USEPA 8260D calibration and IDP acceptance criteria. Nitrogen purging showed a 40–60% reduction in detector response compared to helium but maintained acceptable precision (%RSD within method limits). Frequent maintenance such as source cleaning and calibration may be required when using nitrogen.

Practical Benefits and Applications

The substitution of nitrogen for helium offers significant cost savings and leverages readily available gas supplies. Laboratories performing routine VOC monitoring, QA/QC, or environmental compliance testing can adopt nitrogen purge while ensuring regulatory compliance.

Future Trends and Applications

Ongoing research may focus on automation of temperature and flow adjustments, advanced trap materials to enhance extraction efficiency, integration with high-throughput platforms, and greener approaches by minimizing helium consumption. Further validation across diverse matrices will expand the applicability of nitrogen purge protocols.

Conclusion

Nitrogen is a viable alternative to helium for VOC analysis by purge and trap GC/MS under USEPA 8260D conditions. While reduced sensitivity and increased maintenance should be considered, the cost savings and operational flexibility justify its implementation in many laboratories.

References

USEPA Office of Solid Waste SW-846 Method 8260D Revision 4 February 2017