A Simple, More Reliable Time Integrated Whole Air Sampling Approach To EPA Method TO15 Using Helium Diffusion Sampling

Significance of the Topic

Air monitoring for volatile organic compounds (VOCs) at trace levels is critical for environmental compliance and public health assessments. Traditional vacuum‐driven sampling methods under EPA Methods TO‐14A and TO‐15 can introduce contamination and inconsistencies. A simplified, more reliable sampling approach using helium‐driven diffusion (HDS) offers improved accuracy, reduced sample loss, and extended compound range, enhancing routine air quality monitoring.

Objectives and Study Overview

This study compares the new HDS canister sampling against conventional vacuum canister sampling over 24 hours in indoor and outdoor environments. Key goals were to evaluate precision, reproducibility, and potential contamination sources, and to verify that HDS maintains constant sampling rates without external flow controllers.

Methodology and Instrumentation

The HDS approach relies on helium diffusion through a calibrated orifice integrated into the canister valve to gently draw air until a pre‐set helium/air ratio (typically 50%) is achieved. Sampling rates are preset by orifice size and range from hours to one month without external flow controllers.

Instrumentation:

• Entech 7500A Robotic Canister Autosampler
• Entech 7101AR Preconcentrator (and Entech 7150 novel volume measurement design)
• Agilent 6890/5973 Gas Chromatograph–Mass Spectrometer
• Entech 4601A Dynamic Diluter for standards
• Entech 3120A Canister Cleaner

Sampling was performed using 0.45 L, 1.4 L HDS canisters and 6 L vacuum canisters with CS1200E4 flow controllers. Gravimetric analysis quantified the percentage of air collected.

Main Results and Discussion

HDS and vacuum samplers delivered concentrations in close agreement, within EPA precision requirements, even near reporting limits. Gravimetric checks showed air uptake consistently within 1–2% of targets, unaffected by air movement (0–5 mph). HDS samplers demonstrated stable sampling profiles due to predictable helium diffusion, avoiding complications from external flow paths, filters, and o-rings that can cause carryover or pressure variability.

Benefits and Practical Applications

• Eliminates external flow controllers, reducing contamination risk and simplifying field deployment.
• Allows use of smaller canisters for equivalent performance, lowering shipping and handling costs.
• Enables precise, time‐integrated sampling from light gases (propane) to heavy hydrocarbons (C24).
• Gravimetric determination of sample volume is more accurate than pressure measurement, avoiding canister opening.

Future Trends and Applications

Advances may include integration of real‐time volume measurement across diverse canister sizes, further miniaturization for extended deployments, and adaptation to automated field networks. Enhanced diffusive materials and orifice designs could expand molecular weight ranges and reduce detection limits for emerging contaminants.

Conclusion

Helium diffusion sampling offers a robust, accurate, and user‐friendly alternative to vacuum canister sampling for EPA TO‐15 applications. By eliminating external flow paths and leveraging gravimetric verification, HDS enhances data quality, reduces field errors, and supports high‐fidelity air monitoring across varied environments.

Reference

A Simple, More Reliable Time Integrated Whole Air Sampling Approach to EPA Method TO-15 Using Helium Diffusion Sampling, Application Note A-3732-01; Robinson TX, Casteel C, Cardin D; Entech Instruments, Inc.