Helium Diffusion Sampling for Personal Monitoring by EPA Method TO15

Importance of the Topic

Accurate and reliable personal monitoring of volatile organic compounds (VOCs) in workplace and indoor environments is critical for compliance with regulatory standards and protection of worker health. Conventional active sampling methods often require pumps, power, and complex calibration, while passive badge samplers can suffer from variable orientation, face velocity dependence, and lack of proof of sample collection. The Helium Diffusion Sampling Personal Monitor (HDS-PM) represents an innovative approach that combines the simplicity of passive devices with the constant-rate performance of active TO15 canister sampling.

Objectives and Study Overview

This application note introduces the HDS-PM technique for time-integrated personal air monitoring in accordance with EPA Method TO15. The goals are to describe the operational principles, validate sampling rates against classical vacuum canister collection, demonstrate quality assurance features, and showcase analytical procedures that ensure accurate quantification of workplace VOC exposure over periods from 15 minutes to 8 hours.

Methodology and Instrumentation Used

HDS-PM devices are small stainless-steel canisters pre-cleaned, charged with helium and a bromofluorotoluene (BFT) recovery surrogate under slight positive pressure (≈7 psig), and sealed with a low-flow orifice valve. Upon valve removal, helium diffuses out through the calibrated orifice at a nearly constant rate while drawing in ambient air at a 1:1 volumetric exchange. Orifice sizes accommodate integration times between 15 minutes and 8 hours. After sampling, the canisters are weighed to determine the actual air volume collected (air is 7.2× heavier than helium). Analysis is performed by GC-MS using fluorobenzene internal standard pressurization and either direct loop injection or automated sample extraction with the Entech 7650-M/7200 system.

Instrumentation Used

• Entech HDS Sampler Prep Station (HDS-PS1) with 4× manifold (HDS-PM-4XM)
• Digital Dilution System (DDS) for helium and fluorobenzene pressurization
• Entech GC-MS autosamplers: 7410D, 7650-L10, and 7650-M/7200 sample preparation system
• Analytical balance (±0.1 mg resolution)
• Calibrated absolute pressure gauges (psia)

Main Results and Discussion

Orientation and wind velocity tests showed that HDS-PM collection rates are invariant whether canisters are vertical, horizontal, or exposed to a 2 m/s air flow. Gravimetric corrections account precisely for any minor sampling rate shifts. Co-located sampling of 37 VOCs at 1 ppm v with HDS-PMs (15 min and 8 h) and classical TO15 vacuum canisters demonstrated equivalent results within method specifications, regardless of compound volatility. The built‐in helium diffusion mechanism ensures uniform uptake of light and heavy analytes, eliminating compound-dependent calibration factors common to other diffusive badges.

Benefits and Practical Applications

• Active-rate sampling without pumps or power
• Proof of sample collection via gravimetric measurement and surrogate recovery
• Wide integration times from short STEL measurements (15 min) to full-shift monitoring (8 h)
• Enhanced sensitivity for GC-MS analysis; no solvent extraction
• Compatibility with automated laboratory systems for rapid, reproducible analysis

Future Trends and Applications

Advances may include integration of digital pressure logging for real-time verification, miniaturized orifices for extended sampling windows, multi-analyte surrogates for broader QA coverage, and coupling with wireless data platforms. Adoption of HDS-PM as a standardized personal sampler could streamline industrial hygiene programs and improve exposure assessment precision in diverse occupational settings.

Conclusion

The Helium Diffusion Sampling Personal Monitor bridges the gap between passive and active air sampling by delivering TO15-equivalent performance with minimal field complexity. Its constant-rate helium-driven mechanism, combined with gravimetric proof of sample, surrogate validation, and GC-MS compatibility, positions HDS-PM as a next-generation tool for reliable workplace VOC monitoring.

References

• Cardin DJ, Robinson TX, Cardin DB. Helium Diffusion Sampling for Personal Monitoring by EPA Method TO15. Application Note A-3737-03. Entech Instruments, Inc.; 2003.