Improving Canister Sampling Accuracy and Reliability Using Helium Diffusion Sampling into Silonite Coated Stainless Steel Canisters

Significance of the Topic

Time-integrated sampling of volatile and semi-volatile organic compounds in ambient air is essential for environmental monitoring, source apportionment and regulatory compliance. Conventional canister sampling methods under EPA TO-14A and TO-15 rely on external flow controllers, which can introduce contamination, require complex maintenance and limit the recoverable molecular weight range. The Helium Diffusion Sampling (HDS) technique simplifies the process, reduces potential artifacts, and extends applicability to higher molecular weight compounds without sacrificing accuracy or reliability.

Objectives and Study Overview

This application note evaluates the performance of HDS in Silonite-coated stainless steel canisters and Bottle-Vac samplers. Key aims include:

• Assessing sampling rate consistency and linearity over a 24 h period.
• Validating gravimetric determination of collected air fraction (% air) via weight differences.
• Comparing HDS performance in stagnant, indoor and outdoor (windy) conditions.
• Benchmarking 24 h HDS sampling in 0.45 L and 1.4 L canisters against classical 6 L vacuum sampling with external flow controllers.

Methodology and Instruments

HDS samplers are prefilled with helium to a positive pressure; upon valve removal, helium diffuses out through a calibrated orifice while ambient air diffuses in. Orifice sizes are matched to target integration times (3 h to 1 month) and container volumes (0.45 L, 1.4 L MiniCans; 16 oz and 32 oz Bottle-Vac). Sample collection is driven by the differential diffusion rates of helium and air, requiring no external flow controllers or filters.
Analyses were conducted using:

• Entech 7500A Robotic Canister Autosampler
• Entech 7101AR Preconcentrator
• Agilent 6890 GC coupled to a 5973 MS detector
• Entech 4601A Dynamic Diluter for standard preparation
• Entech 3120A Canister Cleaner for pre-sampling cleaning

Main Results and Discussion

Weight gain profiles for 1.4 L HDS samplers over 24 h showed excellent linearity, with air fractions determined gravimetrically within ±2% of target. Tests under still air (closet), typical indoor movement and 1–5 mph wind yielded identical exchange rates, confirming negligible face velocity effects at 0.01–0.5 cc/min.
A side-by-side comparison of three 6 L vacuum-sampled canisters and five HDS samplers (two 0.45 L, three 1.4 L) collecting 24 h room air demonstrated strong agreement in measured concentrations of VOCs. Differences were within EPA method precision; slight toluene bias in vacuum samplers likely arose from O-ring emissions.

Benefits and Practical Applications

HDS delivers multiple advantages:

• Elimination of external flow controllers and filters reduces contamination risk and maintenance.
• Gravimetric determination of % air avoids field and laboratory pressure checks.
• Compact MiniCans enable deployment in confined spaces and mobile platforms.
• Expanded molecular weight range includes semi-volatile species (pesticides, phthalates, siloxanes, C₂₅ diesel range compounds) when combined with heated preconcentration.
• Simplified workflow accelerates sample handling and supports automated analysis.

Future Trends and Potential Applications

Ongoing developments will further streamline HDS monitoring:

• Integration with advanced autosampler and preconcentrator designs (e.g., Entech 7150) that provide real-time volume measurements insensitive to gas composition.
• Wider adoption in regulatory and industrial air monitoring, leveraging cost reductions and robustness.
• Remote and continuous monitoring schemes using extended-duration (week/month) HDS orifices.
• Further expansion of target analyte lists including higher boiling point organics and emerging pollutants.

Conclusion

Helium Diffusion Sampling into Silonite-coated canisters offers a robust, accurate and contamination-resistant alternative to classical vacuum sampling. By removing external flow controllers, employing simple diffusion orifices and utilizing gravimetric calculations, HDS achieves precise time-integrated air sampling across a broad molecular weight range while streamlining field and laboratory workflows.

References

• Cardin DB, Robinson TX, Cardin DJ. Improving Canister Sampling Accuracy and Reliability Using Helium Diffusion Sampling into Silonite Coated Stainless Steel Canisters. Entech Instruments Application Note A-3731-01.