Indoor Air Monitoring with Helium Diffusion Sampling (HDS)

A breakthrough new technology based on helium diffusion yields a sampler that is small, lightweight, cost effective, universal and defensible.  The HDS monitor delivers highly reproducible results unaffected by factors that create errors in other diffusive samplers such as matrix concentrations, varying humidity and fluctuating face velocities.

The HDS sample is collected based upon the higher diffusion rate of helium relative to air, creating a vacuum that pulls air atmospheres in as helium escapes.  A surrogate recovery compound is added to every HDS Personal Monitor to substantiate the collection of a proper sample in the field.  To prove effective recovery during laboratory analysis an internal standard is added prior to analysis.  Monitors are available in 15 minute, 1-2 hour and 4-8 hour sampling versions.

Before it goes out into the field each HDS monitor is vacuum cleaned and pressurised with helium at 4 to 7psig.  A surrogate chemical, typically bromofluorobenzene (BFB), is also added to validate leak-free operation, ensures the inertness of every sampler, and verifies the sample collection.  To begin sampling the operator simply removes the external valve cap and leaves unattended for the specified sampling time.  The sampler is then sealed with the same external valve cap ready for laboratory analysis.  Laboratory analysis is undertaken through the Entech 7410 or 7500 Autosampler and 7150 Pre-Concentrator connected to a GC/MSD for quantitative and qualitative characterisation of the atmosphere.  Proof that the sample has been properly injected is obtained by  verifiable recovery of an internal standard.

The HDS personal monitor, like the other forms of canister-based sampling in MiniCans and Bottle-Vac samplers, is completely re-useable following analytical grade cleaning and verification.

Soil Vapour Intrusion

Soil vapour intrusion refers to the process by which volatile organic compounds (VOCs) migrate from a sub-surface soil into the indoor air environment of the buildings that reside above. 

Soil vapour or soil-gas is the air found in the pore spaces between soil particles.  Due to the difference in pressure, soil vapour can enter the buildings through voids in slabs or cellar floors or walls, and openings where pipes and electrical wires go through the foundations.  In addition, heating, ventilation or air-conditioning systems may create a negative pressure that can draw soil vapour into the building.

Soil vapour becomes contaminated when volatile compounds evaporate from the sub-surface and enter the soil vapour.  Sources of chemicals may include contaminated soil and ground water, or buried wastes.  If the soil vapour is contaminated and enters a building above, indoor air quality may be affected.

HDS is a simple, unobtrusive, inexpensive and effective way of collecting this kind of sample.