Passive Air Sampling
Passive Air Sampling: What is it?
Passive air sampling is a method of determining airborne concentrations of volatile contaminants using a passive sampling device (PSD). Instead of using bulky pumps and sorbents, such as charcoal in glass tubes, or summa canisters, air samples are collected with a small badge-like device that relies on the diffusion of compounds to a collection surface or sorbent. The diffusion barrier across the badge confers a constant, predetermined effective flow rate that is only slightly affected by temperature and unaffected by pressure (or altitude). The PSD hangs in the sampling area for periods ranging from 8 hours to one month. After the sampling period, it is capped and returned to the laboratory for analysis using traditional methods.
Benefits of Passive Air Sampling
PSDs can be deployed in offices or residential environments, or for occupational personal exposure assessment and in ambient air situations without disrupting day-to-day activities. They are generally very small, silent and unobtrusive since no pump (or power) is needed for sampling. These features allow easier sampling and a denser sampling design to collect contaminants under representative, normal conditions. Passive sampling is more cost effective since less equipment is required. The savings depend on the substance measured. The ease of deployment and retrieval, since no pump or calibration is required, means less professional time to undertake multi-sample surveys. In the laboratory, PSDs are generally less labour-intensive to extract and analyze than traditional sorbent tubes or summa canisters, allowing quicker turnaround times for results. Otherwise, processing in the laboratory is much the same as for traditional sorbent tubes. In addition, failure rates of sampling are much lower and in general, measurement precision is somewhat better than comparable active sampling approaches. In part, this is due to invariant sampling rates compared to pump-based samplers.
In the Workplace
Due to the use of pumps and tubes, occupational air sampling has been difficult in the past. With PSDs, workers no longer wear heavy, noisy pumps nor have to deal with awkward tubing. By attaching a PSD to the lapel or collar of the worker, breathing zone exposures can be measured.
The Scope of PSDs
Like any other sampling tool, PSDs have recommended uses. Samples can effectively be collected to determine VOCs, formaldehyde and ethylene oxide, inorganic compounds, ozone, etc. The upside of this is that many indoor air contaminants fit into these categories and can be measured.
As with active air samples, PSD detection limits are determined by the design of the sampler, the exposure time and the compound-specific diffusion rate. Environmental and analytical factors such as relative humidity, blank levels and measurement sensitivity also affect detection limits as they do for other sampling devices.