Passive sampling (diffusive sampling) onto sorbent media uses the principle of Fick’s First law of Diffusion to target known sample environments. Here analytes migrate to the surface of a sorbent bed at a rate dependent on conditions. As no power source is required, the technique can be used for long-term, time-weighted-average monitoring for determining exposure levels in line with occupational health guidance. When used in environmental air monitoring, their relatively low cost makes them suitable for large-scale deployment.
Axial sampling - Axial diffusive sampling is used for measuring volatile organic compounds (VOCs) in air over a wide concentration range. Depending on the sample concentration axial samplers can be exposed from 8 hours to 4 weeks.
Radial sampling - In radial diffusive samplers, the orientation of the diffusion path is parallel to the radius of the sorbent cartridge. They have a cylindrical diffusive surface area and hence an effective sampling rate typically 100 times that of axial tubes. Due to the higher sampling rate radial samplers are often used for short term deployment (4 hours - 1 week) but are not always suited to sampling high concentration atmospheres.
Passive sampling is used when studying known compounds, and quantitation is possible using validated uptake rates. The technique is widely used for monitoring personal exposure, for large-scale environmental studies, and for indoor air monitoring.
There are two approaches to tube-based passive sampling: axial passive sampling and radial passive sampling. These are explored in Application Note 008, The theory and practice of diffusive monitoring. And in our blog Passive sampling and its pivotal role in greener sampling of VOCs.
Passive sampling is central to a number of important standard methods, including:
- US EPA 325, in which passive samplers are deployed around the perimeter of an industrial site. Tubes are housed in robust, weather-proof hoods containing up to five tubes, with a typical sampling period of 3 – 28 days.
- ISO16017-2, which uses passive sampling for sampling and analysis of VOCs in indoor, ambient and workplace air.
Benefits of passive sampling
- No power source is required, so it can be used for long-term, time-weighted-average monitoring for determining exposure levels in line with occupational health guidance.
- Supplies are easy to transport and use, and suitable for a wide range of common volatile organic air pollutants.
- When used in personal exposure monitoring, passive samplers are silent and unobtrusive.
- When used in environmental air monitoring, their relatively low cost makes them suitable for large-scale deployment.