Diffusive (passive) sampling

The great benefit of diffusive sampling versus active / pumped sampling is that, as no pump is required, operating costs are much reduced.  This means that a larger number of samplers can be deployed, leading to a more accurate picture of VOC concentrations across a region.  Diffusive samplers are also less intrusive than pumps and are generally preferred by workers for personal exposure monitoring.

Axial diffusive samplers have been used for over 20 years in the field of occupational hygiene, and, more recently, for both indoor and ambient environmental air monitoring.  It is in these areas that the performance of traditional axial diffusive samplers has been pushed to the limits and where new radial diffusive samplers have a role to play.


Axial diffusive samplers

Diffusion caps are required for use with stainless steel and inert coated tubes for diffusive (passive) sampling.  Fitted onto the sampling, (grooved) end of the tube they ensure that the diffusive path length (critical for quantitative diffusive monitoring) is correct.

These diffusive samplers have a typical sampling rate equivalent to 0.5 to 1 ml/min (depending on the compound) and are widely used both for 8 hour occupational hygiene exposure and long term (two or four week exposure) ambient air monitoring.  For short term (8 hours) exposure, these tubes have a minimum detection limit of about 10 ppb and for 2 week monitoring, a detection limit of 0.1 ppb.

For traceability purposes diffusion caps may be supplied etched with a unique serial number.

Please note that glass sorbent tubes and SafeLok tubes are not suitable for diffusive sampling.

Radial diffusive samplers

The novelty of radial diffusive samplers is that the orientation of the diffusion path is parallel to the radius of the tube.  During sampling, the sorbent cartridge is contained within a cylindrical, inert, porous polymer diffusive body which provides the diffusion barrier.  The sampling surface area is therefore much larger than that for axial diffusive tubes and this, combined with the short diffusive path length, means they have an effective sampling rate typically 100 times faster than that of normal axial tubes.  Radial diffusive samplers are suitable for short term monitoring (from 30 minutes up to 6 hours) of low concentration atmospheres - a scenario previously unsuitable for passive devices.  For 6 hour exposure times, these tubes have a minimum detection limit of about 0.1 ppb.  Compatible analytes range in volatility from ~n-C₆ to ~n-C₁₀. 

After sample collection, the cartridges are removed from the porous polymer body and inserted into steel carrier tubes for storage and analysis by thermal desorption - GC(-MS).  Careful optimisation of the desorption parameters ensures complete extraction of the retained volatiles leaving the cartridge clean and ready for immediate reuse. 



For part numbers and pricing please contact your local Markes distributor or visit our on-line shop.


Relevant technical support publications from Markes International can be automatically downloaded from this web site (registration required). Examples include:

• Brochure - Thermal Desorption and Air Monitoring Products
• TDTS 1 - Uptake rates on axial tube diffusive samplers
• TDTS 2 - Prediction of uptake rates for diffusive tubes
• TDTS 8 - Principles of diffusive monitoring
• TDTS 10 - Use of diffusive samplers with thermal desorption - capillary GC analysis for monitoring VOCs in ambient air
• TDTS 42 - Uptake rates for radial diffusive samplers
  
  

Other sorbent tubes and accessories available from Markes include Sorbent tubes, Sorbents, SafeLok tubes, Certified Reference Standards, Sorbent tube accessories, TubeTAG

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