Which to choose?
Do you perform aerosol (particulate or mist) sampling? Airborne dust, metals, wood grains, oil or acid mists, or other aerosols? Have you considered the difference between closed-face and open-face sampling?
Filter cassettes generally come in two types: 2-Stage or 3-Stage cassettes. A 2-Stage cassette has a lower section which holds the filter and support pad while the upper section (the second stage) holds the filter/pad in place. There is an inlet/outlet port for each section where the air flows through. A 3-Stage cassette has the same configuration as the 2-Stage with an added middle section – a simple ring that, when the top of the filter is removed, acts as a holder to keep the filter/pad in place during sampling. The 2-Stage cassette can only be used for Closed Face sampling. The 3-Stage cassette can be used for either Closed Faced or Open Faced sampling.
Which Technique Do I Use?
When the Analytical Method you have selected specifies the technique you should follow the method.
When sampling for the respirable fraction, a 3-Stage cassette is required with a cyclone adapter.
If the Analytical Method does not specify the technique – you have a choice to make. Generally, open-face sampling is recommended. There are a few situations where this technique may not be appropriate, such as, mining operations or operations with very high airborne concentrations (e.g., baghouse cleaning). There are alternate techniques that may be more appropriate for these situations.
Why is the technique important?
Studies have indicated that sampling aerosols, particulates specifically, closed-face may be biasing your results low due to failure to capture larger sized particles – a situation that may lead to a compliance issue in the future.
Intuitively, in areas of high concentrations, sampling closed-face can lead to a build-up of the aerosol in a small area directly under the inlet. This build-up may lead to a loss of sample (loose particulate not secured to the filter) or filter damage including the center of the filter being completely destroyed during the sampling event, again, resulting in loss of sample.
Open-face sampling for aerosols (dust, metals, mists, etc.) provides a more evenly distributed sample collection and therefore less opportunity for sample loss due to build-up or filter damage.
How can the Lab help?
Typically, Armstrong Forensic Laboratory provides 3-stage cassettes for all media orders involving aerosol sampling. This allows you, the Client, to make the final decision regarding technique in the field. Unless specifically identified in the analytical method, Armstrong Forensic Laboratory discourages Closed-Face sampling. Open-Face sampling provides more reliability and less potential for sample loss.
The only critical piece of information left is which type of filter to use….. a conversation for another time.