About the author Ben Armstrong

Open faced or Closed face Sampling

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.

Open faced and Closed face Cassettes
Open faced and Closed face Cassettes

Diesel Water Separators

When should you drain your diesel water separator?

Proper maintenance of a diesel engine is incredibly important and can mean the difference between catastrophic loss and operator error. Know when and how to keep your engine in the best condition possible.




For how and why to test your fuel: www.AFLab.com/services/vehicle-fluid/


Winter is coming!


And the American Chemical Society Southwest Regional Meeting (ACS SWRM) – http://www.swrm2014.org/

Our own Dr. Kelly Wouters will be speaking at the conference on Friday November 21st.

The Conference will run from November 19th through the 22nd.


Hope to see you there!

3 Rules

Operate your business like you live your life. Believe in something.

We hold many beliefs. Some can be explained very simply, like the 3 Rules.


3 Rules:

  1. Facts is Facts.
  2. Tell the Truth.
  3. I don’t know is a good answer.
  4. Whether this is the 4th rule or the 0th rule is matter of some debate. That being said. When in trouble, when in doubt, see Rule 1: Facts is facts. 2: Tell the Truth…

We have moved!

We moved digitally, not physically!


All of the convenient shipping program shipping labels still come to the right place.


Facebook – https://www.facebook.com/ArmstrongForensicLaboratoryInc

Linked In – https://www.linkedin.com/company/armstrong-forensic-laboratory

Twitter – https://twitter.com/aflabCOM



Dryer Fires

At Armstrong Forensic Laboratory, we routinely analyze debris recovered from laundry dryer fires.  The clothes dryers we have in our homes heat our clothing to temperatures approaching the ignition temperature of cotton.  When there are traces of other contaminants on the clothes, such as residual cooking oils that have a tendency to self-heat, spontaneous combustion fires can occur. Continue Reading

We help you.

We help you. We help you find the truth. We help you determine the source. We help you identify contaminants. We help you meet the regulation. We help you exceed the standard. We help you keep kids safe. We help you decide. We help you see what’s there. We help you help them.


We are here and we will help you.

AAAI Training Nov 3-6, 2014

Best wishes and congratulations to all the attendees at the AAAI’s Alabama Association of Arson Investigators Fall Training Seminar “From the fire scene to the courtroom.”


OSAC Subcommitee announcement

Congratulations to our own Dr. Andrew T. Armstrong on his appointment to the OSAC subcommittee on Fire Debris and Explosives!


The National Institute on Standards and Technology (NIST) has announced the selections for the Chemistry/Instrumental Analysis Scientific Area Committee Fire Debris and Explosives Subcommittee.

We are delighted to the have the AAFS well represented, and invite you to review the appointments here: