High Capacity Sampling. No Back Section Required.

Occasionally, a thoughtful, careful customer will ask us about the capacity of the Organic Vapor badges (546, 566, and 525).  They know if the capacity of the sampling media is exceeded, they will get a low biased result.  So, they ask:

• Why doesn’t Assay badges have a back up carbon wafer for high capacity sampling?
• How do you know you’ve exceeded capacity of the monitor?

Usually customers are comparing Assay Badges against the 3M 3520, which has a second wafer, or a charcoal tube, which has a back section.  In both cases, when the capacity of the front section is reached, the chemical will start to be collected on the back section, which is your indication the capacity of the media was exceeded.

Since Assay badges do not have a back section, there is no indicator that the capacity of the badge was reached.  But before you reach for media that have a back section, consider this:

The Assay 546 badge has been designed so you would have to have concentrations many times over the PEL before the capacity is exceeded, even for chemicals with high PELs like Toluene and Ethanol.  This makes an additional section unnecessary.

As for the 3520s and the small charcoal tubes, their capacities are much lower, requiring the use of a back section.

For example:   The PEL of Toluene is 200 ppm.

Assay 546 Badge: If you use the 546 monitor for eight hours, the capacity of the 546 badge after 8 hours is 3500 ppm.  That means the concentration would have to be about 17 times the PEL before capacity of the badge would be reached.  And the 546 reporting limit after 8 hours is 0.47 ppm.  That’s over 400 times below the PEL.

Tubes:  The amount of charcoal in a 546 badge is 150 mg, which is the same as the front and back of a charcoal tube.  Let’s assume their capacities in micrograms are about the same.  That means the capacities would be the same  if they sampled at the same rate.  But they don’t.  Typically, the tubes have a pump connected to it which generates a flow of 100 mL/min.  The 546 samples at 2.37 mL/min.  That means the 546 has the capacity of over 40 tubes.  

3M 3520:  Using the data on the 3M website, the 8 hour capacity for toluene is 440 ppm.  That’s double the PEL.  Thank goodness that second wafer is in there.  Because, without it, the capacity would be less than the PEL.   Remember the Assay 546 badge has its capacity at 17 times the PEL.

Not all chemicals have PELs at 200 ppm.  Let’s try a more difficult Chemical:  Ethanol.  PEL 1000 ppm

Here are the numbers:

546:  8 hour capacity:   4100 ppm.  You have to be at levels 4 times the PEL before there are problems.

Tube:  This time, the OSHA method uses a 4X bigger tube and 2X slower sampling rate.  So that’s 8 times the capacity of the small tubes with the 100 mL/min sampling rate.  That puts the capacity right about at the PEL.

 3M 3520:  The capacity in micrograms is rather low:  3500 ug.  That means the capacity is reached at 88 ppm.  No where close to the PEL of 1000 ppm.

So the 546 badge is just better designed for chemicals like Toluene and Ethanol with a high PEL.

Here’s a table that shows how many times over the PEL  the 546 capacity is.  For example, the 8 hour capacity for t-butyl alcohol is 3077 ppm and the PEL is 100 ppm.  So the “Over PEL (factor)” is 31:

Assay Technology has three organic vapor monitors to choose from.  Here’s a summary of how we suggest the badges be used:

• 525:  Use for chemicals with PELs 10 ppm and below.  Also use for STELs and Indoor Air Quality
• 546:  Use for chemicals with PELs 200 ppm and above.
• 566:  Use for chemicals with PELs between 10 ppm and 200 ppm.

Also, the Sampling Guide will show our recommendation for each chemical and we have a blog that discusses which monitor to use.