Acetic Acid Badge – No PTFE allowed

What could be easier than a badge for Acetic Acid? It is such a common household chemical.  You could practically just use your nose. As it turns out, though, Acetic Acid is about as difficult as any chemical out there. We wanted to use our standard 566 badge, which has charcoal encased in PTFE.  But, the recoveries using the usual Ion Chromatography and the water-based desorbtion solution were very poor.  Poor recoveries were observed when experimenting on any commercially available charcoal badge, including ours. The situation was made even worse in that it would be easy for a lab to mistakenly, and unknowingly, report results biased low if they used a charcoal badge with a PTFE binder.  The solution is to use a special carbon without a binder.

**********  Read Dr Maria Peralta’s Report **********

THE PROBLEM

When sampling the air we breathe for chemicals, the collection media must collect and hold the chemical of interest, but then also release the chemical during analysis. The most common organic vapor badges, including our own, have trouble letting go of acetic acid once it is on the badge.

These badges contain charcoal and PTFE. Through a difficult process, the PTFE is used as a binder that allows the mixture to be made into a flexible carbon wafer that is easy to handle in the laboratory. When a PTFE charcoal wafer is used for collecting a common organic chemical like Benzene, the organic chemical easily diffuses past the PTFE and into the charcoal. During the analysis, organic solvent is used to pull the benzene back out of the charcoal and the solution is analyzed by a gas chromatograph. The PTFE is of little concern and recoveries are quite high. greater than 95%.

With acetic acid, the story is different. While the PTFE again allows the acid to diffuse into the charcoal with a vapor exposure, during the analysis using the Ion Chromatography method, the PTFE obstructs the water-based desorbtion solvent and the recoveries are quite poor. When tested, the Assay 566 badge recovery was 46% and a competitor’s PTFE/Charcoal badge was 24%.

In addition, laboratories may mistakenly think that the recovery of acetic acid off of diffusive charcoal badges is much better than actual recoveries. In order to determine the recovery of a chemical from sampling media, labs will dissolve the chemical in a convenient solvent and use a syringe or pipette  to introduce the mixture to the sampling media.  For acetic acid, the solvent is water.  However, the water does not penetrate the PTFE barrier.

Instead, when a known quantity of acetic acid is delivered to the charcoal wafer, the water-based spike beads on top of the wafer.  When the desorbtion solvent is added, it easily mixes in the acetic acid containing water droplet on the surface of the media, and leads to a falsely high recovery.  Usually high recoveries are good recoveries, unless, as in this case, the actually badge material has not absorbed any of the acetic acid. So, to effectively measure recoveries from charcoal media, vapor spikes – not liquid spikes – must be used in order to determine the actual desorbtion efficiency for the IC method.   There is a GC method that doesn’t not have this problem, however the GC method it is less sensitive than the IC method.

THE SOLUTION

The solution is to use:

• carbon specially designed for small organic molecules like Acetic Acid and
• not use a binder, like PTFE. Instead, place only loose carbon inside the badge casing.

The laboratory may not like the loose carbon as well, but since the desorbtion solution can freely mix with the carbon, the recoveries were much improved, 76%. We would have preferred a result above 80%, but considering we started with 24 to 47% recoveries, this is a great improvement.

To learn more about Assay Technology’s Acetic Acid 543 badge, visit our website assaytech.com and read Dr Peralta’s report.