The Acetic Acid Recovery Problem
Sponsored by Assay Technology
What could be easier than developing a badge for acetic acid? It’s such a common household chemical. You could practically just use your nose. But, as it turns out, acetic acid is about as difficult as any chemical out there to sample when using a badge. In fact, if your lab isn’t careful, they could be reporting results that are biased low.
Laboratories have to perform a lot of spike and recovery tests. They use them during method validation to establish an average recovery for the method and during every analytical batch to ensure they have performed the test correctly. There are two kinds of spikes: liquid spikes and vapor spikes. A liquid spike is easy and accurate. The lab will dissolve the chemical in a convenient solvent and use a syringe or pipette to present the mixture to the sampling media. For a vapor spike, the badges are put in a chamber and exposed to a known amount of the chemical of interest. The vapor spike closely mimics an actual exposure, but is difficult to be accurate and takes a lot of time. So, many laboratories use the liquid spike method. In this case, it is essential that laboratories using the ion chromatography (IC) method use a vapor spike in its recovery study.
The Easy Mistake
Most organic vapor badges available contain charcoal encased in polytetrafluoroethylene (PTFE or Teflon or Syncolon). The PTFE allows the carbon to be cut into thin, flexible wafers and does not affect the sampling rate nor the desorption of the chemical of interest. When the solvent used is organic (carbon disulfide, toluene, and so on) this is a fine procedure.
However, for acetic acid, the solvent is water, and water does not penetrate the PTFE barrier. Instead the spike beads on top of the wafer. Using the liquid spike method, you can be fooled into thinking your recoveries are much better than they actually are. (There is a gas chromatography method that does not have this problem; however, it is less sensitive than the IC method.) So the lab may believe the recovery is about 90 percent, but when using the vapor spike method, the average recovery from a PTFE badge is between 27 and 42 percent. That means a lab using liquid spikes and the IC method could be reporting results that are significantly lower, due to these limitations.
The Solution: Innovate
At Assay Technology, since we could not use our normal organic vapor badge, the 566, for acetic acid, we invented a new badge, the 543. The collection media is charcoal specially designed for small organic molecules like acetic acid—and obviously, it did not include a PTFE binder. The laboratory may not like the loose carbon, but since the desorption solution can freely mix with the carbon, the recoveries were much improved (76 percent).
To learn more about Assay Technology’s Acetic Acid 543 badge, visit our website to read Maria Peralta’s report (PDF).