By Grant Lobdell
Regulations concerning per- and polyfluoroalkyl substances (PFAS), a group of thousands of manmade and persistent molecules, continue to grow. Where previous regulations on firefighting foam referenced limits on just two specific molecules within this group, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), some regulations are now calling for a limit on all PFAS.
PFOA and PFOS Analysis
PFOA and PFOS, which I will refer to as PFAS species, are generally isolated using liquid chromatography (LC). Chromatography is a laboratory technique that involves the separation of molecules based on their attraction to the material of a tube they are passed through. This tube is typically referred to as a column. The more attracted the molecules are to the material of the column, the slower they travel through it. Given enough column length, even very small differences in this attraction can add up to a significant change in how long it takes the molecule to pass through the column, which is commonly referred to as retention time. A laboratory can use this to their advantage. Knowing how long it takes each molecule in a solution to pass through a column, the laboratory can put a sample containing multiple molecules into the column and get out a sample each specific molecule at a specific time.
To determine total PFAS content in this way, though, the laboratory would have to isolate and quantify each one of the thousands of PFAS species. The sum of all the PFAS species present in a sample would equal the amount of total PFAS. Unfortunately, there is not a column available which can separate all these species by itself. Many columns would be needed. Furthermore, to determine the retention time of each PFAS species on a column, one would need a pure example of each molecule, some of which are certainly not readily available. Therefore, this speciated approach would not be very economical even if it were possible for total PFAS analysis. If a laboratory is using this approach and reporting total PFAS content, they would simply be looking at just a handful of PFAS species and assuming all others are negligible. Therefore, they could be under reporting total PFAS content.
Total Organic Fluorine
All PFAS, by definition, are organic molecules that contain fluorine chemically bonded to carbon. It is important to note before moving on that organic as it is used here does not refer to whether it is manmade or found in nature. The label organic in chemistry refers to the carbon-based structure of the molecule.
The fluorine found bonded to carbon in PFAS, which is referred to as organic fluorine, is a unique feature shared by all PFAS species. Therefore, it makes for a great indicator when looking at PFAS as a whole. Instead of looking for the thousands of individual PFAS species, a laboratory can look for just organic fluorine, which is much more feasible.
To quantify the amount of fluorine in a sample, a laboratory would use combustion ion chromatography (CIC). In this process, the sample is first combusted to break any carbon-fluorine bonds found in the sample. Given the carbon-fluorine bond is one of the strongest chemical bonds, the sample needs to be heated to beyond 1000°C for it to be broken. Once the sample is combusted and the fluorine is no longer attached to carbon, it can then be quantified by ion chromatography (IC). Ion chromatography is very similar to the liquid chromatography process except that it is looking for just parts of a molecule, not the whole molecule itself. These parts of the molecule in this case are referred to as ions. The fluoride ion is separated from all other ions in solutions in a column. Once isolated, it can then easily be quantified.
The amount of fluoride determined by combustion ion chromatography would be the amount of total fluoride (TF) in the sample. Unfortunately, this value by itself may not accurately reflect total PFAS. Fluoride, after all, can come from more than PFAS. It can be found as a salt or mineral, which are referred to as inorganic molecules given they lack that carbon structure defining organic molecules. People come into contact with these inorganic molecules every day. Toothpaste and municipal water generally contain inorganic fluoride given its ability to help prevent tooth decay. This inorganic fluoride can also be referred to as free fluoride where the free label comes from the fact it is not bonded to carbon.
To get a sense of total PFAS, what is actually needed is total organic fluorine (TOF) not total fluorine (TF). Luckily, it is relatively straight forward to determine total organic fluorine from the total fluorine result. The amount of free fluoride in the sample to begin with just needs to be quantified and subtracted out. There are a variety of ways to determine free fluoride. An ion selective electrode (ISE) could simply be used for this. The sample could also be run through the ion chromatography column again but this time without the combustion step so that the organic fluorine remains attached to carbon and therefore is undetectable by ion chromatography. Regardless of how free fluoride is determined, if it is accounted for and subtracted from the total fluorine result, we are left with the amount of total organic fluorine which is a direct reflection of total PFAS content.
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PFAS Analysis Method |
Pros |
Cons |
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Speciated Approach via LC (Dyne does NOT offer this analysis)* |
Low detection limits (ppt) are possible Can quantify specific PFAS species (i.e. PFOA, PFOS, etc.) |
Does not account for all PFAS species |
|
Total Organic Fluorine (TOF) via CIC (Dyne does NOT offer this analysis)* |
Accounts for all PFAS species |
Low detection limits (ppt) are NOT currently possible Can NOT quantify specific PFAS species (i.e. PFOA, PFOS, etc.) |
*List of laboratories that can perform speciated analysis of PFAS can be found here: https://www.denix.osd.mil/edqw/accreditation/accreditedlabs/ (search AFFF as the Matrix)
Total PFAS in Firefighting Foam
Firefighting foams contain a type of molecules referred to as surfactants which drastically reduce the surface tension of the solution. If you’ve ever observed a droplet of water on a surface and noticed how it clings to itself to form a sphere, you’ve observed water’s high attraction to itself or, in other words, its high surface tension. This surface tension is a hindrance when it comes to fire protection though. In the case of firefighting foam, the goal is to fill the drop of water with air so that it becomes light enough to sit on the surface of a flammable liquid and block the flammable liquid from oxygen in the atmosphere. To do this, water’s high surface tension needs to be reduced with a surfactant(s).
Interestingly, if the surface tension of a firefighting foam solution is low enough, even the unexpanded solution, which is much more dense than the foam blanket, is actually able to sit on top of the flammable liquid. This becomes useful if there is a chance the foam blanket could become disturbed before the fire is out. As the foam blanket drains, instead of sinking to the bottom of the flammable liquid, some of the foam solution will remain on top, effectively forming what the industry calls a film, which continues to deprive the flammable liquid of oxygen. This added layer of protection is only found in certain types of foam products such as AFFF (aqueous film forming foam) and FFFP (film forming fluoroprotein). To achieve a low enough surface tension for film formation, though, these products must contain fluorosurfactants. A fluororsurfactant is a very effective surfactant that contains fluorine bonded to its carbon structure. These fluorosurfactants, therefore, would be classified as PFAS.
These film forming foams generally contain anywhere from 0.1-2% total organic fluorine based on our experience. In contrast, fluorine free foams, which do not contain PFAS and therefore do not form a film, generally contain less than 0.0001 % total organic fluorine. Given that is such a low percent, typically the units are converted to 1 part per million (ppm).
As an example, to demonstrate how one could convert total organic fluorine to total PFAS, let’s take a sample that has 1% total organic fluorine and assume all of that fluorine came from PFOS specifically. Given PFOS is 65% fluorine by mass, then there would then be about 1.5% total PFAS in this sample. The amount of total PFAS will always be a slightly higher than the amount of total organic fluorine given the rest of the molecule’s mass, not just the fluorine, needs to be accounted for. However, note an assumption had to be made here. We needed to know what PFAS species the fluorine came from. Typically, this would not be known and, as explained earlier, cannot always be easily determined when the sample could contain any number of the thousands of molecules classified as PFAS. Therefore, limits on total PFAS would be better discussed in terms of total organic fluorine and not total PFAS to avoid any assumptions.
If you have any questions regarding this article, please contact Dyne Fire Protection Labs at lab@dyneusa.com or (800) 632-2304.
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