The analysis of total organic fluorine (TOF) via combustion ion chromatography (CIC) has emerged as a means of assessing per- and polyfluoroalkyl substances (PFAS) as a whole.
High-performance liquid chromatography (HPLC) techniques have been used for quite some time to effectively isolate a handful of specific per- and polyfluoroalkyl substances (PFAS), such as perfluorooctanesulfonic acid (PFOS) or perfluorooctanoic acid (PFOA), which can then be subsequently quantified (see Figure 1). As regulations continue to grow to limit more and more of the chemicals categorized as PFAS, so too have analytical techniques.
High-performance liquid chromatography is quite capable of isolating specific PFAS, even down to parts per trillion (ppt) in some cases (see Table 1), but it simply cannot isolate them all. As a result, some regulations have already started limiting the amount of total organic fluorine (TOF) instead. One such example is UL 162 Foam Equipment and Liquid Concentrates, which is the standard used by Underwriters Laboratories (UL), a global safety certification company, to evaluate firefighting foam concentrates. In the most recent revision, UL 162 states that ‘for a foam liquid concentrate formulated such that it does not contain intentionally added per- and/or poly-fluorinated substances (PFAS) [i.e. synthetic fluorine free foam, SFFF, as defined by the National Fire Protection Association], the concentration of permissible PFAS concentration is < 1mg/L [i.e. parts per million, ppm] Total Organic Fluorine (TOF) when measured by combustion ion chromatography.’ To put this into perspective, note that most aqueous film forming foam (AFFF) typically contain approximately 5,000–20,000mg/L (ppm) TOF depending on the product.
The term organic fluorine refers to fluorine bonded directly to carbon. Since PFAS are made of fluorine to carbon bonds, they innately contain organic fluorine. To quantify the amount of organic fluorine, a technique called combustion ion chromatography (CIC) is used. That carbon to fluorine bond, which is one of the strongest bonds known to man, must be broken by combusting the sample at temperatures above 1,000°C. All the fluoride in solution after combustion can then be separated out by ion chromatography and subsequently quantified to give a total fluorine (TF) concentration (see Figure 2).
Note, however, that not all fluoride in solution is from PFAS. There can be ionic fluoride, or free fluoride (FF) in the sense that it is not bonded to carbon, in solution as well. For example, ionic fluoride is commonly added to municipal water supplies for dental hygiene. Therefore, to determine the total organic fluorine concentration, one must also determine the amount of free fluoride as well and then subtract it from the total fluorine concentration (see equation 1). Luckily, there are several well-developed methods for determining free fluoride ranging from the use of ion chromatography (IC) to ion selective electrodes (ISE).
Equation 1: Total Organic Fluorine (TOF) = Total Fluorine (TF) – Free Fluoride (FF)
Dyne Fire Protection Labs is currently equipped and capable of conducting TOF analysis for a variety of matrices.
For more information, go to www.dyneusa.com