Firefighting formulations without per- and polyfluoroalkyl substances (PFAS) have been developed to replace PFAS-containing aqueous film-forming foam (AFFF); however, there are no rapid and cost-effective methods to quantify the PFAS content in PFAS-free firefighting formulations and thereby validate the safety of these products. This project will develop a paradigm-shifting analytical method and employ it for total PFAS quantification in PFAS-free firefighting formulations down to the sub-ppb level. This innovative analytical method will significantly lower the cost and time for PFAS analysis in firefighting formulations. This project consists of two interconnected objectives: 1) determine the limit of quantitation of surface-enhanced Raman spectroscopy (SERS) for total PFAS analysis in methanol, and 2) quantify PFAS in PFAS-free firefighting formulations down to 0.1 μg/L using the optimized SERS protocol.

Raman spectroscopy provides abundant information about vibrational bonds and exhibits potential for multiplexed detection because of the narrow Raman bands. The intensities of normal Raman scatterings are intrinsically low, which restricts traditional Raman spectroscopy to probing highly concentrated samples. To overcome this disadvantage, ultrasensitive SERS will be used for PFAS analysis in firefighting formulations. SERS is an emerging tool for chemical analysis, but has not been applied for label-free PFAS analysis, which may be attributed to the low Raman cross sections of PFAS, a lack of PFAS referencing spectra, and the mediocre reproducibility of SERS. We aim to overcome these disadvantages by developing a comprehensive and innovative SERS protocol for PFAS quantification.

First, we take advantage of the Marangoni flow in a drying droplet and the unique “non-sticky” behavior of PFAS to concentrate PFAS into a tiny spot for Raman measurement. This unique PFAS concentration method has not been reported previously and is expected to significantly improve the sensitivity of PFAS analysis by SERS. Second, we will create the first Raman spectrum library of PFAS and elucidate how the PFAS Raman fingerprints vary as a function of their carbon-fluorine chain lengths, head groups, and molecular geometries. This Raman spectrum library will serve as a reference for future studies on PFAS analysis and carbon-fluorine bond characterization. Finally, we will employ hot spot normalization (HSN) to improve the quantitative performance of SERS. HSN-SERS will be used to calibrate the SERS signals collected from the heterogeneously distributed hot spots and improve measurement reproducibility. Using the optimized SERS protocol and the identified common Raman features of alkyl C-F moieties, we will quantify the total PFAS content in 10 PFAS-free fighting formulations.

A paradigm-shifting analytical platform that allows rapid, inexpensive, and field-deployable analysis of PFAS in firefighting formulations will be achieved. This analytical method can overcome the intrinsic disadvantages of liquid chromatography-tandem mass spectrometry and total organic fluorine assay. Because it is an optical technology, the samples will not contact the instrument and thus do not need any pretreatment. The analysis does not need to be conducted by well-trained personnel in a highly specialized laboratory. The platform being developed could ultimately be deployed as a standard method for rapid validation of the “PFAS-free” firefighting formulations and other similar products. (Anticipated Project Completion - 2025)