Common and Distinctive Raman Spectral Features of Per- and Polyfluoroalkyl Substances for Their Identification and Distinction

As per- and polyfluoroalkyl substances (PFAS) pose a great threat to public health, the U.S. Environmental Protection Agency established the draft Method 1633 to detect PFAS in various environmental media based on solid-phase extraction and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Though precise and sensitive, the operational complexity and high cost of the standard method hinder the regular monitoring of PFAS. Raman spectroscopy is a promising complementary tool for LC-MS/MS because of its fingerprinting ability for trace analysis, low operational cost, and fitness for field-deployable applications. For the practical application of Raman spectroscopy, a well-established Raman library is a prerequisite due to the current lack of comprehensive and reproducible spectral data on PFAS. In this study, we propose a simple method to concentrate PFAS in organic solvents and to establish a Raman library for PFAS. We drop-coated PFAS solutions onto aluminum foil and gold-coated glass slides and dried them to concentrate and secure solid PFAS. The different functional groups and chain lengths affected the evaporation and crystallization behavior of PFAS. Raman maps were then collected from the sessile drops using a 532 nm laser and a confocal Raman spectrometer. Regardless of the chemical structure of PFAS, they shared common Raman peaks at approximately 725, 385, and 310 cm-1, while having varying peak-to-peak ratios according to their chain length, head groups, and the extent of fluorination. Principal component analysis was performed on wavenumbers from 200-1,000 cm-1 and 1,100-1,600 to differentiate PFASs from non-fluorinated alkyl acids and PFAS with varying functional groups. To the best of our knowledge, this research created a novel experiment-based reproducible Raman spectral library of PFAS that will be a foundation for simple and facile PFAS screening using Raman spectroscopy.