Quantitation of Total PFAS Including Trifluoroacetic Acid with Fluorine Nuclear Magnetic Resonance Spectroscopy

Abstract

Fluorine nuclear magnetic resonance (19F-NMR) spectroscopy has been shown to be a powerful tool capable of quantifying the total per- and polyfluoroalkyl substances (PFAS) in a complex sample. The technique relies on the characteristic terminal −CF3 shift (−82.4 ppm) in the alkyl chain for quantification and does not introduce bias due to sample preparation or matrix effects. Traditional quantitative analytical techniques for PFAS, such as liquid chromatography–mass spectrometry (LC-MS) and combustion ion chromatography (CIC), contain inherent limitations that make total fluorine analysis challenging. Here, we report a sensitive 19F-NMR method for the analysis of total PFAS, with a limit of detection of 99.97 nM, or 50 μg/L perfluorosulfonic acid. To demonstrate the capabilities of 19F-NMR, the technique was compared to two commonly used methods for PFAS analysis: total oxidizable precursor (TOP) assay and LC-high resolution MS analysis for targeted quantification and suspect screening. In both cases, the 19F-NMR analyses detected higher total PFAS quantities than either the TOP assay (63%) or LC-MS analyses (65%), suggesting that LC-MS and TOP assays can lead to underreporting of PFAS. Importantly, the 19F-NMR detected trifluoroacetic acid at a concentration more than five times the total PFAS concentration quantified using LC-MS in the wastewater sample. Therefore, the use of 19F-NMR to quantify the total PFAS in highly complex samples can be used to complement classic TOP or LC-MS approaches for more accurate reporting of PFAS contamination in the environment.

Identifying Unknown Fluorine-Containing Compounds in Environmental Samples Using 19F NMR and Spectral Database Matching

“…19 F NMR spectroscopy has a demonstrated unique ability to provide both quantitative and structural information for known and unknown compounds within a sample. − There are a multitude of advantages when using 19 F NMR for examining environmental and biological samples. 19 F NMR exhibits minimal background, owing to a lack of naturally occurring fluorinated species.…”

“…19 F NMR exhibits minimal background, owing to a lack of naturally occurring fluorinated species. This results in relatively clean spectra that are straightforward to interpret and does not require extensive sample cleanup to negate matrix effects, with 19 F NMR demonstrating less susceptibility to matrix components …”

“…Importantly, 19 F NMR does not require any anticipation of which types of carbon–fluorine bonds or functional groups may be present in the sample; unlike mass spectrometry, there is no requirement for optimization of ionization efficiency or accounting for in-source fragmentation . It has been previously demonstrated that 19 F NMR has been successfully used in a variety of environmental applications for the measurement of PFAS and other fluorinated species. − ,, Some examples include measurement of the perfluorinated carboxylic acids in lake water, measurement of trifluoroacetic acid in environmental samples, ,, and measurement of total organic fluorine in a number of environmental and biological samples …”

“…This results in relatively clean spectra that are straightforward to interpret and does not require extensive sample cleanup to negate matrix effects, with 19 F NMR demonstrating less susceptibility to matrix components. 13 19 F NMR also has a large spectral window, with key sample resonances spanning more than 300 ppm, leading to inherently higher-resolution spectra than those achievable through proton experiments. Importantly, 19 F NMR does not require any anticipation of which types of carbon−fluorine bonds or functional groups may be present in the sample; unlike mass spectrometry, there is no requirement for optimization of ionization efficiency or accounting for in-source fragmentation.…”

“…12 It has been previously demonstrated that 19 F NMR has been successfully used in a variety of environmental applications for the measurement of PFAS and other fluorinated species. [9][10][11][12]15,16 Some examples include measurement of the perfluorinated carboxylic acids in lake water, 9 measurement of trifluoroacetic acid in environmental samples, 10,11,13 and measurement of total organic fluorine in a number of environmental and biological samples. 12 In this study, the NMR spectra of 135 fluorine-containing compounds were acquired with the goal of producing a reference dataset of the spectra for the analysis of fluorinecontaining compounds in environmental samples.…”