On the Unimolecular Breakdown Products of Short Deprotonated Per‐ and Poly‐Fluorinated Acids and Alcohols

ABSTRACT

The strong C–F bond found in per‐ and poly‐fluorinated alkyl substances (PFAS) makes them resistant to degradation and thus persistent in the environment. One of the most common methods for quantifying PFAS in environmental matrices is to use tandem mass spectrometry. However, the dissociation of ions made by deprotonating PFAS alcohols and acids has only been qualitatively explored. In this study, we investigated the breakdown of deprotonated 2,2,3,3,3‐pentafluoropropionic acid (1, m/z 163), 3,3,3‐trifluoropropionic acid (2, m/z 127), 2,2,3,3,3‐pentafluoro‐1‐propanol (3, m/z 149), 3,3,3‐trifluoro‐1‐propanol (4, m/z 113), and trifluoromethanesulfonic acid (5) by energy‐resolved collision‐induced dissociation (CID) tandem mass spectrometry and density functional theory (M06/6‐311+G(d,p)). Ion 1 loses CO₂ at low lab‐frame collision energy. Ion 2 also loses CO₂ to form the 1,1,1‐trifluoroethane ion (m/z 83) and 1,2‐difluoroethylene to form FCO₂ ⁻ (m/z 63). RRKM calculations for the two reactions show that m/z 83 has a higher entropy of activation driving its formation. Ion 3 undergoes the loss of CH₂O to form the pentafluoroethyl anion (m/z 119) and the loss of HF to form CF₂CF₂COH⁻ (m/z 129). Ion 4 produced four fragment ions with two primary reactions making CF₃CHCH⁻ (m/z 95) + H₂O and CF₂CHCOH₂ ⁻ (m/z 93) + HF, which go on to dissociate further to produce CF₃ ⁻ (m/z 69) + HCCH + H₂O and CF₂CH⁻ (m/z 63) + CH₂O + HF. At low collision energy, m/z 95 dominates due to a lower energy transition state, but as internal energy increases, m/z 93 takes over as its transition state has a more favorable entropy. Ion 5 produced FSO₃ ⁻ (m/z 99), SO₃ ⁻ (m/z 80), and CF₃ ⁻ (m/z 69). SO₃ ⁻ was the most abundant fragment due to its higher electron affinity.

1. Introduction

Per‐ and poly‐fluoroalkyl substances, PFAS, are compounds that have been used in many products, including nonstick coatings, water‐resistant clothes, and food packaging [1]. The C–F bond found in PFAS is strong and resistant to degradation in the environment, which can lead to bioaccumulation [1]. PFAS are a diverse class of compounds that vary in their carbon backbone as well as functional groups. The acidic functional groups can be sulfonic, carboxylic, and phosphonic acids. Recently, shorter‐chain PFAS have been introduced to prevent bioaccumulation, but recent studies show that these shorter‐chain PFAS are just as toxic and very mobile in the environment [2]. These shorter‐chain PFAS are also harder to detect and therefore make their removal challenging [2].

PFAS with acidic head groups, like carboxylic and sulfonic acids, are usually analyzed by liquid chromatography–tandem mass spectrometry (LC‐MS/MS) in negative mode [3]. The mechanisms for the formation of the observed fragment ions in MS/MS experiments have not been explored in detail (but more recently, the dissociation of deprotonated tri‐ and penta‐fluoropropionic acid ions has been investigated) [4]. To investigate these pathways, five short‐chain PFAS—2,2,3,3,3‐pentafluoropropionic acid (1H), 3,3,3‐trifluoropropionic acid (2H), 2,2,3,3,3‐pentafluoro‐1‐propanol (3H), 3,3,3‐trifluoro‐1‐propanol (4H), and trifluoromethanesulfonic acid (5H)—were selected. Their deprotonated forms are shown in Figure 1. The unimolecular chemistry of each of these deprotonated ions was explored with tandem mass spectrometry and theory.

FIGURE 1.

The deprotonated anions of the five short‐chain PFAS 2,2,3,3,3‐pentafluoropropionic acid (1), 3,3,3‐trifluoropropionic acid (2), 2,2,3,3,3‐pentafluoro‐1‐propanol (3), 3,3,3‐trifluoro‐1‐propanol (4), and trifluoromethanesulfonic acid (5).

2. Experimental Methods

2,2,3,3,3‐Pentafluoropropionic acid (97%, Sigma‐Aldrich, Oakville, Ontario, Canada), 3,3,3‐trifluoropropionic acid (98%, Sigma‐Aldrich, Oakville, Ontario, Canada), 2,2,3,3,3‐pentafluoro‐1‐propanol (97%, Sigma‐Aldrich, Oakville, Ontario, Canada), 3,3,3‐trifluoro‐1‐propanol (Sigma‐Aldrich, Oakville, Ontario, Canada), and trifluoromethanesulfonic acid (Reagent grade, 98%, Sigma‐Aldrich, Oakville, Ontario, Canada) were purchased and prepared in five individual solutions at a concentration of 1 ppm with a mixture of 9:1 water (Optima, $\ge$ 99.9%, Fisher Scientific, Fair Lawn, New Jersey, United States), and methanol (Optima, Fisher Scientific, Fair Lawn, New Jersey, United States). Product ion scans were performed on a Micromass Quattro Ultima triple quadrupole mass spectrometer with a Z‐Spray ion source and the MassLynx operating software. The samples were introduced into the electrospray ionization (ESI) source using a syringe pump with a flow rate of ~ 30 μL/min. The desolvation gas used was nitrogen (244 L/h), the collision gas was argon, and all solutions were analyzed in negative ion mode. The source and desolvation temperatures were 100 °C and 150 °C, respectively. For each compound, the collision energy was varied from 0 to 30 eV (E _lab ). Breakdown curves were generated by plotting the relative abundance of precursor and fragment ions as a function of center‐of‐mass collision energy, which was derived from the lab‐frame collision energy by equation 1.

$$E_{\mathit{com}}=\left(\frac{m_{\mathit{Ar}}}{m_{\mathit{Ar}}+{\mathrm{m}}_{\mathit{\text{PFAS}}}}\right)E_{\mathit{lab}}$$ (1)

where m _Ar and m _PFAS are the masses of argon and the ion in question.

3. Computational Methods

Optimized structures (minima and transition states) and vibrational frequencies were obtained using the M06/6‐311+G(d,p) level of density functional theory (DFT) [5, 6] with the Gaussian 16 suite of programs [7]. Transition states were confirmed with the intrinsic reaction coordinate method [7]. Rice–Ramsperger–Kassel–Marcus (RRKM) theory was used to determine the microcanonical rate constants, k(E), of the unimolecular reactions using the following equation [8, 9].

$$\mathrm{k}\left(\mathrm{E}\right)=\frac{\mathrm{\sigma }{\mathrm{N}}^{‡}\left(\mathrm{E}-{\mathrm{E}}_0\right)}{\mathrm{hp}\left(\mathrm{E}\right)}$$

where σ is the reaction degeneracy, h is the Planck's constant, E ₀ is the 0 K activation energy of the reaction, $N^{‡}$ is the number of vibrational states for the transition state, and $p\left(E\right)$ the density of vibrational states for the reactant, both calculated using the Beyer and Swinhart direct count algorithm [10].

4. Results and Discussion

The breakdown diagrams and representative CID mass spectra for ions 1–5 are shown in Figure 2. Ion 1 was found to lose only 44 Da, corresponding to CO₂, forming the CF₃CF₂ ⁻ anion (m/z 119), Figure 2A. The modest E _com for this process is consistent with the facile loss of CO₂ found from thermal degradation studies of perfluorooctanoic acid [1]. Ion 2 had two breakdown products, m/z 83 and m/z 63 (Figure 2B). The former is the loss of CO₂ to form CF₃CH₂ ⁻, while m/z 63 is FCO₂ ⁻ (loss of CF₂CH₂). Alternatively, m/z 63 could result from HF loss from m/z 83, but the breakdown diagram is not consistent with a sequential reaction, but rather two competing parallel reactions from the precursor ion having similar energy onsets. The relative abundance of the two channels is reversed from that found by Lee et al. [4] presumably due to a different collision energy regime.

FIGURE 2.

Breakdown curves and representative mass spectra for 1–5. The representative mass spectra were acquired at E_lab = 9 eV (A), E_lab = 10 eV (B), E_lab = 7 eV (C), E_lab = 11 eV (D), E_lab = 24 eV (E).

Ion 3 forms a major product with m/z 119, but there is a discernible signal with m/z 129 in the spectra (Figure 2C). The fragment ion with m/z 119 can only be the CF₃CF₂ ⁻ anion, resulting from the loss of formaldehyde (CH₂O) from the precursor ion, while m/z 129 must be due to the loss of HF. Interestingly, the production of CH₂O from neutral PFAS has also been found in the gas phase kinetics literature [11]. Ion 4 produces the most complex unimolecular behavior featuring four fragment ions with m/z 95, 93, 69, and 63 (Figure 2D). Loss of water can lead to CF₃CHCH⁻ (m/z 95) and loss of HF to CF₂CHCOH₂ ⁻ (m/z 93). CF₃ ⁻ (m/z 69) and CF₂CH⁻ (m/z 63) appear to be sequential reaction products (based on their higher onset energies) that likely originate by the loss of C₂H₂ from m/z 95 and CH₂O from m/z 93. Ion 5 formed FSO₃ ⁻ (m/z 99), SO₃ ⁻ (m/z 80), and CF₃ ⁻ (m/z 69) (Figure 2E). The removal of SO₃ has also been shown from perfluorooctane sulfonate due to an oxidation breakdown method [1].

Each of these systems has been computationally explored to derive minimum energy reaction pathways (MERPs). The reaction pathway for the loss of CO₂ from 1 (Figure 3) shows a reaction with no reverse barrier and an endothermicity of 1.39 eV.

FIGURE 3.

Energy pathway showing the dissociation of 1 into CO₂ and the pentafluoroethyl anion.

The calculated MERPs for 2 are shown in Figure 4. Again, loss of CO₂ was found to have no reverse energy barrier, and the calculated threshold was 1.49 eV. The assignment of m/z 63 to FCO₂ ⁻ is supported by the competitive reaction leading to F‐transfer, Figure 4, and previous IRMPD experiments [4]. The sequential loss of HF from m/z 83 is too high in energy (Figure 4). The competition between forming m/z 83 and m/z 63 is governed by their relative activation entropies (Δ^‡S (m/z 83) = 64 J K⁻¹ mol⁻¹ compared to 32 J K⁻¹ mol⁻¹ for m/z 63), Figure 4B. Lee et al. employed a linear ion trap and found m/z 63 to be the dominant fragment ion [4], which is only consistent with their experiment accessing lower internal energy ions than the current one.

FIGURE 4.

(A) Minimum energy reaction pathway for the dissociation of 2 and (B) the RRKM k(E) vs. E curves for forming m/z 63 and m/z 83. The greater Δ^‡S for m/z 83 means this channel becomes predominant with increasing internal (and thus collision) energy.

Ion 3 loses HF in a minor process to form CF₃CFCHO⁻ (m/z 129) and CH₂O to form CF₃CF₂ ⁻ (m/z 119), Figure 5, the dominant product observed in Figure 2. The lower energy HF‐loss products are formed over the high‐energy transition state 3TS‐A1.

FIGURE 5.

Energy pathway showing the dissociation of 3.

The MERPs for 4 are shown in Figure 6. Water loss to form m/z 95 (Figure 6B, orange) is preceded by a 1,3‐H transfer to the O atom, followed by a 1,2‐H shift leading to H₂O. Competing with this reaction at low E _com is HF loss (making m/z 93), Figure 6A (gray). The fact that m/z 95 and 93 are formed competitively, but with m/z 95 at a higher abundance at lower collision energy, is due to the lower relative energy of 4TS‐B1. 4TS‐A1 is less entropically hindered, though, so as internal energy increases, the rate constant for forming m/z 93 outcompetes that for m/z 95, and the former ion becomes the dominant reaction observed in the CID BD.

FIGURE 6.

Energy pathways showing the dissociation of 3.

Subsequent reaction can form CF₃ ⁻ (m/z 69) + C₂H₂ (Figure 6B, purple) from m/z 95. CF₂CH⁻ (m/z 63) can be formed by CH₂O loss from m/z 93 (Figure 6A, blue).

Ion 5 formed FSO₃ ⁻ (m/z 99), SO₃ ⁻ (m/z 80), and CF₃ ⁻ (m/z 69), Figure 7, all requiring ~ 3 eV or greater to be formed, which is consistent with the higher E _com needed to observe these reactions compared to the other four precursor ions (Figure 2E). SO₃ ⁻ (m/z 80) and CF₃ ⁻ (m/z 69) represent the same bond cleavage reaction, with the charge preferentially residing on the product with higher electron affinity (2.064 eV [12] for SO₃ ^• and 1.82 eV [13] for CF₃ ^•). FSO₃ ⁻ (m/z 99) is also observed in low abundance due to the lower Δ^‡S for this reaction caused by the F‐transfer transition state 5TS‐B1.

FIGURE 7.

Energy pathway showing the dissociation of 5.

5. Conclusion

The four oxygenated PFAS anions 1–4 exhibited the reactions of CO₂, HF, and CH₂O loss, similar to what has been observed in the thermal degradation of such species [1, 12]. There was qualitative agreement between the observed E _com onset energies for fragmentation of the ions and the corresponding calculated energy requirements. For the acids, increasing hydrogen content increased the number of competing reactions beyond simple CO₂ loss. This was also true for the alcohols, with the pentafluorinated 3 primarily losing formaldehyde, whereas the trifluorinated 4 exhibited two major dissociation channels, producing HF and water. Thus, it may be possible to gain insight into thermal degradation processes and products of PFAS by examining the tandem mass spectra of the corresponding anions.

Radnoff A., Charpentier‐St‐Pierre S., and Mayer P., “On the Unimolecular Breakdown Products of Short Deprotonated Per‐ and Poly‐Fluorinated Acids and Alcohols,” Journal of Mass Spectrometry 60, no. 10 (2025): e5175, 10.1002/jms.5175.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.