DOI: 10.5061/dryad.d51c5b0cr
title: Data from: Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry

description: Thermal destruction is a critical cornerstone of addressing the rampant
 contamination of natural resources with per- and polyfluoroalkyl
 substances (PFAS). However, grave concerns associated with stack emissions
 from incineration exist because mechanistic studies have thus far relied
 on ex-situ analyses of end-products and theoretical calculations. In this
 manuscript, we used synchrotron-based vacuum ultraviolet photoionization
 mass spectrometry to study the pyrolysis of a representative PFAS –
 perfluorohexanoic acid, and for the first time provide direct evidence of
 fluorocarbon radicals and intermediates. A key reaction pathway from
 perfluorocarboxylic acids to ketenes via acyl fluorides is proposed. We
 furthermore propose CF2/CF3-radical-centered pyrolysis mechanisms and
 explain their roles in the formation of other products which may form in
 full-scale incinerators. These results have not only unveiled the role of
 radicals and intermediates in thermal PFAS decomposition and recombination
 mechanisms, but also provide unique insight into improving the safety and
 viability of industrial PFAS incineration.
description: The data for this experiment were collected using the synchrotron
 vacuum ultraviolet photoionization mass spectrometer at the National
 Synchrotron Radiation Laboratory, Hefei, China. The entire analytical
 detection process consists of four main steps: vaporization of the
 reactant, thermal decomposition, photoionization, and reflectron
 time-of-flight mass spectrometry (Re-TOF-MS) analysis. During operation,
 0.3% PFHxA vapor was seeded in argon carrier gas, passing through an
 alumina laminar flow tube with a temperature range from 400 to 975 °C. The
 residence time is calculated to be around 10 milliseconds as molecules
 traverse through the flow tube, where the high temperature caused thermal
 decomposition of PFHxA molecules. Successively, the molecules expanded
 into vacuum to generate a supersonic molecular beam. The beam containing
 all thermal decomposition intermediates and products then entered the
 interaction region and was intersected by VUV photons from the
 synchrotron.
description: # Direct measurement of fluorocarbon radicals in the thermal destruction
 of perfluorohexanoic acid using photoionization mass spectrometry
 [https://doi.org/10.5061/dryad.d51c5b0cr](https://doi.org/10.5061/dryad.d51c5b0cr) ## **Dataset overview** This file contains all experimental data for the manuscript "Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry", *Sci. Adv.*, 2025, DOI: 10.1126/sciadv.adt3363, by M.-G. Xu, C. Huang, L. Zhao, A. K. Rappé, E. M. Kennedy, M. Stockenhuber, J. C. Mackie, N. H. Weber, J. A. Lucas, M. Ahmed, J. Blotevogel, and W. Lu. This repository includes (1) an OriginLab file for plotting Figures 2, 3, and 5 in the main text, and Supporting Figures S2, S3, S4, and S5; and (2) all raw ASCII data files collected for this experiment, including folder "`C6F12O`" (perfluorohexanoyl fluoride) used to draw Figure S5; "`difluoroacetic acid`" used to draw Figure S4; "`hydration products confirmation`" the interplay between water content and the hydrolysis products signal intensity; "`PFHxA`" and "`PFHxA+H2O`" the data for the main body of the experiment. The values are signal count numbers at each pixel and were used to draw Figures 2, S3, S4, and S5. Integrated peak areas that were derived from these files were used to draw Figures 3 and S2. For the x-axis (mass-to-charge ratio), follow y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in each folder, x is the number of row (pixel), and y is the mass-to-charge ratio. ### **Corresponding author information** Name: Long Zhao; Wenchao Lu ORCID: 0000-0002-6585-0028; 0000-0002-3798-5128 Affiliation: University of Science and Technology of China; CSIRO Environment Email: [zhaolong@ustc.edu.cn](mailto:zhaolong@ustc.edu.cn); [wenchao.lu@csiro.au](mailto:wenchao.lu@csiro.au) ### Funding information This work is supported by the U.S. Department of Defense’s Strategic Environmental Research and Development Program (SERDP) under Projects ER21-1019, ER24-4068, and ER24-4073. MGX, CH, and LZ are supported by the Combustion Beamline at the National Synchrotron Radiation Laboratory, Hefei, China, and by the National Natural Science Foundation of China via contract number 12205302. MA is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, through the Gas Phase Chemical Physics program of the Chemical Sciences Division. WL also acknowledges travel funding (AS/IA233/21635) provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron, part of ANSTO, and funded by the Australian Government. ### Detailed Description of "*Figure\_Data.opju*" `├── Fig_2` Mass spectra for the pyrolysis of PFHxA. From top to bottom: (**A**) Mass spectrum collected at 11.0 eV, 400 °C showing the background of the system; (**B**) mass spectrum at 11.0 eV, 950 °C showing the thermal decomposition products marked in red; (**C**) mass spectrum at 14.0 eV, 400 °C showing the photodissociation fragments of reactant PFHxA, marked in yellow; (**D**) mass spectrum at 14.0 eV, 950 °C showing all thermal decomposition products marked in red against background in gray; and (**E**) mass spectrum at 14.0 eV, 950 °C with the addition of equal amount of water vapor to PFHxA. Additional species are marked in blue, in comparison with pyrolysis products (pale red peaks) and background (gray peaks). All identifiable pyrolysis products were labeled and characterized by *m*/*z* and ionization energies. `├── Fig_2 - Copy` This is a duplicate for Fig_2 but removing most texts, for replotting according to the requirements of the journal. `├── Fig_2_data` This is the raw data workbook for plotting all 5 frames of Fig_2. `├── Fig_3` Temperature dependence curves for each pyrolysis product and some background species. (**A**) Temperature dependence curves (temperature versus integrated peak intensity) for fluorocarbon-based products (CF2, CF3•, CF2CO, CnF2n (n = 2 – 3) and CnF2n+1• (n = 2 – 4)) at 13.0 eV, and **(B)** carbon oxides and C4F9COF at 14.0 eV (CO at 14.7 eV). Background gases (O2) and photodissociation products (C5F10 and •COOH) are shown in **(C)**; (**D**) Temperature dependence curves with the addition of equal amount of water vapor to PFHxA for fluorocarbon-based products (CF2, CF3•, CF2CO, CnF2n (n = 2 – 3), CnF2n+1• (n = 2 – 4)) at 13.0 eV, and **(E)** carbon oxides, C4F9COF, and emerging hydrolysis products at 14.0 eV (CO at 14.7 eV). The values at 400 °C from pyrolysis results are used as the baseline. Background gases (O2) and photodissociation products (C5F10 and •COOH) are shown in **(F)**. C5F10 from pyrolysis and hydrolysis are normalized based on the value at 700 °C and 13.0 eV, referring to the same injection amount. The ratio between dry and hydrolysis conditions is also shown in **(F)**. `├── Fig_3 - Copy` This is a duplicate for Fig_3 but removing most texts, for replotting according to the requirements of the journal. `├── Fig_3_data` This is the raw data workbook for plotting all 6 frames of Fig_3. `├── Fig_5` Temperatures for 50% progress of each decomposition pathway within 10 ms (T50). Calculated free energies of activation as a function of temperature for α-HF elimination (blue solid line), β-HF elimination (blue short dash), and β C-C bond cleavage for PFHxA (green solid line). The black line is the maximum free energy of activation required to achieve 50% destruction in 10 milliseconds of residence time (t50). The temperatures at the crossing points of the respective lowest free energy of activation and the black line represent the T50. `├── Fig_5_data` This is the raw data workbook for plotting Fig_5. `├── Fig_S2_PIE (plot)` Ionization threshold measurements and photoionization efficiency curves for all thermal products from the pyrolysis of PFHxA in this study. `├── Fig_S2_PIE (workbook)` This is the raw data workbook for plotting Fig_S2. `├── Fig_S3` Mass spectrum collected for PFHxA at 18.0 eV and 950°C, indicating the absence of HF, F, and F2. The peak of nominal mass 40 is argon (*m*/*z* = 39.962). `├── Fig_S3&MS data` This is the raw data workbook for plotting Fig_S3. `├── Fig_S4` Mass spectrum for the pyrolysis products of difluoroacetic acid, CF2HCOOH (*m*/*z* = 96), recorded at 13.0 eV and various temperatures, indicating no water elimination product CF2=C=O observed. `├── Fig_S4_data` This is the raw data workbook for plotting Fig_S4. `├── Fig_S5` Mass spectrum for the pyrolysis products of perfluorohexanoyl fluoride, C5F11COF (*m*/*z* = 316), recorded at 13.0 eV and various temperatures, highlighting the formation of CF2=C=O via pyrolysis. `├── Fig_S5_data` This is the raw data workbook for plotting Fig_S5. ### Detailed description of "*raw data.7z*" The tree structure for the primary and secondary directories is shown bellow (files not shown): `raw data` `├── C6F12O` `│   └── PIE13.8-14.1` `├── difluoroacetic acid` ` │   └── PIE10.9-11.5` ` ├── hydration products confirmation` `│   ├── 20240710` `│   └── C6F11O2H-before chamber cleaning` `├── PFHxA` `│   ├── Control` `│   ├── PIE(10-12)` `│   ├── PIE(10-12) rescan from 11.65 to 11.85` `│   ├── Product Confirmation - 230727` `│   ├── STABILITY` `│   └── Temperature Scan` `└── PFHxA+H2O` `    ├── PIE(9-12eV)` `    ├── Stability Test` `    └── Temperature Scan` A detailed description of each of the five sub-directories is provided below: ###### Folder "C6F12O" `├── C6F12O` `    ├── PIE13.8-14.1` `    │   ├── ...` `    ├── 11.0eV-120s-C6F12O-850C.888` `    ├── 11.0eV-120s-C6F12O-850C.asc` `    ├── ...` `    ├── C6F12O stability.PNG` `    └── GUI Translation.PNG` This folder contains all raw mass spectrum ASCII data of the pyrolysis of perfluorohexanoyl fluoride (C6F12O). The two .PNG files are the screenshots of the graphic user interface (GUI) indicating the stability of the signal, together with a translation for the labels written in simplified Chinese. The folder "`PIE13.8-14.1`" records the photoionization efficiency measurements from 13.8 to 14.1 eV. The data were used to draw Figure S5 in the main text. For the x-axis (mass-to-charge ratio), follow y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in the .opju file, x is the number of row (pixel), and y is the mass-to-charge ratio. ###### Folder "difluoroacetic acid" `├── difluoroacetic acid` `    ├── PIE10.9-11.5` `    │   ├── ...` `    ├── 12.5eV-120s-180C-blank.888` `    ├── 12.5eV-120s-180C-blank.asc` `    ├── ...` `    ├── CF2HCOOH - including x-axis.opju` `    └── GUI Translation.PNG` This folder contains all raw mass spectrum ASCII data of the pyrolysis of difluoroacetic acid (CF2HCOOH). The folder "PIE10.9-11.5" records the photoionization efficiency measurements from 10.9 to 11.5 eV. The data were used to draw Figure S4 in the main text. For the x-axis (mass-to-charge ratio), follow y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in the .opju file, x is the number of row (pixel), and y is the mass-to-charge ratio. ###### Folder "hydration products confirmation" (files not shown) `├── hydration products confirmation` `    ├── 20240710` `    │   ├── addition of water` `    │   ├── after baking the chamber` `    │   ├── comparison` `    │   ├── dry (trace water remained)` `    │   ├── GUI Translation.PNG` `    │   └── x-axis coefficients.txt ` `    └── C6F11O2H-before chamber cleaning` `        ├── ...` This folder contains all raw mass spectrum ASCII data for the confirmation of new pyrolysis products after the addition of water vapor. It includes two sub-directories, "`20240710`" and "`C6F11O2H-before chamber cleaning`". There are four sub-directories in "`20240710`". In "`addition of water`", mass spectra were collected by double and quadruple the injection amount of water vapor. Then the machine chamber was baked overnight, and the products were measured the next morning while all mass spectrum files are stored in "`after baking the chamber`". The comparison between before baking and after baking the chamber was summarized in the folder "`comparison`". The "`dry (trace water remained)`" folder contains the mass spectrum files with water vapor removed from the system (although trace water may remain). The PNG file is the screenshot of the graphic user interface (GUI) showing the translation for the labels written in simplified Chinese. The file "`x-axis coefficients.txt`" records mass spectrum coefficients for the x-axis (mass-to-charge ratio) calibration, follow y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in this file, x is the number of row (pixel), and y is the mass-to-charge ratio. The folder "`C6F11O2H-before chamber cleaning`", records an invalid set of water addition experiments for the confirmation of new products. The vacuum chamber was contaminated, and only very low signals were observed.  ###### Folder "PFHxA" (files not shown) `├── PFHxA` `    ├── Control` `    │   ├── 400` `    │   └── 925` `    ├── PIE(10-12)` `    ├── PIE(10-12) rescan from 11.65 to 11.85` `    ├── Product Confirmation - 230727` `    │   ├── PIE(9.0-10.05)` `    │   ├── PIE(9.2-11.6)` `    │   ├── PIE(9.5-10)` `    │   └── stability` `    ├── STABILITY` `    └── Temperature Scan` `        ├── 11.0eV` `        ├── 11.5eV` `        ├── 12.0eV` `        ├── 12.5eV` `        ├── 13.0eV` `        ├── 13.5eV` `        ├── 14.0eV` `        └── 14.7eV` This folder contains all raw mass spectrum ASCII data of the main part of this research, the pyrolysis experiments of perfluorohexanoic acid (PFHxA). It contains 5 sub-directories:  The folder "`control`" provides the mass spectra that compare the initiation temperature of the pyrolyzer (400°C) vs. high temperature at 925°C, to have a quick view of pyrolysis products. The folders "`PIE(10-12)`" and "`PIE(10-12) rescan from 11.65 to 11.85`" record the photoionization efficiency measurements from 10.0 to 12.0 eV, and a finer scan from 11.65 to 11.85 eV. The folder "`Product Confirmation - 230727`" measures the mass spectra at high photon energies to confirm the formation of high-ionization-energy species such as hydrogen fluoride (ionization energy = 16.00 eV). However, no formation of HF was found, and the resulting mass spectrum was plotted in Figure S3. The folder "`STABILITY`" provides a series of stability test experiments at each experimental temperature, from 400 to 975°C. The results were recorded as screenshots (.PNG files), together with a translation for the labels written in simplified Chinese. The folder "`Temperature Scan`" contains mass spectrum files recorded at 400°C and from 700 to 975 °C for every 50°C. Different photon energies from the synchrotron were used, from 11.0 to 14.7 eV for every 0.5 eV. Each file contains the information, including the photon energy, photocurrent, undulator settings, collection time, pyrolyzer position, and temperature. Lastly, the file "`blank - chamber pressure.txt`" provides the vacuum chamber pressure data during the experiment, and the file "`x-axis coefficients.txt`" records mass spectrum coefficients for the x-axis (mass-to-charge ratio) calibration, following y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in this file, x is the number of row (pixel), and y is the mass-to-charge ratio. ###### Folder "PFHxA+H2O" (files not shown) `└── PFHxA+H2O` `    ├── PIE(9-12eV)` `    ├── Stability Test` `    └── Temperature Scan` `        ├── 11.0eV` `        ├── 11.5eV` `        ├── 12.0eV` `        ├── 12.5eV` `        ├── 13.0eV` `        ├── 13.5eV` `        ├── 14.0eV` `        └── 14.7eV` This folder contains all raw mass spectrum ASCII data of the main part of this research, the pyrolysis experiments of perfluorohexanoic acid (PFHxA) with the addition of water vapor. It contains 3 sub-directories:  The folder "`PIE(9-12eV)`" records the photoionization efficiency measurements from 9.0 to 12.0 eV. The folder "`Stability Test`" provides a series of stability test experiments at each experimental temperature, from 700 to 975°C. The results were recorded as screenshots (.PNG files), together with a translation for the labels written in simplified Chinese. The folder "`Temperature Scan`" contains mass spectrum files recorded from 700 to 975 °C for every 50°C. Different photon energies from the synchrotron are used, from 11.0 to 14.7 eV for every 0.5 eV. Each file contains the information, including the photon energy, photocurrent, undulator settings, collection time, pyrolyzer position, and temperature. Lastly, a file "`x-axis coefficients.txt`" records mass spectrum coefficients for the x-axis (mass-to-charge ratio) calibration, following y = A + Bx + Cx^2, where A, B, and C are coefficients that are provided in this file, x is the number of row (pixel), and y is the mass-to-charge ratio.