Revolutionizing PFAS Air Detection

Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals extensively used in various industrial and consumer products due to their water- and oil-resistant properties.¹ Despite being manufactured since the 1940s, these substances have only recently garnered global attention because of their detrimental effects on human health.

Known as “forever chemicals” for their persistence and bioaccumulation, PFAS have been detected in air, oceans, soils, surface waters, and even in remote Arctic regions.² Additionally, high indoor concentrations have been detected, representing a significant human exposure pathway.³

According to the US Environmental Protection Agency’s CompTox database, nearly 15,000 synthetic PFAS chemicals exist. The rise of new PFAS and next-generation PFAS-like compounds presents significant challenges for environmental scientists and regulators.

Keeping up with new sources, developing dedicated analytical methods, and understanding their toxicological effects is difficult. Consequently, most of these substances are not yet regulated, especially considering their direct measurement in ambient air.

The low concentration of PFAS complicates their analysis, requiring highly sensitive detection methods. Traditional gas-phase sampling methods rely on offline analytical techniques involving passive or active air sampling devices that collect samples onto filters or sorbents for subsequent laboratory analysis.^[4]

However, these methods have limitations, such as long collection periods and time-intensive sample analysis, which hinder the ability to address issues directly at the source. Furthermore, offline methods typically only detect known compounds, limiting their ability to identify new contaminants.

Real-time measurement techniques, allowing for the detection of a wide variety of compounds, can provide insights into the release, sources, and transport mechanisms of these pollutants.

Direct PFAS Analysis via Chemical Ionization

Chemical ionization (CI) using iodide as a reagent ion has recently shown promise for real-time PFAS detection in air.^[4–6] The TOFWERK Vocus Aim Reactor (adduct ionization mechanism) uses a soft ionization approach to minimize fragmentation and preserve the parent molecular ion.

Coupled with a TOFWERK time-of-flight mass spectrometer, this method enables accurate assignment of molecular formulas with a one-second time resolution and unprecedented sensitivity.

The study discussed here demonstrates the Vocus Aim Reactor’s applicability for real-time quantitative PFAS detection in air, aiming to introduce this innovative technique to a broader audience beyond the scientific community. Unlike traditional methods, this technique directly analyzes air without the need for sample collection or pre-separation.

Reliable calibration is essential for quantitative data. Chemical ionization can be calibrated using a multi-component gas cylinder for compounds with high vapor pressure and limited reactivity. Alternatively, certified permeation tubes with known permeation rates can be employed; however, these standards have limited availability.

For less volatile molecules, solutions of known concentration can be prepared and evaporated using commercially available liquid calibration systems (LCS), though these systems have surfaces where low volatile molecules can adhere, causing long response times and potential permanent contamination from toxic compounds like PFAS.^[7]

The objective of this study was to demonstrate a standardized and robust calibration method, including calibration factors, detection limits, and humidity dependencies for emerging contaminants, and compare the method to a recent calibration approach.^[3]

Two ion chemistries in the Vocus Aim Reactor, iodide and nitrate, were also evaluated, extending this approach to other emerging contaminants like pesticides, which pose similar challenges in gas-phase measurement and calibration due to high toxicity and contamination issues.

Experimental

Calibration measurements were performed using a Vocus 2R equipped with an Aim Reactor at a measuring frequency of 0.5 Hz.^[8[ The reactor operated at 50 mbar and 50 °C for both iodide and nitrate ion chemistries.

To investigate solvent dependency, liquid calibration standards of PFAS were prepared in methanol, methyl acetate, and dichloromethane at concentrations ranging from 0.2 to 4 mg L^-1.

A 250 μL glass syringe (Hamilton) and a syringe pump (KD Scientific) were used to compare single and continuous injections of liquid standards, assessing precision, stability and reproducibility.

Samples were introduced into a 130 °C heated 0.5-inch OD Sulfinert tubing at a 90° angle through a GC septum into the injection point. Tubing was encapsulated with a heating element, allowing for easy sonication or replacement of the tubing itself if required. The injector was continuously flushed with two sLPM of UHP N₂, with excess flow directed to the exhaust.

Figure 1 outlines the experimental setup used during the calibration, which was performed in both iodide and nitrate modes.

Figure 1. Experimental set up used in the calibration of the PFAS. Image Credit: TOFWERK

Table 1. Iodide CIMS emerging contaminants calibration summary. All calibration factors are reported in counts per second (cps) per part per trillion by volume (pptv) and normalized per million counts of total reagent ion signal. Source: TOFWERK

Results

Table 1 presents calibration factors and limits of detections (LODs) for two fluorotelomer alcohols (FTOHs), 11 perfluorinated carboxylic acids (PFCAs), pentachlorophenol and 4, 4 DDT, measured in nitrogen and ambient air to assess potential interferences due to the dilution medium.

The 1 Hz LODs were within 10 % between matrices and were in the low parts-per-trillion (ppt) range. By increasing the average time to one minute, the LODs were reduced significantly.

As shown in Figure 2, calibration curves for selected PFAS using iodide as the reagent ion and the syringe pump method showed good linearity (R² > 0.98) even at lower ppt levels, indicating the AIM reactor’s excellent detection capabilities.

Figure 2. TOFWERK iodide Aim calibration curves for selective PFAS. Image Credit: TOFWERK

As shown in Figure 3, sensitivity measurements for PFCAs using NO₃^- as the reagent ion were generally within 20 % of those obtained with the iodide mode, proving nitrate’s effectiveness in detecting this subgroup of PFAS. In both cases, sensitivity decreased with increasing PFCA molecule size, likely linked to their efficient gas-phase transfer.

Figure 3. Measured PFCAs sensitivities using iodide CIMS (red full circles) and in nitrate CIMS (blue open circles). Image Credit: TOFWERK

The comparison of single syringe injections and continuous syringe pump injections is shown in Figure 4. Generally, the two methods agreed within 30 % for the volatile fraction of the PFAS; however, high variability in the signal response was observed for some molecules using single injections of the same volume and concentration, as demonstrated in Figure 4c.

Despite the simplicity and cost-effectiveness of direct injection, syringe pump injections produced more consistent and accurate results in response to changes in injected flow rates, mitigating operator-induced errors. As a result, all further results rely on the syringe pump method.

Figure 4. Comparison between direct injections (a & c) and syringe pump approach (b & d) for 6:2 FTOH and selected PFCAs. For manual injections, the same volumes of solution with increasing concentration were used. For syringe pump injection one concentration solution with varying injection rates was used. Calculated mixing ratios of 6:2 FTOH in the air are highlighted by blue shaded area. Image Credit: TOFWERK

Understanding how sensitivity changes with varying humidity levels is crucial for accurate measurements in environments with fluctuating relative humidity (RH), like ambient air. Water can significantly impact the sensitivity of species detected by CI, with RH-dependent sensitivities varying across compound classes. As a result, measurements in environments with fluctuating humidity require many time-consuming calibrations.

Using a water vapor control system with a regulated flow of 5 sccm acetonitrile as the dopant molecule, the Vocus Aim Reactor mitigates water dependence. The dopant displaces water molecules that would usually attach to the reagent ions, ensuring that the modified reaction mechanism does not depend strongly on humidity.

Figure 5a displays the change in relative sensitivity for selected PFAS as a function of increasing humidity while using dopant, demonstrating a minute decreasing trend. However, the interpretation of this trend is complicated by the inherent measurement error and reproducibility issues. In addition, sensitivity variation increases with increasing humidity levels, further complicating this trend.

Figure 5b shows the impact of matrix solvent selection on the sensitivity of PFAS detection. A significant deviation with dichloromethane, attributed to its higher volatility, was observed. Differences between methanol and ethyl acetate were minimal, the latter being preferable due to its lower toxicity.

Figure 5. a) Sensitivity normalized to dry conditions as a function of increasing humidity in relative (25 °C) and absolute values. The error bars represent the standard deviation, calculated from nine measurements conducted on different days. b) Sensitivity differences for various solvents. Image Credit: TOFWERK

Conclusion

The Vocus Aim Reactor demonstrates real-time detection and quantification of PFAS and offers temporal resolution exceeding that of traditional offline techniques. With sensitivities ranging from 0.5 to 5 ncps/ppt and detection limits down to hundreds of ppq, this method provides high sensitivity and specificity.

Despite the detection limits exceeding the typical requirements for background monitoring of ambient air, the speed and precision of the Vocus Aim Reactor make it perfectly suited for real-time identification of volatile PFAS sources or locating potential leaks where concentrations are much higher.

The Vocus Aim Reactor is suitable for measurements in environments with fluctuating humidity and has the potential to enhance understanding of PFAS environmental pathways through atmospheric chamber experiments, consumer product evaluation, indoor air monitoring, and material emission testing. Additionally, it can facilitate regulatory monitoring of flue gases to ensure compliance with emission standards.

Acknowledgments

Produced from materials originally authored by Spiro Jorga and Veronika Pospisilova from TOFWERK.

References and Further Reading

1. Glüge, J., et al. (2020). An overview of the uses of per- and polyfluoroalkyl substances (PFAS). Environ. Sci. Process. Impacts, 22, pp.2345–2373. https://doi.org/10.1039/D0EM00291G
2. Evich, M. G., et al. (2022). Per- and polyfluoroalkyl substances in the environment. Science, 375, eabg9065. https://doi.org/10.1126/science.abg9065
3. Davern, M. J., et al. (2024). External liquid calibration method for iodide chemical ionization mass spectrometry enables quantification of gas-phase per- and polyfluoroalkyl substances (PFAS) dynamics in indoor air. The Analyst, 10.1039.D4AN00100A. https://doi.org/10.1039/D4AN00100A
4. Barber, J. L., et al. (2007). Analysis of per- and polyfluorinated alkyl substances in air samples from Northwest Europe. J. Environ. Monit., 9, pp.530–541. https://doi.org/10.1039/B701417A
5. Riedel, T. P., et al. (2019). Gas-Phase Detection of Fluorotelomer Alcohols and Other Oxygenated Per- and Polyfluoroalkyl Substances by Chemical Ionization Mass Spectrometry. Environ. Sci. Technol. Lett., 6, pp.289–293. https://doi.org/10.1021/acs.estlett.9b00196
6. Bowers, et al. (2023). Evaluation of iodide chemical ionization mass spectrometry for gas and aerosol-phase per- and polyfluoroalkyl substances (PFAS) analysis. Environ. Sci. Process. Impacts, 25, pp.277–287. https://doi.org/10.1039/D2EM00275B
7. Mattila, J. M., et al. (2023). Tubing material considerably affects measurement delays of gas-phase oxygenated per- and polyfluoroalkyl substances. Journal Of The Air & Waste Management Association, 73(5), pp.335–344. https://doi.org/10.1080/10962247.2023.2174612
8. Riva, M., et al. (2024). Evaluation of a reduced pressure chemical ion reactor utilizing adduct ionization for the detection of gaseous organic and inorganic species. EGUsphere. https://doi.org/10.5194/egusphere-2024-945

This information has been sourced, reviewed and adapted from materials provided by TOFWERK.

For more information on this source, please visit TOFWERK.