Using the Vocus Aim Reactor to Detect PFAS in the Air

^{Per- and poly-fluoroalkyl substances (PFAS) are synthetic chemicals that are commonly used in a range of consumer and industrial products due to their water- and oil-resistant properties.1}

Although initial production of these chemicals began way back in the 1940s, they have attracted international attention in the last ten years due to their negative effects on human health and the environment.

The molecules, which have been branded as “forever chemicals” due to their opposition to natural breakdown and their ability to bioaccumulate, have been found in the air, soils, surface waters, and oceans as far as the Arctic.² 

Furthermore, elevated concentrations of PFAS have been detected indoors, representing an important human exposure pathway.³

The chemical database maintained by the United States Environmental Protection Agency states that there are almost 15,000 synthetic PFAS chemicals. The emergence of new PFAS alongside the release of next-generation PFAS-like compounds are substantial obstacles for environmental scientists and regulators.

Keeping pace with new sources, developing dedicated analytical methods, and understanding their toxicological effects is challenging. As a result, the majority of these substances are not yet subject to regulatory monitoring under current environmental laws, especially considering their direct measurement in ambient air.

The low concentration of such compounds makes analysis complicated; assessment requires highly sensitive investigative approaches.

Traditional techniques for gas-phase sampling have primarily relied on offline analytical approaches, which involve the use of active or passive air-sampling devices that collect samples onto filters and/or sorbents with subsequent laboratory analysis.⁴

While important, such techniques have significant limitations in directly addressing the issue at the source.

For instance, the lengthy duration of sample collection combined with the labor-intensive process of analyzing these samples makes it challenging to fully grasp the origins and behavior of air pollutants.

Moreover, offline methods usually focus on detecting specific, pre-identified compounds, which restricts their capacity to uncover new or emerging pollutants.

Real-time measurement techniques can be used to better understand how these contaminants are released into the air and to explore their sources and transport mechanisms. These techniques enable the detection of a broad spectrum of compounds, providing more comprehensive insights.

Direct PFAS Analysis using Chemical Ionization

Chemical ionization using iodide as a reagent ion has recently materialized as a capable technique for the detection of PFAS in air in real time.^4–6 The versatile TOFWERK Vocus Aim Reactor (VAR) and its adduct ionization mechanism uses a soft ionization approach, diminishing fragmentation and preserving the parent molecular ion.

When coupled with a TOFWERK time-of-flight mass spectrometer, the approach supports an accurate and sensitive assignment of molecular formulas of the detected ions with a time resolution of one second.

This study demonstrates the ability of the VAR for real-time quantitative PFAS detection in air and aims to showcase this innovative technique to a broad audience beyond the scientific community. The technique directly analyzes air without the need for sample collection or pre-separation, distinguishing it from traditional methods.

To achieve accurate quantitative measurements, a dependable calibration method for PFAS is crucial. Typically, chemical ionization can be calibrated using a multicomponent gas cylinder for compounds with sufficient vapor pressure and minimal reactivity.

Alternatively, certified permeation tubes with known permeation rates are an option, though these standards are not always readily available.

For less volatile molecules, known concentration solutions can be prepared and vaporized using commercial liquid calibration systems (LCS). However, these systems often have extensive surfaces where low volatility molecules can adhere, leading to prolonged response times and potential permanent contamination from toxic substances like PFAS.⁷

To overcome these challenges, the goal is to establish a standardized and reliable calibration method, including factors such as calibration coefficients, detection limits, and humidity effects for these emerging contaminants, and to compare this method with a recently developed calibration approach.³

Additionally, two ion chemistries were assessed in the Vocus Aim Reactor: iodide and nitrate. This methodology is also extended to other emerging contaminants, such as pesticides, which present similar challenges in gas-phase measurement and calibration due to their high toxicity and contamination issues.

Experimental

Calibration measurements were conducted using a Vocus 2R with an Aim Reactor, operating at a measurement frequency of 0.5 Hz.⁸ The device was operated at 50 mbar and a temperature of 50 °C for chemistries of both iodide and nitrate ions.

Liquid calibration standards of PFAS were prepared in methyl acetate, methanol, and dichloromethane, with concentrations ranging from 0.2 to 4 mg L^⁻¹, to examine potential solvent dependency in the results.

To assess the differences between single and continuous liquid standard injections, a 250 μL glass syringe (Hamilton) and syringe pump (KD Scientific) were employed.

The injection rates tested ranged from 20 μL to 800 μL per hour. Single injections involved the one-time introduction of a specific volume of the liquid standard, while continuous injections sustained a steady flow of the liquid standard over time.

The comparison was designed to determine which method provided more consistent and accurate tuning results by assessing stability, precision, and reproducibility.

The sample was introduced into a half-inch OD Sulfinert tubing, heated to 130 °C at an angle of 90 degrees through a GC septum into the injection point. The tubing had a heating element which allowed for easy sonication or the replacement of the tubing itself if necessary.

The injector was continuously flushed with 2 sLPM of UHP N₂ and excess flow was directed to the exhaust. Figure 1 displays the experimental setup. As the Aim Reactor allows switching between reagent ions, a subset of the chemicals was calibrated in both nitrate and iodide 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

Using the calibration setup described earlier, Table 1 presents the calibration factors and limits of detection (LODs) for two fluorotelomer alcohols (FTOHs), eleven perfluorinated carboxylic acids (PFCAs), pentachlorophenol, and 4,4'-DDT.

LODs were determined in both nitrogen and ambient air to evaluate potential interferences associated with the dilution medium of the injected solutions.

At a measurement frequency of 1 Hz, the LODs were consistent within 10 % between the two matrices and were found to be in the low parts per trillion (ppt) range. Extending the averaging time to 1 minute resulted in LODs that were reduced by nearly an order of magnitude.

Figure 2 presents the calibration curves for 8:2 FTOH, PFBA, PFHxA, and PFOA obtained from a single experiment using iodide as the reagent ion and the syringe pump method.

The calibration showed strong linearity (R^² > 0.98) for the PFAS compounds, even at low parts-per-trillion levels, demonstrating the AIM reactor's exceptional sensitivity in detecting PFAS in the sub-ppt range.

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

The sensitivity of PFCAs was also investigated, using NO^₃⁻ as the reagent ion. The measured sensitivities were generally within 20 % of those obtained with the iodide mode (Figure 3), demonstrating that using the nitrate reagent ion is similarly effective in detecting this subgroup of PFAS.

For both ion chemistries, the measured sensitivities decrease as the size of the PFCA molecule increases. This is likely to be linked to their ability for efficient transfer to gas phase via this approach.

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

Figure 4 shows the results of the evaluation of the two tuning approaches (single syringe injections of the same volume and the continuous syringe pump injection method). Largely, these two methods are at agreement within 30 % for the volatile fraction of the PFAS.

For certain molecules, high variability in the signal response was perceived with single injections of the same volume and concentration (Figure 4c). The direct injection method is relatively simple and cost-effective but presents variability due to human involvement.

The syringe pump technique, in contrast, showed improved stability in response to changes in injected flow rates and additionally alleviated operator-induced errors. Therefore, all further results reported here rely on tuning using the syringe pump approach.

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

In order to take precise measurements in environments where relative humidity (RH) fluctuates, e.g. in ambient air, an understanding of how sensitivity deviates with changing humidity levels is vital.

Water can meaningfully impact the sensitivity of species detected by CI, with RH-dependent sensitivities fluctuating across compound classes. Resultantly, measurements in environments with variable humidity require time-consuming calibrations for different RH conditions.

To alleviate this dependence, a water vapor control system, consisting of a regulated flow of 5 sccm of acetonitrile as a dopant molecule, is used in the VAR.

The dopant displaces the water molecules that normally attach to the reagent ions. This ensures that the modified reaction mechanism no longer depends drastically on varying water vapor conditions.

Figure 5a demonstrates the relative sensitivity change for selected PFAS as a function of increasing humidity while using a dopant. The outcomes show a very small decreasing trend, but it is important to consider that the interpretation of this trend is complex. It is complicated by the inherent error of the measurement, particularly regarding the reproducibility across replicates, and the variation in sensitivity increases with rising humidity levels.

Figure 5b shows the impact of the matrix solvent selection on the sensitivity of PFAS discovery. A significant deviation with dichloromethane can be observed, which is likely attributable to its higher volatility. Dissimilarities between ethyl acetate and methanol were minimal, with the former emerging as the preferable choice due to 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

Conclusions

Here, Tofwerk demonstrates the capability of the Vocus Aim Reactor to provide real-time detection and quantification of PFAS, offering temporal resolution beyond traditional offline techniques.

With sensitivities ranging from 0.5 to 5 ncps/ppt and detection limits extending to hundreds of ppq levels, this technique delivers high sensitivity and specificity.

While these detection limits exceed typical requirements for background monitoring of ambient air, the speed and precision of the Vocus Aim Reactor make it ideal for applications such as real-time identification of volatile PFAS sources or locating potential leaks where concentrations are expected to be significantly higher.

Tofwerk has also shown that the Aim is suitable for measurements in ambient environments with fluctuating humidity.

This technique also has the potential for advancing our current understanding of PFAS environmental pathways through atmospheric chamber experiments, indoor air monitoring, consumer product evaluations, and material emission testing. Additionally, it can support 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.