Detection and quantification of Poly- and Perfluoroalkyl Substances (PFAS)

The detection and quantification of Poly- and Perfluoroalkyl Substances (PFAS) has recently become one of the hottest topics in analytical chemistry [1]. Many of these omnipresent "forever chemicals" are connected to serious health concerns and thus have been subject to increasingly stringent restrictions [1]. So far, the vast majority of analytical PFAS studies have been performed offline, utilizing time-consuming GC and LC based techniques [2]. While these methods may be the gold-standard for spot sampling, they render online compound monitoring impossible.
Here, we present first results of a comprehensive series of real-time direct-injection PTR-MS studies on PFAS: findings on the ion chemistry, experimentally determined Limits-of-Detection (LoDs) and a proof-of-concept indoor air study. 
We utilized an off-the-shelf PTR-TOF 6000 with a mass resolution of about 6000 m/Δm and a sensitivity of about 2000 cps/ppbv. As all IONICON PTR-TOFMS instruments come with TRU-E/N® ion chemistry, also for this feasibility study no compound specific calibration is necessary for quantification.

PFTBA (C₁₂F₂₇N) is the most potent greenhouse gas known so far. It is available as a gas standard at a certified concentration of 980 pptv in an N₂ matrix and was dynamically diluted in order to determine the Limits-of-Detection (LoDs).

Dissociative PTR reactions of H₃O⁺ with PFTBA result in a product ion at m/z 413.977 (C₈NF₁₆⁺). The high sensitivity of the PTR-TOF 6000 in combination with the extremely broad transmission range of IONICON's hexapole ion guides and the low chemical background in this high mass region result in excellent LoDs:

• 2.4 pptv after only 1 s of integration time
• 280 ppqv with 1 min integration time.

With NO⁺ and NH₄⁺ (produced via our patented ammonia-free method [3]) reagent ions no PFTBA product ions can be observed. O₂⁺ reagent ions react via dissociative charge transfer to product ions at m/z 501.971 (C₉NF₂₀⁺) and m/z 263.978 (C₅NF₁₀⁺).

A petri dish wetted with FTOH (C₈H₅F₁₃O) was used to create an initial indoor air concentration of 6.7 ppbv in an office room of about 30 m³ (LxWxH: 410x250x290 cm; RT: 23-26°C; two walls consisting of windows) with low ventilation.

8.8 min after the start of the experiment the petri dish was removed. Subsequently the indoor air was monitored for 48 h at an increased integration time (set from initially 1 s to 30 s). 20 min into the experiment the measured concentration dropped to 50% and about 5 h later to 10% of the initial 6.7 pptv. Interestingly, 24 h after the start direct sunlight warmed the surfaces in the room, which caused an instant concentration increase by a factor of 2.3. Opening the windows for a short time and thus venting the room after 48 h resulted in only a short dip in FTOH concentration, with 85 pptv being present after 63 h. This is still five times higher than the already elevated (due to previous experiments) background of 17 pptv at the beginning of the experiment.

In parallel to FTOH also PFBA (C₄HF₇O₂) was released into indoor air using a wetted petri dish. PFBA is of particular interest as this compound is known to accumulate in the lung and increased plasma levels are associated with a greater severity of COVID-19 prognosis [4].

However, in contrast to FTOH, indoor air concentration of PFBA dropped to 50% of the initial 676 pptv already after 10 min and to 10% after 25 min. 2 h into the experiment the concentration was below 10 pptv and did not increase when the sun was heating up the room.

Conclusion

The three examples presented demonstrate the excellent performance of IONICON's PTR-TOF 6000 for real-time quantification of PFAS in air. Additionally, an in-depth study on a much wider range of PFAS will soon be published.

For rare cases where the PTR-TOF 6000's performance should still be insufficient, we recommend the brand-new FUSION PTR-TOF 10k with 20-40 times higher sensitivity and about twice the mass resolution. This Next-Gen PTR-TOF comes with the novel Fast-SRI ion source for nearly instant reagent ion switching and the ultra-clean FUSION reaction chamber for LoD's <200 ppqv. Certainly, this instrument is TRU-E/N certified!

References

[1] echa.europa.eu/hot-topics/perfluoroalkyl-chemicals-pfas, European Chemicals Agency (17.11.2022).

[2] S. F. Nakayama et al., Worldwide trends in tracing poly- and perfluoroalkyl substances (PFAS) in the environment. TrAC Trends in Analytical Chemistry 2019, 121, 115410.

[3] US11342171 (B2), CN111386590 (B), EP3503161 (B1).

[4] P. Grandjean et al., Severity of COVID-19 at elevated exposure to perfluorinated alkylates. PLoS ONE 2020, 15(12).