Toward Better Detection of PFAS in Consumer Electronic Products: Optimizing the Accuracy of a Total Fluorine Test Method in Materials Containing Polymeric PFAS

Per- and polyfluoroalkyl substances (PFAS) are a class of thousands of synthetic chemicals used in electronics and many other industries because of their unique performance properties such as flame retardancy, water/oil repellency, thermal resistance, and friction reduction. Their extreme persistence in the environment and other toxicological properties have raised increasing concerns regarding both ecological and human health impacts, prompting heightened consumer awareness and regulatory scrutiny. Accurately quantifying PFAS content in consumer products such as electronics is therefore essential. However, accurate measurement of PFAS in common materials in electronics is still quite challenging. Current total fluorine test methods, such as combustion ion chromatography (CIC) and oxygen bomb ion chromatography (IC), have not been validated nor optimized for analysis of total fluorine in materials with polymeric PFAS content in the low parts-per-million (ppm) range. The aim of this article is to provide a validated and optimized CIC-based method for accurate measurement of total fluorine in materials with polymeric PFAS content in the low-ppm range, which could be used in the consumer electronics and potentially other industries. This, in turn, can aid companies in tracking and mitigating PFAS ahead of regulations.

Per- and polyfluoroalkyl substances (PFAS) are a group of chemicals that have been widely used in a variety of consumer products and industrial applications such as adhesives, coatings, inks, and plastics because of their exceptional chemical and heat resistance properties (Figure 1) (1).

However, PFAS in certain use cases have been linked to a range of health and environmental concerns. They can persist in the environment for long periods of time and accumulate in biological tissues. As a result of these concerns, several jurisdictions have proposed or passed laws restricting the use of PFAS (2–4). A pre-published draft for the European Union PFAS restriction has stated a limit for total fluorine of 50 parts-per-million (ppm) (5). If total fluorine content exceeds 50 ppm, the manufacturer, importer or downstream user may be required to provide proof that the fluorine measured is not related to PFAS.

Combustion ion chromatography (CIC) is an emerging technique, particularly useful for measuring total fluorine content in materials, which can be indicative of PFAS presence (6,7). It is particularly useful for the comprehensive assessment of fluorine including both known and unknown PFAS. However, in our literature search, we could not find studies that provide sensitivity, accuracy and variance of total fluorine in polymeric PFAS reference materials using the CIC technique.

The purpose of this article is to provide a validated and optimized CIC based test method for total fluorine in polymeric materials at the <50 ppm level.

Experimental
Chemicals
Deionized water (DI, 18 M Ω.cm resistivity, Milli-Q), Seven Anion Standard (Thermo Scientific P/N: 057590), Perfluorooctanoic acid (PFOA, AccuStandard, PFOA-001N), Perfluorooctanesulfonic acid (PFOS, AccuStandard, PFOS-001S), and custom-made reference material plaques (~100 – 1500 ppm PTFE doped in PC; PTFE is a synthetic fluoropolymer of tetrafluoroethylene and PC represents polycarbonate).

Equipment
A Dionex Integrion high-pressure ion chromatography (HPIC) system was used in this study. The system is an integrated ion chromatograph that includes: Dionex Integrion HPIC System Pump, Detector Compartment Temperature Control, Conductivity Detector, Dionex EGC 500 KOH Eluent Generator Catridge (P/N: 075778), Dionex CR-ATC 600 (P/N 088662), Thermo Scientific DionexAERS500 Anion Electrolytically Regenerated Suppressor (4 mm, P/N 082540), IonPac AS18-4 μm column (4 mm × 150 mm, P/N: 076034), Dionex IonPac AG18-4 μm guard column (4 mm × 30 mm, P/N: 076035).

A Mitsubishi Automatic Quick Furnace AQF-2100H system with Solid Auto Sampler ASC-240S, Horizontal Furnace HF-210, Pyrolysis Set, and Gas Absorption unit (10 mL). A ceramic (mullite) pyrolysis tube was used for testing of materials in this study.

Results and Discussion
Polymeric materials are oxidized in the CIC instrument by oxygen at >900 °C. Fluorine-containing substances form hydrogen fluoride (HF) or fluorine gas (F₂). Loss of hydrogen fluoride through the reaction with the pyrolysis tube (quartz glass or ceramic tube) is avoided by continuously adding water during combustion (hydropyrolysis). The volatile pyrolysis products are then absorbed into an aqueous solution and subsequently detected as fluoride anions by ion chromatography (IC) (6,7).

There have been very few studies that determine the combustion efficiencies of polymeric PFAS. We conducted a study to determine the combustion efficiency of currently available methods (6,7,10). Polymeric PFAS reference materials were used for this study.

Combustion Efficiency Evaluation
Four reference materials with known amounts of fluorine were evaluated using the CIC method described in an application note (11):

Inorganic fluorine: 20 ppm solution in water.

Small molecule organic PFOA: 20 ppm solution in methanol.

Small molecule organic PFOS: 20 ppm solution in methanol.

Polymetric PFAS: 125 ppm PTFE doped in a PC plaque.

The amount of fluorine detected in these materials was estimated using an inorganic fluoride anion calibration curve (Appendices A and B). Results are shown in Figure 2.

These data show that inorganic fluorine has a much better recovery as compared to materials containing organic fluorine. PTFE has the lowest recovery (~80%), suggesting that polymeric materials tend to have incomplete combustion in the furnace.

These results also show the importance of analyzing known polymeric PFAS reference material using the CIC technique. Without this study, we may underestimate the total fluorine contained in materials due to incomplete combustion resulting in poor recovery.

Optimization of Combustion Process
The following parameters were optimized to improve combustion efficiency of polymeric PFAS.

Oxygen/Argon ratio: More oxygen and less argon tends to reduce the combustion debris accumulated in the system, extending the lifetime of combustion tubes.

Sample boat speed: Slower sample boat speed allows samples to combust more efficiently.

End time: Longer end time tends to provide better recovery.

Temperature: Increasing the combustion temperature, if possible, can help the sample burn completely.

Pre-treatment: Cryo-milling of solid material into finer particles increases the surface area, hence improving the combustion efficiency.

Combustion aid: Introducing a combustion aid can help ignite and sustain the combustion of stubborn samples. Parafilm with alcohol is recommended with its minimal background interference. Tungsten oxide, iron oxide, and benzoic acid can be used as well.

Additionally, the following approaches can improve test sensitivity and provide a lower detection limit of fluorine:

Absorption tube: A smaller-size absorption tube (10 mL) allows lower detection limits for fluoride anions.

Sample load amount: Optimal amount of sample should be loaded into the furnace boat. A higher sample amount increases absolute fluoride signal, resulting in better sensitivity.

Total fluorine recovery studies were performed after optimization of the combustion method. Four concentrations of PTFE/PC plaques with a fluorine amount ranging from 95 ppm to 1140 ppm were tested. Approximately 100 mg of sample was loaded in a quartz sample boat and introduced into the system. The optimized CIC method is listed in Table I.

Results presented in Figure 3 show that total fluorine recovery from all four reference samples improved significantly. For a 95 ppm sample, the recovery was improved by 15% just by optimizing the combustion parameters, i.e., without sample preparation changes. All four PTFE/PC plaques achieved ~95% recovery.

Validation of Optimized Method
To evaluate the accuracy and precision of the optimized method, validation studies were conducted by analyzing four concentrations of PTFE doped in PC plaques, six replicates at each concentration. Results are shown in Table II.

The percentage recoveries of total fluorine measured at 95 ppm, 190 ppm, 380 ppm and 1140 ppm levels are all within ±10% of the expected amounts, which met our success criteria of 85%-115%.

The precision (repeatability) of the method was assessed by calculating the percent relative standard deviation (% RSD) of the measured values. These were found to be <10%, which again met our success criteria of 15%.

The method detection limit (MDL) was calculated by multiplying standard deviation (STD) of six replicates of the lowest concentration PTFE/PC plaques (95 ppm total fluorine) by 3.365, which is a student’s t-value for six replicates with 99% confidence. Estimated MDL for PTFE/PC plaques was found to be ~15 ppm.

Inter-Laboratory Test
An independent inter-laboratory test (also called round-robin test) was conducted with three external laboratories to assess how closely various laboratories match against each other. PTFE-doped PC plaques were distributed to all participating laboratories, with each independently conducted the CIC test for total fluorine content. The success criteria for percent recoveries were set to within 85%–115% of the expected value. Initial results were discouraging, as percentage recoveries at participating laboratories showed significant disparities (Figure 4).

Laboratory 2 showed less than 60% recovery. We then worked with these laboratrories to optimize the combustion furnace process. After that, percentage recoveries improved significantly at all participating laboratories and met our success criteria (Figure 4). In fact, all participating laboratories recoveries were within ± 10% of the expected amount.

Conclusions
CIC is a valuable technique for the detection of PFAS in materials. It offers a broad assessment of PFAS, useful in environmental monitoring and regulatory compliance contexts. This article demonstrates that total fluorine can be precisely and accurately measured in polymetric materials. The combustion process has been optimized to ensure efficient oxidation of polymeric PFAS. This optimized testing protocol will be useful as an indicator for PFAS and mapping uses in electronic products, validating uses of PFAS within supply chains, and supporting more accurate compliance verification for the upcoming EU regulation (5).

Acknowledgments
We thank Charlene Wall-Warren (Director, Environmental Technologies at Apple).

Appendices

Appendix A: Inorganic fluorine calibration curve used for quantification of total fluorine content.

Appendix B: PTFE/PC plaques total fluorine concentration.

References

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