Commonly referred to as “forever chemicals,” per- and polyfluoroalkyl substances (PFAS) are ubiquitous and environmentally persistent industrial chemical contaminants found in soil, sediment, fresh water sources, oceans, and other matrices. While PFAS chemicals have been around since the 1940s, there has been renewed emphasis recently to limit exposure due to identified negative effects on human health.
PFAS can be found in a variety of matrices, with its presence in drinking water and the food supply a direct route into the human body. PFAS contamination in food can come from multiple sources, including: accumulation in aquatic and terrestrial food chains, contaminated agricultural soil, grains and feed, environmental leaching from firefighting foams and—primarily—via materials that touch food, such as grease-proof food packaging and nonstick cookware.
To address concerns around PFAS in food packaging, in February 2024, the U.S. Food and Drug Administration (FDA) announced that substances containing PFAS used as grease-proofing agents on paper and paperboard for food contact use are no longer being sold by manufacturers into the U.S. market. In an update published just last month, the FDA issued a notice that 35 food contact notifications (FCNs) related to PFAS-containing food contact substances as grease-proofers applied to paper and paperboard food packaging are no longer effective due to abandonment— when a manufacturer or supplier has ceased or will cease the production, supply, or use of the food contact substance for its intended use.
Still, given the ubiquitousness of PFAS, the chemicals find a way into our drinking water and food supply. It’s due to this persistence—and ability to contaminate sensitive matrices like infant formula, for example—that ultra-sensitive testing methods are critical for quantification and screening.
Currently, liquid chromatography tandem mass spectrometry (LC-MS/MS) is the gold standard for PFAS analysis due to its sensitivity, specificity and versatility. The technology’s versatility ensure it is compatible with various sample types and sample preparation methods, including complex matrices.
“LC-MS/MS also allows for the simultaneous detection and quantification of multiple PFAS compounds in a single run, enabling very low concentrations to be measured—often in the range of parts per trillion, which is important for regulatory compliance,” said Estelle Riché, Global Senior Application Specialist at MilliporeSigma.
Method development
In April 2024, the FDA published method C-010.031, a procedure for measuring 30 PFAS in food and feed using LC-MS/MS. The method was single laboratory validated in multiple food matrices, including lettuce, chocolate milk, salmon, bread, eggs, clams, blueberries, silage and corn snaplage. The matrices were chosen based on those that have been previously validated, those with known interferences (chocolate milk, eggs) and those with known analytical challenges due to matrix effects and interferences (silage, corn snaplage). The method performed well and has been received well in the industry as the go-to when analyzing PFAS in food and feed.
“Analysts shall be able to identify chromatographic and mass spectrometric interferences during sample analysis and take necessary actions following validated procedures for their correction to achieve reliable identification and quantification,” the authors write in their study.
They also caution about contamination and the importance of controlling contamination during sensitive PFAS analyses.
“PFAS chemicals are prevalent in all laboratory environments and special care must be taken to prevent false positives due to accidental and/or routine laboratory contamination,” write the authors.
Indeed, background contamination is an often-overlooked challenge in PFAS testing. Plastic housing, reagents and solvents, laboratory equipment and more, can and do leach PFAS that inadvertently interfere with analysis results. It is one of the reasons why the authors suggest complete method blanks be performed and analyzed daily, preferably in the same instrument sequence as analyzed samples. It’s also why they suggest a delay column be used between the mobile phase mixer and sample injector to temporarily trap any system-related interferences, which results in their elution at a later retention time than the analyte. This helps eliminate contamination from instrument tubing, mobile phase solvents and solvent bottles.
Ultrapure water for PFAS testing
Reagent water plays a critical role in trace analyses of PFAS: it is used in many steps of the analytical workflow, from rinsing glassware and SPE cartridges to preparing mobile phases, calibration standards and blanks. Even though using a delay column can alleviate some background issues, water is still used in steps where the delay column does not help, such as sample preparation.
In a recent experimental2, scientists at MilliporeSigma in Burlington, Mass., tested a Milli-Q IQ 7000 system with a polisher specifically designed for sensitive organic analyses (LC-Pak polisher) to see if it could: 1) remove traces of PFAS from tap water and 2) release no PFAS into purified water.
According to the study results, a few PFAS compounds— PFHxA, PFHpA, PFOA, PFNA)—were detected in the tap water of the laboratory, albeit at very low levels. However, no detectable levels of PFAS were found in the ultrapure water delivered by the Milli-Q IQ 7000 system fitted with an LC-Pak polisher.
The study also showed that most PFAS molecules are large enough to be retained by reverse osmosis (RO) membranes and are also efficiently retained by activated carbon. In addition, since PFAS are charged molecules, they can be removed by ion-exchange resins and electrodeionization (EDI). LC-MS analysis requires ultrapure water that is reliably very low in organics and ions. These interfering trace contaminants are removed from good-quality pure water by activated carbon, photo-oxidation and ion-exchange resins. These purification technologies also help to remove PFAS even further.
When performing LC-MS or LC-MS/MS, it is also recommended to use a point-of-use polisher containing C18 reverse-phase silica. Such a polisher ensures that no traces of organics (PFAS or others) can interfere with these sensitive analyses.
Ultimately, this study demonstrates that even if some PFAS molecules may be present in the water feeding a water purification system, carefully selecting the system can ensure no detectable amounts of PFAS will be present in the ultrapure water produced.
References
1. Genualdi, G & deJager, L. (2024). Determination of 30 Per and Polyfluoroalkyl Substances (PFAS) in Food and Feed using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS). FDA. https://www.fda.gov/media/131510/download
2. Riche, E., Renard, P., Royer, J.C.. (2024). Obtaining Ultrapure Water for Sensitive PFAS Analysis by LC-MS. MilliporeSigma. https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/analytical-chemistry/small-molecule-hplc/obtaining-ultrapure-water-sensitive-pfas-analysis-lc-ms?