Strong, Sticky, and Tricky to Measure

When researchers measure PFAS in water, the first step is ensuring that their sampling tools and lab instruments are free of PFAS. The useful characteristics that have made this group of per- and polyfluoroalkyl substances so commonplace in household items mean that these compounds are also used in Teflon-lined water sampling containers or bottle lids, lab tubing, and the waterproof jackets researchers might wear for rainy fieldwork.

The US Environmental Protection Agency and the researchers they collaborate with follow established, dependable methods for detecting PFAS in drinking water samples. Through these methods, they could even measure a single drop of PFAS in the water from 20 Olympic-sized swimming pools—a detection level known as parts per trillion.

But detecting PFAS in the waters of the Chesapeake Bay is a more complicated matter, says Emily Majcher, a hydrologist with the US Geological Survey and co-chair of the Chesapeake Bay Program’s Toxic Contaminants Workgroup.

Sampling Bay water is more challenging in part due to the complexity of what’s in the water—a chemistry soup filled with nutrients, ions, and organic compounds in flux from the creatures in the Bay. Unlike freshwater lakes or rivers, the Bay and its tributaries have a saltwater gradient that can affect how compounds like PFAS interact with suspended particles and the ions dissolved in the water.

Part of Majcher’s focus in leading the Toxic Contaminant Workgroup has been encouraging standard sampling and analysis protocols for easier comparison across projects and regions. The group is also creating a collaborative map of different PFAS projects so that researchers can investigate more efficiently and collaborate between the many projects that have sprung up.

“It’s a really challenging time because there are analytical challenges and hurdles to overcome while we're still really trying to elucidate those [PFAS] pathways,” Majcher says. “There's this big explosion of studies and investigations that have been going on.”

The namesake chemical bond found in all PFAS—a fluorine atom bonded to a carbon atom—is one of the strongest organic bonds found in nature. The strength of this bond is the main reason why PFAS can linger in the water or soil and make their way into fish tissue—and into the birds or humans eating those fish.

Although plants like marigolds can produce toxic pesticides naturally as a defense mechanism against deer and other predators, there’s a natural pathway for these molecules to break down. Then, the molecule’s components can be reassembled and recycled for other uses.

“All of these chemicals that are produced in nature have a pathway of recycling where the carbon goes back to carbon dioxide, the hydrogen and oxygen goes back to water, and then something else reproduces those chemicals from the basic elements,” says Upal Ghosh, a professor of chemical and environmental engineering at the University of Maryland, Baltimore County.

Unlike other human-introduced contaminants in the environment, such as hydrophobic PCBs that only accumulate in fish fat, many of the common PFAS also have components that allow them to interact with both water and fats.

PFAS with eight or more carbon atoms linked together to form a molecular “tail” have been found to accumulate in fish and humans more readily and can interfere with human health. There are two main components of these longer PFAS—the molecule’s “head” with the carbon-fluorine bond that can interact with water, and the chain of carbons that form the “tail.” Combined, these traits allow PFAS to move through soils and waterways into organisms without breaking down.

In this way, PFAS are almost like a strong magnet, with two sides that each have opposite pulls. This allows PFAS to interact with a wider variety of molecules in fish and humans—in particular, proteins and blood. Unlike most pesticides, whose shapes are designed to bind to specific proteins and inhibit specific functions, PFAS can bind to many proteins.

Field sampling for PFAS in aquatic environments generally requires a sampling kit with a PFAS-free container to collect the sample in. These one-off samples, often called “active” or “grab” samples, provide researchers with an idea of PFAS concentrations in a specific part of a water body for a specific period of time. To get a more holistic understanding of the PFAS upstream and downstream, or how PFAS concentrations change over time, researchers must come back multiple times—and the time and effort adds up.

Michella Salvitti, a PhD student at the University of Maryland Eastern Shore who is studying PFAS around the Bay, had to carefully limit sampling locations in her project. With five water samples taken from each site, the time required for sampling can limit the scope of many projects. Time isn’t the only cost: The equipment and chemicals needed to process the sample in the laboratory can cost upward of $200-400 per sample.

Some researchers seek to improve the classic grab sampling methods. To make active sampling easier and more cost-effective for PFAS, Emanuela Gionfriddo, an analytical chemist at the University at Buffalo, has spent part of her career developing a small microfiber that can bind and extract PFAS from water samples.

The centimeter-long fiber, made from metal alloys, is the width of two strands of hair. It is carefully coated with chemicals that PFAS in a sample of water will bind to, like sprinkles stick on a chocolate-covered pretzel. A researcher using the fiber inserts the fiber into water for about 20 minutes, allowing PFAS from the water to adhere to the fiber. In the lab, this device can be inserted directly into laboratory instruments for analysis, which reduces the potential for contamination of the sample. Normally, a water sample needs an extra processing step before analysis.

Another benefit to sampling with Gionfriddo’s fibers is that they can be re-used and coated with different polymers to bind to specific PFAS from a sample. Gionfriddo painstakingly validated her method, cross-checking it against other measurement methods. Now, Gionfriddo’s lab members are using the method as an efficient way to measure low concentrations of PFAS in their water sampling.

Sometimes, a single grab sample, be it a traditional water sample or an advanced method like Gionfriddo’s device, is all researchers need to get a snapshot of PFAS levels in the water, often to pair with fish tissue samples or compare PFAS across geographic regions. But to understand how PFAS concentrations vary over a span of time, researchers would need to come back multiple times to sample a site—and analyze each of those samples individually. The time required to do this is a drawback of active sampling.

To get a more holistic sense of the average PFAS concentrations in a water body over time, environmental engineer Lee Blaney and his laboratory are developing passive samplers. These circular devices remain in the water for a longer stretch of time. As the samplers remain in the water, they record PFAS concentrations in the water that reflect a longer stretch of time: a panorama compared to the one-time snapshot of a grab sample.

The ion-exchange membranes Blaney’s team uses contain positive charges that are anchored to a membrane on the device. Initially, the fixed positive charges in the membrane contain chloride, an ion commonly found in Bay water. When the passive sampler is set in the water, PFAS molecules with a higher affinity for the positively charged sites replace chloride and bind to the membrane. This same ion-exchange chemistry is employed in some filters to treat PFAS-impacted drinking water.

Back in the lab, researchers use the corresponding chemical reactions to release PFAS from the sampler, measure PFAS levels, and back-calculate the PFAS concentrations in the water body where the sampler was deployed.

Part of the challenge is developing passive samplers that accumulate all PFAS of concern. Short-chain PFAS don’t have the same affinity for conventional passive samplers that work well to capture long-chain PFAS. Blaney and his lab are testing new ion-exchange membranes that improve uptake of short-chain PFAS, so that their concentrations in water bodies can be more accurately and sensitively measured.

To develop additional compounds that can catch PFAS in passive samplers, Upal Ghosh has taken another chemical engineering approach. Ghosh has used molecules that are known to bind to PFAS in organisms, like components of pig blood, isolated these compounds, and used them to bind PFAS in passive samplers.

While passive samplers provide researchers with a better understanding of overall PFAS levels in a water body, they aren’t perfect. They require calibration, since the researchers calculate the concentrations of PFAS in the water based on chemical reaction rates of PFAS binding to the passive sampler, which depend on temperature, flow, pH, and salinity of the water.

Passive samplers can also smooth out “spikes” in PFAS levels, meaning the peaks in PFAS levels that are recorded by the sampler are not as large as the true peak level in the water body. Still, Lee says, passive samplers have a higher chance of capturing a spike in PFAS levels than a one-off grab sample because of their longer timeframe in the water.

Not all labs are equipped to process PFAS samples—and those that can have undergone rigorous review to ensure that any PFAS they detect are coming from the samples they process, and not residual PFAS from the equipment they’re using.

The “gold standard” of analysis used by federal agencies, commercial labs, and most academic labs for detecting and identifying PFAS is liquid chromatography paired with tandem mass spectrometry. These two analytical methods combined, often referred to as LC-MS/MS, allow researchers to separate out the different molecular components of a sample, and then analyze the mass of a particular molecule to determine its chemical structure and quantity in the sample.

Liquid chromatography with tandem mass spectrometry allows researchers to measure how much of a particular PFAS is in a sample, even in very small quantities. To identify and distinguish different compounds, researchers compare their samples against standards with pure, known quantities of specific PFAS. Researchers can also use these standards to compare against an unknown sample to determine the exact concentrations of PFAS in field samples. This method can be useful for researchers like Blaney, who needs to identify the different types of PFAS present in a sample.

“One of the things I'm really interested in is sampling from places where you don't expect to find contaminants, because maybe there's something there that we're missing,” Blaney says.

A limiting factor in PFAS analysis is that there are thousands of variations of PFAS, but standards for only about 200 specific compounds. Although these standards include many of the PFAS that are known to affect human health, additional standards could help researchers gain a more holistic understanding of the compounds circulating in the water or sediment. Researchers can run the standards through their own instruments so they know exactly how each PFAS would appear in the readouts, adding additional certainty to their measurements.

For even more refined analysis, some researchers like Carrie McDonough, an assistant professor of chemistry at Carnegie Mellon University, turn to another spectrometry method called high-resolution mass spectrometry. This method allows researchers to differentiate between molecules with similar masses, and can help to identify compounds that don’t have analytical standards, like many types of PFAS. Similar to how a microscope at higher resolution allows researchers to get a more in-depth view, high-resolution mass spectrometry gives researchers more refined peaks from their samples. The refined analysis also allows researchers to work toward identifying unknown PFAS without a standard.

Through new field sampling methods like passive sampling and detailed laboratory analysis, researchers are gaining a better understanding of PFAS in the Bay. With these technological developments, PFAS are steadily becoming less tricky to measure.