IN THE NEWS - CBEE pioneers PFAS Analysis in Chesapeake

CBEE IN THE NEWS: 

Chesapeake Quarterly‘s Complicated Contaminants: Finding PFAS in the Chesapeake Bay,  Volume 23, Number 1 | May 2024

EXCERPT FROM: Strong, Sticky, and Tricky to Measure

By Madeleine Jepsen | April 25, 2024

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A Breakdown of the Chemicals that Don’t Break Down 

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.

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Passive Sampling Provides a Panoramic Picture of PFAS

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.

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Back in the Lab

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.

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Photo Credit: Chesapeake Quarterly Cover photo by Jay Fleming