New Strategies To Detect “Forever Chemicals”
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At first glance, a cardboard pizza box seems innocent enough. It needs to keep the large slab of dough, toppings and melted cheese on its journey from the oven to your table. Made of cardboard, the box can rapidly biodegrade, ideal for a single-use item. But a pizza’s inherent greasiness – and hence, its good taste – means oil can soak through its temporary paperboard home and make it look less appealing. A thin layer of grease proofing on the pizza box seems the perfect solution, until scientists realized that the degreasing chemicals often used are per- and polyfluoroalkyl substances, aka PFAS. In these synthetic organic compounds, scientists partially or completely substituted hydrogen atoms with fluorine. The nearly unbreakable carbon-fluorine bond in PFAS made them perfect for firefighting foams, paints, clothing, food containers and even dental floss.
But the same chemical properties that made PFAS so desirable to industry mean that these compounds accumulate in bodies and the environment. In recent years, researchers have linked PFAS to a range of health problems, including various types of cancer, immunosuppression, thyroid problems, low birth weights and liver issues. Knowledge of PFAS dangers combined with improved testing methods mean that consumers are pushing food manufacturers to reduce or even eliminate PFAS. The U.S. Environmental Protection Agency (EPA) and Department of Defense (DoD) have followed on the heels of this concern by creating validated laboratory analytical methods to test for PFAS in the environment, including wastewater, surface water and soils. Out of 5,000–6,000 potential PFAS chemicals, the EPA has developed standards to test for roughly 40 of them to date.
Choosing the right analytical method
As awareness of PFAS contamination has grown and detection technologies have improved, scientists need to determine the best way to test for ever more minute quantities of PFAS in the environment. What’s more, these tests need to be quick, easy and cheap, so they can be carried out on a broad scale. Advancements in PFAS testing promise to improve both scientific rigor and consumer safety.
Historically, chemists have used several strategies to identify and quantify specific PFAS compounds in environmental samples. Many techniques rely upon the characteristic carbon-fluorine bond found in all PFAS but only rarely seen in the natural environment. The oldest technique is mass spectrometry (MS), which measures a molecule’s mass by calculating its deflection through a magnetic field. Coupling the mass analysis capabilities of MS or tandem MS with an initial gas or liquid chromatography (LC) step for physical separation allows researchers to analyze more complex mixtures of chemicals.
Choosing the right analytical method depends on both the specific PFAS chemicals that might be contaminating a sample and the sample itself. The majority of PFAS have a short chain ranging in length from 4 to 18 carbons. This makes them semi-soluble in water and non-volatile, ideal for LC/MS. Over the years, improvements in instrumentation have made LC/MS cheaper, more sensitive and more suitable for different sample types.
Challenges of testing
The replacement of legacy PFAS chemicals such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) with newer alternatives has also complicated the testing picture. The huge range of potential PFAS chemicals as well as their precursors and degradation products in the environment means that the number of contaminants far outstrips the known analytical standards. As a result, researchers have developed untargeted screening methods to identify novel PFAS chemicals. Advances in quadrupole time-of-flight instrumentation has allowed researchers to screen for a wide range of samples more quickly and efficiently than traditional LC/MS.
Regardless of the method chosen, screening for PFAS compounds remains challenging. Their ubiquity in the environment – scientists found PFAS in polar bears living in the remote Arctic – means that cross-contamination is a major issue.
Because PFAS are so ubiquitous they are found in common components of analytical instrumentation such as the very LC/MS, solvents, pipettes, vials and collection bottles that are utilized for testing and subsequently can result in false positives.
For many analytical chemists, sample preparation remains the most challenging part of analysis. The process is relatively straightforward for biological samples such as blood, serum and urine. To measure PFAS in blood, for example, people can add an acid to coagulate the proteins. These can be spun down and removed, and the resulting liquid directly injected to the LC/MS or be further processed to remove lipids. Solid tissue samples are often chopped up and digested with enzymes.
Environmental and food samples are more challenging. The darker lignins in sewage sludge and swamp water, for example, can create interferences in MS, that may require a charcoal cleaning after extraction. Vegetables need to be chopped and have salts added, before being spun down, adding more resin, and being spun down again before the sample is injected. Fatty foods such as butter or some types of fish may also require solid phase extraction and clean up.
Some of the biggest advancements in PFAS analysis haven’t been in instrumentation but rather sample preparation. It doesn’t sound like a big improvement but combining two resins in a single cartridge greatly reduces cost, time and chances of error. This is especially important for high-throughput labs that are processing 5,000 to 6,000 samples each month, and using these new products can be a tremendous time-saver. For industry, the sample prep part of the process has always been the biggest bottleneck, since it takes the most time and can’t necessarily be automated.
Newer sample prep techniques don’t just save time, they also save money. The more samples you can process, the cheaper the cost per sample. Sample prep for water, tissues and foods and food contact materials still remain a challenge, but incremental improvements are enabling samples to be analyzed faster and easier, ultimately improving human health overall.
About the author:
Richard Jack, is global market development manager – food and environmental at Phenomenex.