New strategies for detecting “eternal chemicals”
At first glance, a cardboard pizza box looks quite innocent. It needs to keep the large slab of dough, toppings, and cheese melted on its way from the oven to your table. Made of cardboard, the box biodegrades quickly, ideal for a single-use item. But the inherent fat in a pizza – and therefore its good taste – means that oil can penetrate through its temporary cardboard housing and make it less appealing. A thin layer of grease proofing on the pizza box seems like the perfect solution, until scientists realize that the degreasing chemicals often used are per- and polyfluoroalkyl substances, aka PFAS. In these synthetic organic compounds, scientists have partially or completely replaced the hydrogen atoms with fluorine. The nearly unbreakable carbon-fluorine bond of PFAS makes them ideal for fire-fighting foams, paints, clothing, food containers, and even dental floss.
But the same chemical properties that have made PFAS so desirable to industry mean that these compounds build up 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 weight and liver problems. Knowledge of the dangers of PFAS combined with improved testing methods means that consumers are pushing food manufacturers to reduce or even eliminate PFAS. The U.S. Environmental Protection Agency (EPA) and the Department of Defense (DoD) have followed up on this concern by create validated laboratory analytical methods to test for PFAS in the environment, including wastewater, surface water and soil. Out of 5,000 to 6,000 potential PFAS chemicals, the EPA has developed standards to test about 40 of them so far.
Choose the right analysis method
As awareness of PFAS contamination has grown and detection technologies have improved, scientists must determine the best way to test for ever smaller amounts of PFAS in the environment. Moreover, these tests must be quick, easy and cheap, so that they can be carried out on a large scale. Advances 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 on the characteristic carbon-fluorine bond present in all PFAS, but rarely seen in the natural environment. The oldest technique is mass spectrometry (MS), which measures the mass of a molecule 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 can contaminate a sample and the sample itself. The majority of PFAS have a short chain whose length varies from 4 to 18 carbon atoms. This makes them semi-water soluble 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 types of samples.
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 wide range of potential PFAS chemicals and their precursors and breakdown products in the environment means that the the number of contaminants far exceeds known analytical standards. As a result, researchers have developed non-target screening methods to identify new PFAS chemicals. Advances in quadrupole time-of-flight instrumentation have allowed researchers to screen a wide range of samples faster and more efficiently than traditional LC/MS.
Whichever method is chosen, screening for PFAS compounds remains difficult. Their ubiquity in the environment – scientists have found PFAS in polar bears living in the remote Arctic – means cross-contamination is a major problem.
Because PFAS are so ubiquitous, they are found in common components of analytical instrumentation such as LC/MS, solvents, pipettes, vials, and collection vials that are used for testing and can by result in false positives.
For many analytical chemists, sample preparation remains the most difficult part of analysis. The process is relatively simple for biological samples such as blood, serum and urine. To measure PFAS in blood, for example, people can add an acid to coagulate proteins. These can be centrifuged and removed, and the resulting liquid injected directly into the LC/MS or further processed to remove lipids. Solid tissue samples are often minced and digested with enzymes.
Environmental and food samples are more difficult. Darker lignins in sewage sludge and marsh water, for example, can create interferences in MS, which may require carbon cleaning after extraction. Vegetables should be chopped and added salts, then centrifuged, added more resin and centrifuged again before sample injection. Fatty foods such as butter or certain types of fish may also require solid phase extraction and cleaning.
Some of the greatest advances in PFAS analysis are not in instrumentation, but rather in sample preparation. It doesn’t sound like a big improvement, but combining two resins in one cartridge dramatically reduces cost, time, and chance of error. This is especially important for high throughput labs that process 5,000 to 6,000 samples each month, and using these new products can be a huge time saver. For industry, the sample preparation part of the process has always been the biggest bottleneck, as it takes the most time and cannot necessarily be automated.
New sample preparation techniques not only save time, they also save money. The more samples you can process, the lower the cost per sample. Sample preparation for water, tissue and food and food contact materials remains a challenge, but incremental improvements are making it possible to analyze samples faster and easier, improving human health as a whole. together.
About the Author:
Richard Jack, is Head of Global Market Development – Food and Environment at Phenomenex.