Detection of perfluorocarboxylic acids in soil samples

Naturally occurring fluorinated compounds tend to only contain one fluorine atom. This distinction makes it obvious that all other compounds containing two and up to a multitude of fluorine atoms are usually man-made (1). Perfluoroalkyl substances (PFAS), as a group of compounds, amass over 4500 substances with varying structure numbers of fluorine atoms bound to a carbon backbone, of which approximately 25 % are registered with the US EPA Toxic Substances Control Act. Perfluoro carboxylic acids (PFCAs) in particular were produced by electrochemical fluorination from 1947 onwards (2). The intrinsic properties of PFCAs, e.g., the hydrophobic and lipophobic behaviour, find application in a multitude of industries, posing an environmental risk globally.
However, detection of trace amounts of PFCAs in the environment still presents difficulties with environmentally friendly extraction practices and affordable and efficient detection in complex matrices.
In this project we show the application of a sonication-assisted extraction procedure requiring minimal solvent usage for extracting PFCAs from soil, which minimises the risk of cross-contamination. Soil samples from agricultural land were collected over a period of two years, dried, processed, extracted and analysed before statistical evaluation.
The trace analysis of compounds was performed using GC-MS analysis and LC-Orbitrap-MS for comparison. GC-MS analysis was performed in a 27-minute run on a DB-5 column (30 m x 0.25 mm x 0.25 µm). MS detection was performed with an electron impact (EI) source rather than chemical ionisation (CI) that is often favoured in relevant literature. The nature of the setup allowed the analysis of PFHpA (C7) up to PFDoA (C12) of perfluoro carboxylic acids. The LC-MS analysis by contrast allowed the analysis of PFBA (C4) up to a chain length of PFDoA (C12) on a C18 Accucire (RP) column (100 mm x 2.1 mm x 2.6 µm) within 22 minutes.
Preliminary results (LC-Orbitrap-MS) showed the presence of PFHpA (1.1 – 22.2 ng/g), PFOA (2.7 ng/g), PFNA (0.1 – 0.7 ng/g), PFDA (0.1 – 1.8 ng/g) and PFDoA (0.7 ng/g). PFBA, PFPeA, PFHxA and PFUnA were not detected in the analysis of the selected soil samples.
The presence of PFCAs in an agricultural frame, irrespective of the low-level concentrations, implicates the possibility of the introduction of contaminants into the food chain (e.g., the absorption into plant material, or the ingestion of soil and plants by livestock) and should be investigated further.