PFAS
What are PFAS in drinking water?
PFAS are a large family of fluorinated compounds measured at nanogram-per-litre concentrations. Here is what the term covers and how laboratories approach it.
4 February 2026 · 6 min read
A family, not a single substance
PFAS stands for per- and polyfluoroalkyl substances. The term covers thousands of synthetic compounds that share strong carbon–fluorine bonds, which is why they resist heat, oil and water.
Because PFAS is a family rather than one chemical, any measurement is defined by the specific compounds a method targets. Two reports that both say "PFAS" can cover very different compound lists.
Why the concentrations are so small
PFAS in drinking water are typically discussed in nanograms per litre — parts per trillion. That is roughly a thousand times lower than the units used for many metals.
Working at these levels changes the whole analytical chain: sample containers, blanks, laboratory materials and instrument sensitivity all have to be controlled, because trace contamination during handling can matter as much as the water itself.
How PFAS are measured
The usual approach is liquid chromatography with tandem mass spectrometry (LC-MS/MS), with isotopically labelled internal standards added to each sample to correct for losses during preparation.
The report should state the target compound list, the limit of detection for each compound and the quality control applied. Without that context, a number is difficult to act on.
Reading a PFAS result
A non-detect means the compound was not measured above the stated detection limit by that method. It is not a statement that the compound is absent at every concentration.
The profile across compounds is often more informative than any single value, because related PFAS tend to occur together and can point to a plausible origin.
Common questions
- Are PFAS the same as PFOA and PFOS?
- No. PFOA and PFOS are two individual compounds within the wider PFAS family. Most methods report them separately as well as within any defined sum parameter.
- What units are PFAS reported in?
- Usually nanograms per litre (ng/L), also written as parts per trillion, with a detection limit stated for each compound.