What does it cost me to p .95 reliably differentiate 0.04 ppt from 0.02 ppt?
In water? Doesn't seem like an issue.
In pure water it's not an issue. In drinking water which has all sorts of "stuff", getting very to that level of precision isn't easy.
I was browsing test kits where you send a sample back to a lab. They're $79 - $300 and advertise 1 or 2 parts per trillion. Looks like getting to 0.02 ppt requires some very specialized equipment, and would probably be optimized for continuous monitoring of a water supply.
It's more meaningful to get a blood test which Quest Health offers without a prescription in the US, although it's not cheap. Obviously the blood test is very precise, but it tests only for a few types of PFAS, whereas thousands of types exist in the environment.