Paraformaldehyde Solution (PFA), 4% (w/v)
Dissolve 4 g of PFA (e.g., Sigma-Aldrich 8.18715) in 100 mL of phosphate-buffered saline (PBS) with thimerosal. Heat to 60°C and let stir for ∼30 min. After the solution becomes transparent, pour it into a volumetric flask, let it cool in the refrigerator, and adjust the volume to 100 mL with PBS with thimerosal. Use fresh or store for up to 1 yr at −20°C, and thaw the needed volume just before use.
