Paraformaldehyde in 1X PBS, RNase-free (4% PFA)

Recipe

Reagent	Quantity	Final concentration
Paraformaldehyde powder (EM grade; Polysciences or equivalent)	4 g	4%
DEPC-treated H₂O, sterile	80 mL
NaOH (10 M)	2 drops
Phosphate-buffered saline (PBS), RNase free (10X, pH 7.4)	10 mL	1X

Incubate the paraformaldehyde in DEPC-treated H₂O, treated with 2 drops of 10 M NaOH, at 65°C, mixing occasionally until the powder is completely dissolved. Chill the solution on ice and then add the 10X PBS. Adjust the volume to 100 mL with DEPC-treated H₂O. Prepare fresh before use.