Paraformaldehyde in PBT-H, RNase-free (4% PFA in PBT-H)

Recipe

Reagent	Quantity	Final concentration
Paraformaldehyde powder (EM grade; Polysciences or equivalent)	4 g	4%
DEPC-treated H₂O, sterile	80 mL
Tween 20	0.1 mL	0.1%
NaOH (10 M)	2 drops
Phosphate-buffered saline (PBS), RNase-free (10X, pH 7.4)	10 mL	1X

Incubate the paraformaldehyde in H₂O and Tween, 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 10X PBS.
PBT-H is PBS with 0.1% Tween 20.