4% PFA (paraformaldehyde) fixative recipe
Fixes proteins in place before immunostaining cells or tissue: 4% paraformaldehyde in PBS.
🧮 Volume calculator
| Component | Amount | Final concentration (1X) |
|---|---|---|
| Paraformaldehyde (powder) | 4 g | 4% (w/v) |
| 1X PBS | as needed | to final volume |
| 1 N NaOH | as needed | a few drops until clear |
Steps
- In a fume hood, warm about 80% of the PBS to 60 °C (do not boil).
- Add PFA powder and stir, adding 1 N NaOH dropwise until the solution clears.
- Cool, adjust to pH 7.2–7.4 and bring to volume with PBS.
- Filter and use within a week at 4 °C, or aliquot and freeze at −20 °C.
Good to know
- Diluting methanol-free 16% formaldehyde ampoules 1:4 in PBS is convenient and consistent.
⚠ Safety PFA/formaldehyde is toxic and a suspected carcinogen — always handle in a fume hood with gloves and goggles.
FAQ
Are formalin and PFA the same?
Formalin is 37% formaldehyde solution containing methanol; PFA is its polymer dissolved without methanol, which better preserves fluorescent proteins.
This is a widely used standard composition. Amounts depend on the hydrate form and purity of your reagents — check the formula weight (FW) on the bottle and follow your institution's safety rules and the manufacturers' instructions.