Hallo Rebecca, it depends on the kind of tissue. Most commen fixativ is eithe  4% bufferedFormalin (ca.38 %) or better 4% Paraformaldehyd in PBS pH 7.4. If you are woking with animals I recommand to perfuse them with this fixativ. After the perfusoin  stay 1 additional day in the fixativ and then put it in 30% succrose until it will sank on the bottom of the vial.

Hello!

I thought I will just share my protocol, which seems to be working for me:

I normally work with animals, so, I will first profuse the animal with fixative (4% PFA or 4% formalin) and quickly harvest the tissue of interest. Then, the harvested tissues are incubated in 4% fixative for a max of 6 hrs (This is because over fixation can also cause autofluorescence during IHC-and this can be very annoying). After fixing, tissue can be cryoprotected in 20% sucrose for overnight or till it sinks to the bottom.

Hope this helps :)

The 4% PFA needs to be fresh as it only lasts for 5-7 days and it needs to be diluted in a buffer. We make up 20% then dilute with a stock buffer and pH to 7.4 before making up to the final volume.

What tissue? What species? How big a bit of tissue? Beware 4% PFA (does go off), 4% formaldehyde and 10% formalin are all the same concentration of formaldehyde. But its  easy to get the concentration wrong. I use 10% neutral buffered formalin (10% NBF) as it avoids the creation of formalin pigment artefact. 

No fixative is used for frozen section

But NO is used for blocking & embedding