Turn ON the CO 2 cylinder tap. Use short bursts until the sample is completely frozen. Turn OFF the CO 2 cylinder tap. Remove the safety lid, release the chuck and place it in the cryostat. Allow to equilibrate for 10 - 15 minutes before cutting.
Muscle is the exception when freezing tissue for cryotomy.
It must be frozen using cold iso-pentane or ice crystal artefact will occur. Take unfixed muscle and cut to an appropriate size and orientation. Put a small amount of liquid nitrogen in a thermos flask - about 5cm deep. Wait until the iso-pentane is syrupy and almost solidifying. If it does solidify take it out of the nitrogen and stir it with a plastic spatula until it is mostly liquid again.