Ironically, one of the recognized ways to euthanize pinks are through freezing. They have a low oxygen affinity and resist hypoxia, so CO2 may not work if you don't leave them in longer than older rats. The low oxygen affinity is due to being able to tolerate mom sitting on them.

And yes. There are recognized euthanasia procedures. Many non recognized ways are illegal and/or dubbed to be animal cruelty. Of course all this can be subjected, but there is an 'official' veterinary approved list out there