Argon is the new CO2, apparently it produces less distress when used on rodents etc.
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"Therefore, based on the findings of these studies the
recommended anaesthetic agent for rats is halothane and for mice
is enflurane as at appropriate concentrations they induced a
rapid and effective induction with the minimum of distress. The
recommended method of euthanasia using a single agent would be
argon. However, inducing unconsciousness with a volatile liquid
anaesthetic (eg halothane and enflurane) and subsequently
rapidly killing with carbon dioxide after the animals are
unconscious can be considered more humane than argon alone. As
once an animal is unconscious then exposure to carbon dioxide,
which is an effective killing agent, is not a welfare issue."