CO2 is not a humane method of putting rats to sleep. A lot of research has been done, some of which can be found in the reference section.
As mia9_28 wrote, please see
viewtopic.php?f=26&t=9582
for the info you are requesting
Make sure you discuss the procedure long before you need to have a rattie pts, and as mia9_28 mentioned .... make sure you are present to ensure it is done right and to comfort your baby.
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I hold my rat in my arms with the "mask" re the isofluorane gas and oxygen mix used during surgery, placed near their face (not over their face) ..... while I hold my rat in my arms and pet him/her. After s/he becomes unconscious, my vet makes sure my rat is deeply unconsciuos by gently pinching the foot to check for a reflex and by touching their eye to check for a reflex ... I always request that they are under at least as deeply as they would need to be for surgery.
Then the vet injects the correct dossage of sodium pentobarbital using a 27 - 29 gauge needle. The injection is given in the lower right side of the abdomen, although recently my vet has started injecting into the liver.
My rat lays in my arm, still unconscious and still getting the isofluorane gas oxygen mixture. I hold and cuddle my rattie ... eventually they stop breathing and then their heart stops. It takes about 20 to 40 minutes.
My vet checks to make sure my rattie's heart has stopped.
Some vets like to administer a sedative prior to the gas. If they do, make sure they use a 27 - 29 gauge needle as the higher the gauge, the smaller the needle. I have found it is much better just to give the gas mixture. I gave also read that it is better not to give a sedative to a rat experiencing difficulty breathing as it may make it harder for them to breath.
You may want to order Debbie D.'s Rat Health Care book (
http://www.ratfanclub.org)
It is updated yearly, and costs less then $10. It is a valuable resource, just ignore the section on home euthanasia using C)2 gas as it is inhumane.