As I indicated in my welcome page, I have worked for 30 years at the Medical College of Wisconsin. we have a breeding colony of Wag/rij/MCW rats they are into their 70th generation.
Our ratties are gassed with a nose cone, Isoflurane, then we just continue until they have passed (absence of breath,heart pulse) This doesn't take long, as we use an overdose of the Isoflurane and they just pass away, it is very peaceful.
I actually saw a very nice youtube video of a rat being put to sleep, using the nose cone.
It is sad when we have to, but these little dollies just don't live long enough, so we need to enjoy them each and every day.
There are other methods, such as C02, but I find the Isoflurane doesn't scare them as much, they tend to just take in breaths and drift off, where as with the C02 they tend to struggle for breath more.