We also freeze small aliquots of ab after finding out that repeated
opening of the ab vials caused the ab to weaken before we were finished
with it.  But instead of using centrifuge vials - we use cryrovial by
Nalgene because they have conical shape on the inside but are flat
bottomed on the outside.  Makes for much easier handling.  Also, by
pre-measuring ab - you know exactly how much you have on hand rather
than estimating visually and coming up short one morning!  Our ab has
kept well for up to a year with no problems.