Is there any way you can pre-weigh streptozotocin out into tubes of smaller quantities and then dissolve in the buffer in the animal room just prior to injection?
To maintain the powder in active form (dry), make sure that the bottle/vial is at room temp before opening. Weigh approximately the desired amount (or slightly more) in individual tube/vials by difference on a reliable balance, label tubes with the actual amounts and seal them as best as possible. As an added measure of safety, it is a good idea to seal caps/threads with Parafilm. Make solution just prior to use at a fixed concentration such that the volume needed for injection can be easily calculated without the use of calculator and wasting time.