The liquid N (Pisabental) comes in a 100ml glass vial With a flip off top & typical rubber stopper and metal clamp seal. You could (assuming you have a syringe) simply remove the flip off top, withdraw say, 1ml for testing and the seal and stopper would still be in place. Yes you would have punctured the rubber stopper, but Those are multi dose vials & the rubber stopper reseals after a syringe is inserted & withdrawn.
However if you completely remove the seal & rubber... the shelf life will be significantly decreased as oxygen get to the pentobarbital. I can't say for certain, but you would be decreasing the shelf life from Several years to perhaps 30 days.