When confronted with the task of ensuring that the headspace of a vial contains a customer specified gas there are several options available. However if the evacuation of atmospheric gasses and replacement with a specified gas is critical and must be verified, the challenge requires specialized equipment.
Most gassing systems used today are passive systems which flush a vial with gas in order to displace the atmospheric gas with the new gas. This is a generally reliable system for open vials when the evacuation of the original gas is not 100% critical. When the removal of the original gas is critical to the function of the product in the vial, more aggressive techniques must be employed to ensure the removal of the original atmosphere.