Abstract: A hollow container defining a sample chamber is formed of at least two parts which fit together and have apertures therein. One of the parts is displaceable relative to the other part between a first relative disposition of the parts in which the apertures are open so as to provide a clear path for air to pass through the container and a second relative disposition of the parts in which the apertures are closed so that there is no path for air to pass through and a volume of air is trapped in the chamber. In one embodiment part is axially moveable relative to the other part. In another embodiment where the parts are tubular, one of the parts is rotatable relative to the other.