AbstractAn apparatus for checking the contents of closed containers, such as baggage, for the presence of substances, such as explosives and drugs, which are capable of emitting characteristic vapours. The baggage is located in a compartment having means to promote the diffusion of the vapours out of the baggage and into the compartment. Such means can be by vacuum pumping, by vibration of the atmosphere within the compartment or a combination of both. The vapours diffusing into the compartment are drawn at substantially constant flow rate into a detector assembly sensitive to the presence of the vapours. Initially the concentration of vapours in the atmosphere evacuated from the compartment is low and a major proportion of this evacuated flow by-passes the detector assembly. As the pressure in the compartment is reduced there is an increase in the concentration of the vapours in the atmosphere and at a lower pressure limit all the exhaust from the compartment passes into the detector.