A vapour generator for a detection apparatus, the generator comprising: an impeder; and a vapour source comprising a vapour chamber coupled by a flow path to provide vapour through the impeder to an outlet for dispensing vapour to the detection apparatus, wherein the impeder comprises an absorption assembly, the absorption assembly comprising: a first vapour permeable passage arranged to enable vapour to be driven from the source to the outlet, the first vapour permeable passage comprising a material adapted to take up the vapour to impede diffusion of the vapour from the source to the outlet, wherein the absorption assembly comprises a passage inlet configured to receive flows of vapour from the vapour chamber, the first vapour-permeable passage having a first end in fluid communication with the passage inlet and a second end in fluid communication with the outlet, and a sink separated from the outlet by the first vapour permeable passage wherein the sink comprises at least one second vapour permeable passage in fluid communication with the passage inlet, the second vapour permeable passage comprising a material adapted to take up the vapour to divert diffusion of vapour away from the outlet. an impeder; and a vapour source comprising a vapour chamber coupled by a flow path to provide vapour through the impeder to an outlet for dispensing vapour to the detection apparatus, wherein the impeder comprises an absorption assembly, the absorption assembly comprising: a first vapour permeable passage arranged to enable vapour to be driven from the source to the outlet, the first vapour permeable passage comprising a material adapted to take up the vapour to impede diffusion of the vapour from the source to the outlet, wherein the absorption assembly comprises a passage inlet configured to receive flows of vapour from the vapour chamber, the first vapour-permeable passage having a first end in fluid communication with the passage inlet and a second end in fluid communication with the outlet, and a sink separated from the outlet by the first vapour permeable passage wherein the sink comprises at least one second vapour permeable passage in fluid communication with the passage inlet, the second vapour permeable passage comprising a material adapted to take up the vapour to divert diffusion of vapour away from the outlet. a first vapour permeable passage arranged to enable vapour to be driven from the source to the outlet, the first vapour permeable passage comprising a material adapted to take up the vapour to impede diffusion of the vapour from the source to the outlet, wherein the absorption assembly comprises a passage inlet configured to receive flows of vapour from the vapour chamber, the first vapour-permeable passage having a first end in fluid communication with the passage inlet and a second end in fluid communication with the outlet, and a sink separated from the outlet by the first vapour permeable passage wherein the sink comprises at least one second vapour permeable passage in fluid communication with the passage inlet, the second vapour permeable passage comprising a material adapted to take up the vapour to divert diffusion of vapour away from the outlet.