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 . <EOS>