An apparatus for collecting volatile compounds in human breath includes a device for discriminating between alveolar and non-alveolar portions of exhaled breath by real-time measurement of a physical characteristic of said exhaled breath a device for measuring volume of exhaled breath a chamber with piston or similar compressible device with clean internal surfaces designed to collect a precise volume of the alveolar portion of the exhaled breath a pump to draw the exhaled breath collected in the chamber through at least one sorbent tube a subsystem for introducing a clean compressed gas into the chamber to expand it, and for purging the internal tubing of the system and a subsystem for selectively collecting a room air sample. A manifold for receiving at least one sorbent tube includes an input block having at least one open-topped channel for receiving an input end of a sorbent tube, and an input port within the open-topped channel configured to be in fluid communication with an input end of the sorbent tube an output block having at least one open-topped channel for receiving an output end of a sorbent tube, and an output port configured to be in fluid communication with an output end of the sorbent tube and a locking lever associated with at least one of the input and output blocks for moving the input and output blocks linearly towards and away from each other between fully open and closed positions.
