An apparatus for detecting a presence of at least one analyte in a gas sample. The apparatus comprises a pump for drawing a gas sample from an ambient air, a passage having first and second ends, a chamber connected to the first end and containing a concentrating element for collecting at least one analyte from the gas sample, a chromatographic separator connected to a second end of the passage, and a gas source for streaming a carrier gas via the chamber to transfer the at least one analyte toward at least one chemical detector, via the chromatographic separator, in a first direction. The pump draws the gas sample via the chamber in a second direction and the first and second directions are substantially opposing to one another.