Abstract: Environmental monitoring of low concentration of organic compounds in a medium such as ground water or another environmental fluid is effected by using a diffusion cell (112) having a polymeric membrane formed into a helical tube (125) and immersed in the environmental fluid so that organic compounds at ppb and ppt concentrations diffuse into and through the polymeric material. A carrier gas in the polymeric tube acquires a concentration of the organic compounds substantially in equilibrium with the concentration of the organic compounds in the environmental fluid. For the purpose of permitting monitoring with detectors (116) which are not sensitive to extremely low levels of organic compounds, a concentrating device such as a thermal desorption tube (114) can be provided for processing the gas and periodic thermal activation to discharge therefrom organic compounds to be detected at a sensor.