the context of air pollution monitoring as a consequence of the wide range of possible compounds present at relatively low concentrations Methods include GC with preconcentration, in which ambient air is collected for a period of time (typically ambient 30 min to an hour) on a sorbent or in a cold trap, before being thermally volarilized onto a GC column. Elution of the mixture through the GC column with a carrier gas such as helium separates the various VOC components, which can be identified through their characteristic retention times, and quantified as they exit the column with a flame ionization detector (FID) or mass spectrometer. The preconcentration stage permits detection of small molecules at levels down to 0.1 ppb or lower. passive diffusion tubes are so used for VOC measurement,particularly for the large less-volatile species (Co and e.g., benzene and the BTEX compounds)