Used to determine the amount of a compound in various media such as the air, water or blood. Though there are many gas-phase atmospheric components that are most easily separated by gas chromatography, some of the urban air compounds are best analysed using HPLC. Examples are the multi-ringed aromatic compounds created in urban environments and condensed on urban particulates. Samples of the desired media (air, water, etc. ) are dissolved in solution and then separated by injecting the solution into mobile phase solution which is separated via interaction with a stationary phase. As the solution passes through the stationary phase, the components are separated based upon different partitioning behaviours between the components in the mobile phase and the stationary phase. This allows each component to be analysed separately as they pass through the detector.