HPLC - High-performance liquid chromatography (HPLC) is used to determine the amount of a compound in various media such as the air, water or blood. Samples of the desired media (air, water, etc.) are dissolved in solution and then the components are separated by injecting the solution through a mobile phase solution and then a stationary phase solution. As the solution passes through the stationary phase, the components are separated out due to different partitioning behaviors between the components in the mobile phase and the stationary phase. This allows each component to be analyzed separately as they pass through the detector. 
Atmospheric Chemistry Glossary