Solid Phase Extraction is used in the chemical analysis of fluid samples by removing unwanted constituents and keeping only the analytes of interest. It is a crucial sample preparation step for liquid and gas chromatography. Typical applications include the analysis of pesticide and various pollutants in water, drug in food and blood, as well as oil and grease in environmental samples. Original Sample =❯  Solid Phase Extraction  =❯  Analysis. The working principle behind SPE is the adsorption of analytes to the solid phase (sorbent material in SPE cartridge), thus removing them from the original liquid phase (sample fluid). The type of SPE cartridge has to be selected based on the chemical and physical properties of the analytes.