Supercritical fluids can be used to extract analytes from samples. The main advantages of using supercritical fluids for extractions is that they are inexpensive, contaminant free, and less costly to dispose safely than organic solvents. For these reasons supercritical fluid CO2 is the reagent used to extract caffeine from coffee.
The properties of supercritical fluids also provide some advantages for analytical extractions. Supercritical fluids can have solvating powers similar to organic solvents, but with higher diffusivities, lower viscosity, and lower surface tension. The solvating power can be adjusted by changing the pressure or temperature, or by adding modifiers to the supercritical fluid. A common modifier is methanol (typically 1-10%) which increases the polarity of supercritical CO2.
A supercritical-fluid extractor consists of a tank of the mobile phase, usually CO2, a pump to pressurize the gas, an oven containing the extraction vessel, a restrictor to maintain a high pressure in the extraction line, and a trapping vessel. Analytes are trapped by letting the solute-containing supercritical fluid decompress into an empty vial, through a solvent, or onto a solid sorbent material.
Extractions are done in dynamic, static, or combination modes. In a dynamic extraction the supercritical fluid continuously flows through the sample in the extraction vessel and out the restrictor to the trapping vessel. In static mode the supercritical fluid circulates in a loop containing the extraction vessel for some period of time before being released through the restrictor to the trapping vessel. In the combination mode, a static extraction is performed for some period of time, followed by a dynamic extraction.
Pictures of supercritical fluid extractors