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Supercritical fluids are produced by heating a gas above its critical temperature or compressing a liquid above its critical pressure. The critical temperature of a substance is the temperature above which a liquid phase cannot exist, regardless of pressure. The vapour pressure of a substance at its critical temperature is its critical pressure. At temperatures and pressures above but close to its critical temperature and pressure (the critical point), a substance is called a supercritical fluid.
Under these conditions, the molar volume is the same whether the original form was a liquid or a gas. Supercritical fluids have densities, viscosities and other properties that are intermediate between those of the substance in its gaseous and in its liquid state. Carbon dioxide is the most commonly used supercritical fluid because of its low critical temperature (31 oC), inertness, low toxicity and reactivity and high purity at low cost. Carbon dioxide does not dissolve polar compounds so when analysing that type of compounds methanol, cyclic ethers, water or formic acid can be added to the carbon dioxide.
Supercritical fluid chromatography
Supercritical fluid chromatography (SFC) is a hybrid of gas and liquid chromatography. SFC is of importance because it permits the separation and determination of a group of compounds that are not conveniently handled by either gas or liquid chromatography. These compounds are either nonvolatile or thermally labile so that gas chromatography cannot be used and they do not contain functional groups that make possible detection by liquid chromatography. SFC has been applied to a wide variety of materials including natural prodcuts, drugs, foods, pesticides and herbicides, fossil fuels, explosives and propellants.
A supercritical fluid chromatography instrument consists of a mobile phase container, an injector, a column in an oven, a restrictor and a detector. The components are similar to those of a gas chromatograph with exception of the restrictor. The restrictor is needed to maintain the pressure above the critical point. If the detector is a gas-phase detector working at atmospheric pressure (e.g., Flame Ionization Detector FID) the restrictor is placed before the detector. When using a detector that works under supercritical conditions (e.g., Ultra Violet detector UV) the restrictor is placed after the detector.
The most widely used mobile phase for supercritical fluid chromatography is carbon dioxide because it is an excellent solvent for a variety of organic molecules. A number of other substances have served as mobile phase including ethane, pentane, nitrous oxide, dichlorofluormethane, diethylether, ammonia and tetrahydrofuran.
Both open-tubular and packed columns are used for SFC. The open-tubular columns are most useful for separations requiring high-efficiency separations and for complex samples. Packed columns are most useful for high-speed separations requiring a moderate column efficiency and for samples containing fewer components.
When using capillary columns and carbon dioxide practically all Gas Chromatography (GC) detectors and many High Performance Liquid Chromatography (HPLC) detectors can be used. With packed columns and organic modifiers the number of detectors available is more limited. The Flame Ionization Detector (FID) is the most frequently used detector. Other detectors that often are used are Flame Photometric Detector (FPD), Electron Capture Detector ECD and Mass Spectrometer (MS).
Supercritical fluid extraction
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.
Supercritical fluid extraction (SFE) is based on the principle that solubilities in a supercritical fluid increase dramatically with increasing density, and that different solutes have different solubilities at the same conditions.
SFE is an important method for large-scale purification of complex liquid or solid matrices, such as polluted streams. The major advantage of this method is that the supercritical fluid can easily be removed after extraction by lowering the temperature or pressure or both. The supercritical fluid becomes a gas, and the extracted species condense into a liquid or solid. The problem of removing the extracting liquid is eliminated. An example of the SFE method is the removal of caffeine from coffee.
The main components of the SFE instrument are a pump, an extraction chamber, a recovery chamber and a collection device.
In order to generate a supercritical fluid, carbon dioxide is pressurized above its critical pressure in a pump. The mixture to be separated is placed in the extraction chamber and put in contact with the supercritical fluid. One of the elements (component A) in the mixture dissolves better in the critical fluid and leaves the residue enriched in the other components. The loaded solvent is then transferred to a recovery chamber, where component A is recovered by lowering the solvent's density. This density can be achieved by raising the temperature at constant pressure but more often it is achieve by reducing the pressure at constant temperature. After depressurizing, two methods have been adopted for collection of the extracted analyses, these are on-line or off-line SFE.
In on-line SFE, the extracted analytes are direct coupled to a chromatographic separation system such as SFC, gas chromatography (GC) or high performance liquid chromatography (HPLC) with appropriate detection. Directly coupled GC is limited to volatile compounds while SFE-SFC can be used for higher molecular weights.
The off-line approach allows the extraction and concentration of analytes for subsequent HPLC analysis.