Supercritical fluid extraction (SFE) is a relatively new technique in the field of analytical chemistry, having evolved in the last decade as an alternative method for preparing samples prior to analysis. SFE offers many advantages compared to traditional sample preparation techniques (like liquid solvent extraction in a Soxhlet apparatus), such as rapidity, inexpensiveness, quantitative recovery of analytes without chemical degradation (oxygen free extraction). Moreover, as no solvent is used for the extraction, no additional laboratory wastes are generated. A supercritical fluid is a substance that, at temperatures and pressures greater than its critical temperature and temperature and pressure, it a gaslike, compressible fluid that takes the shape and fills its container. It is not a liquid (an incompressible fluid that takes the shape of the bottom of its container) but has liquid like densities (0.1 to 1 g/ml) and solvating power. Supercritical fluids have both gas- and liquid-like properties. They possess gaslike mass transfer properties and the solvation characteristics of liquids. Their high diffusivity allows them to penetrate solid materials, and their liquid like densities enable them to dissolve analytes from a solid matrix. Supercritical fluids are compressible, and small pressure changes produce significant changes in their density and in their ability to solubilize compounds. Carbon dioxide, the supercritical fluid most frequently used in SFE, is chemically inert, non toxic, and available in a fairly pure state at a reasonable cost. It can be easily removed from the collected analyte because of its low boiling point, and it causes no disposal, problems. Furthermore, its critical temperature and pressure are low (31.1°C and 72.8 atm, respectively). Carbon dioxide is an apolar media; it can be advantageously used in the elution of non polar substances. To enhance the solvating power of CO2 for polar or high molecular mass analytes, miscible organic entrainers, known as modifiers, are commonly added to SF mobile phases. The essential tools for high performance supercritical fluid extractors include: a CO2 source, a pumping system (liquid CO2), an extractor thimble, a restriction device, an analyte collection device, a temperature-control systes for several zones, and an overall system controller (figura) The solvent strength of a supercritical fluid can easily be controlled by changing the pressure (which acts on lipophilicity) and, to a lesser extent, the temperature. The right combination of the parameters makes possible to perform selective extractions with a single supercritical fluid. As pressure and density increase, the solvating power of the fluid increases. For CO2, low pressures such as 75 bar (1090 psi, 80°C, d= 0.15 g/ml) correlate to a solvent power analogous to pentane. For higher pressures such as 380 bar (5550 psi, 40°C, d=0.95 g/ml), the solvent power is more similar to liquid solvents such as methylene chloride, carbon tetrachloride, toluene and benzene. When selecting the pressure for an extraction, a good rule to thumb is that the lower the analyte’s molecular weight, the less the pressure-density is required for compound solubilization. A second consideration for selecting a pressure level relates to polarity and polar interactions: the more polar an analyte is or the more tightly it is bound to the matrix, the greater the pressure-density required for the dissolution and removal of the analyte from the matrix. As little information is yet available about the preconditions a substance must fulfil to be extracted with supercritical carbon dioxide, formulated generalized extraction rules assert that only hydrocarbons and other typically lipophilic organic compounds of relatively low polarity and low molecular weight can be extracted in the lower pressure range (70-100 bar); the introduction of strongly polar functional groups (-OH and -COOH) makes the extraction more difficult. Temperature is also an important but complex parameter for controlling the extraction. Temperature and density vary inversely and non linearly. As temperature increases (at constant pressure), the density decreses. the experience suggests that choosing a low temperature-high density (relatively low pressure) combination often yields fast, efficient extractions. Selectivity can be achieved at low temperatures (32-60°C) and low densities. (A low temperature-density procedure is useful for thermally sensitive compounds). By controlling the polarity and the density of the fluid, it is possible to control the solvating power. Thus samples can be fractionated based on the solubility of the solutes in the fluid.

In our application on phenolic components from Ginkgo biloba (transparency), the extraction of ginkgolic acids at higher densities could be performed by considering two facts: in spite of polar groups both in ginkgolic acids and cardanols, the presence of a long alkyl chain increases the solubility of these compounds in the range of densities of carbon dioxide; moreover, the co-presence of long chain fatty acids may act as "modifier" creating a multicomponent mixture which effects the equilibrium data for the solid-fluid system, thus causing substantial increasing in the component solubilities of all species. This observation holds a substantial role when the extraction of substances present in plant material should be performed. In this case, for example, chlorophylls and very polar substances remain unextracted in the plant material, thus rendering the process free from coloured contaminants and making possible the recovery of more polar substances by successive liquid extraction.

Ginkgolic acids

In the preparative scale, SFE offers many advantages in comparison with traditional extraction procedures: no chemical solvents are used, thus avoiding evaporation steps, recycling, storing and wasting. Extractable material can be recovered (dry) after allowing the supercritical fluid reach its normal conditions.