Supercritical Fluid Extraction natural compounds

Supercritical fluid extraction (SFE) has been extensively used for the extraction of volatile components such as essential oils, flavours and aromas from plant materials on an industrial as well as an analytical scale (61). The extract thus obtained is usually analysed by GC. Off-line SFE-GC is frequently employed, but on-line SEE-GC has also been used. The direct coupling of SEE with supercritical fluid chromatography (SEC) has also been successfully caried out. Coupling SEE with SEC provides several advantages for the separation and detection of organic substances low temperatures can be used for both SEE and SEC, so they are well suited for the analysis of natural materials that contain compounds which are temperature-sensitive, such as flavours and fragrances. 

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

As its name suggests, supercritical fluid extraction (SEE) relies on the solubilizing properties of supercritical fluids. The lower viscosities and higher diffusion rates of supercritical fluids, when compared with those of liquids, make them ideal for the extraction of diffusion-controlled matrices, such as plant tissues. Advantages of the method are lower solvent consumption, controllable selectivity, and less thermal or chemical degradation than methods such as Soxhlet extraction. Numerous applications in the extraction of natural products have been reported, with supercritical carbon dioxide being the most widely used extraction solvent. However, to allow for the extraction of polar compounds such as flavonoids, polar solvents (like methanol) have to be added as modifiers. There is consequently a substantial reduction in selectivity. This explains why there are relatively few applications to polyphenols in the literature. Even with pressures of up to 689 bar and 20% modifier (usually methanol) in the extraction fluid, yields of polyphenolic compounds remain low, as shown for marigold Calendula officinalis, Asteraceae) and chamomile Matricaria recutita, Asteraceae). " ... 

Extraction with supercritical CO2 is a technical process of increasing importance. It provides a mild and rapid technique for the extraction of low- or medium-polarity substances. Supercritical CO2 is used for supercritical fluid extraction (SFE) in important technical processes such as the decaffeination of coffee and the extraction of hops, as well as the extraction of naturally occurring compounds from biomaterials. As many applications are performed in the pharmaceutical, polymer, environmental and nutritional fields, direct on-line SFE-NMR would be an ideal tool to monitor the various extraction processes. 

Thermoanalysis methods such as pyrolysis-gas chromatography-mass spectrometry [GC-MS] and thermogravimetry mass spectrometry have been used to characterise hydrocarbon sludges from petrochemical plants and polluted soils [26,27]. In combination with conventional extraction and supercritical fluid extraction followed by [ GC-MS ], over 100 constituents were identified in samples. White et al. [28] also applied pyrolysis-[GC-MS] to the determination of hydrocarbons and showed that the analysis can be complicated by the presence of natural organic matter. White [28] inferred the presence of biogenic compounds in Alaskan soil. 

Supercritical fluid extraction(SFE) combined with five types of bioassay tests is extensively applied to explore some bioactive substances from thirty types of natural resources available in Korean peninsula. To evaluate comparatively the economic viability of the SFE, organic liquid solvent extraction(LSE) with n-hexane, chloroform and methanol was also performed. To characterize the extracts, GC and HPLC are employed. Also, the column chromatography is used to isolate some target compounds from the total extracts. For all the samples, the optimum SFE condition for each sample which gives maximum yield and cytotoxicity were discussed. 

In the Orient, numerous natural products have long been used in folk medicine. Also, in recent years some pharmaceutical industries and universities in Korea are placing their efforts on the quantification of cytotoxic phytochemicals from the natural resources. As a part of those efforts, the present authors are involved on the two-fold critical evaluations of the possible implementation of the supercritical fluid extraction(SFE) to obtain extracts from the natural resources one is the establishment of the optimum SFE condition for each sample resource which gives maximum extraction yield and cytotoxicity, and the other is the high-purity isolation of some specific compounds from the SFE total extracts. 

Steam distillation is the main commercial extraction procedure for the production of essential oils from almost any type of plant material. Solvent extraction is also used commercially and yields a resinoid, concrete or absolute according to the solvents and techniques used (see Chapter 4). Both steam distillation and solvent extraction are used on a laboratory scale to produce oils and extracts for analysis. Other methods of extraction, such as supercritical fluid extraction (SFE), which uses supercritical CO2 as the extraction solvent, are now being developed and used on both commercial and laboratory scales. The extracts produced by SFE may contain different materials from the steam-distilled oil because of the solvating power of C02 and the lower extraction temperature, which reduces thermal degradation. The C02 extract may therefore have an odour closer to that of the original material and may contain different fragrant compounds. The choice of extraction procedure depends on the nature and amount of material available, and the qualities desired in the extract. Solvent extraction is better suited to small sample amounts or volatile materi-... 

Analysis of plants normally involves a sample preparation stage such as extraction or distillation followed by analysis with gas chromatography or liquid chromatography. The common methods used currently for the isolation of essential oils from natural products are steam distillation and solvent extraction (Ozel Kaymaz, 2004). Losses of some volatile compounds, low extraction efficiency, degradation of xmsaturated compounds through thermal or hydrolytic effects, and toxic solvent residue in the extract may be encountered with these extraction methods. Recently, more efficient extraction methods, such as supercritical fluid extraction (SFE) (Simandi et al., 1998) and accelerated solvent extraction (ASE) (Schafer, 1998) have been used for the isolation of organic compounds from various plants. Subcritical or superheated water extraction (SWE) is non-toxic, readily available, cheap, safe, non-flammable and is a recyclable option. 

The use of essential oils is increasing because of the increase in the number of their apphcations and in the framework of natural and environmentally friendly materials. Many times the analysis of their components is quite complex due to the high number and the diversity of compounds in their composition. In this entry a general overview of the extraction methods is given by comparing conventional hquid-liquid and sohd-hquid methods with new alternative ones, such as supercritical fluid extraction and microwave-assisted extraction. Gas chromatography methods and examples are treated and important issues such as detection systems, modem hbraries for compounds identification, as well as multidimensional or hyphenated techniques are discussed. The use of these modem techniques and methods has improved resolution and sensitivity in essential oils determination and could open the possibihty of future work in this area of chromatography. 

Recently, there has been an increasing interest in the use of supercritical fluid extraction (SEE) with carbon dioxide (CO2) as a solvent. This process uses the properties of gases above their critical points to extract selective soluble components from a raw material. Carbon dioxide is an ideal solvent for the extraction of natural products because it is nontoxic, nonexplosive, readily available, and easy to remove from extracted products [3,6]. SFE has the abihty to use low temperatures, leading to less deterioration of the thermally labile components in the extract. In addition, SFE is typically carried out in the absence of air which also ensures minimal alteration of the active ingredients and preservation of the curative properties [46, 47]. SC CO2 is generally efficient in the purification and fractionation of hydrophobic compounds, such as flavonoids and cinnamic acid derivatives from plant matrixes [49]. 

Finally, a more modem way of extraction is supercritical fluid extraction. In this process, carbon dioxide in supercritical conditions is used to extract the hydrophobic aromatic compounds from the raw material. It does not alter the nature of the aromatic compounds as it takes place at a low temperature and CO2 vaporizes after depressurization. However, it is an expensive process and needs special instmmentation [2]. 

In recent years, many research activities have focused on Natural Products (NPs) derived from Traditional Chinese medicines (TCMs), thus making a renaissance in the drug discovery process ofTCMs. Maximizing the diversity of extracts from those plants is the key for the chemical biology process. Methods for the preparation and pretreatment of plant extracts are very important for further purification and discovery of active compounds present in minor quantities. In this chapter, two methods of extraction, including one of the most broadly applicable method (solvent extraction) and one newly developed technique (supercritical fluid extraction), have been described in detail. 

Supercritical fluids can be used to extract substances from natural products, as solvents or as anti-solvents to micronize drugs and biodegradable polymers, encapsulate drugs in polymeric matrices, resolve racemic mixtures of pharmacologically active compounds, fractionate mixtures of polymer and proteins, and sterilize bacterial organisms. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

---