Supercritical fluid extraction disadvantages

These disadvantages (mentioned above) can be avoided by using supercritical fluid extraction. 

Supercritical fluid extraction (SFE) is a method that circumvents some problems associated with conventional separation techniques. Carbon dioxide, as an inert, inexpensive, nonflammable, and environmentally acceptable gas is the solvent of choice because of its moderate critical temperature and pressure (76). SFE has been used effectively to refine marine oils and remove cholesterol, polychlorinated biphenyls (PCB), Vitamin E, and other components (77). The disadvantages of this process include the use of extremely high pressures and the high capital cost. 

This chapter discusses the principal aspects of the technique in its two modes, the devices typically employed by each and their amenability to coupling with subsequent operations of the analytical process. Also, the main analytical applications of both modes in analytical chemistry are described, and their advantages and disadvantages with respect to alternative techniques such as Soxhlet, ultrasound-assisted, microwave-assisted or supercritical fluid extraction, discussed. 

Based on the number of reported applications, the unequivocal establishment of its scope, advantages and disadvantages from the results of such applications, and the inception of automated coupled systems [132,133], supercritical fluid extraction has already come of age. 

A disadvantage of supercritical fluids for extraction is that most common fluids used for extraction (carbon dioxide, nitrous oxide, sulfur hexafluoride, etc.) are weak solvents, limiting the polarity and molecular weight range of analytes that can be efficiently extracted. Also, for trace analysis the availability of fluids of adeguate ptirity may be a problem. 

Supercritical fluids, in general, are inexpensive, contaminant-free, and less costly to dispose of safely than conventional organic solvents. Extracts are obtained under mild conditions that minimize thermal degradation, they are usually solvent-free or in a concentrated form, and no evaporation steps are needed prior to the final assay. However, the disadvantages of SFE should not be also ignored. As with all extraction methods, there are analytes and matrices for which SFE is not suitable. Some compounds are insoluble and may need solvent extraction. Aqueous matrices can cause problems and samples may need to be freeze-dried. Like all single-step extractions, SFE has only limited selectivity, but some distinction can be made between analytes by careful selection of temperature and pressure. 

In the static addition mode, a preset volume of modifier highly dependent on sample size is directly added to the sample, held in the cell, prior to extraction. The principal disadvantage of this mode is that, as the supercritical fluid begins to circulate through the sample, the modifier is swept from the extraction cell, so the matrix is brought out of contact with the modified supercritical fluid. Although dynamic addition is usually more effective than static addition — the modifier is continuously passed through the sample in the former — Ashraf and Taylor [35,36] found the static addition of methanol... 

To take away the mystery from the word supercritical, it should be recognized that a supercritical fluid can be used like any other solvent for maceration and percolation processes. The only restriction is that it must be handled under high pressure, a fact that requires a special and expensive design of the extraction apparatus. This obvious disadvantage however is compensated by many benefits as demonstrated below. Supercritical C02-extraction of botanical materials is today a well-developed and reliable procedure applied on an industrial scale for about 20 years. The equipment is available turn-key from various suppliers in multi-purpose design or tailor made for special applications. 

Considerable research is being done to increase the utility of CO2 as a solvent. In the area of pharmaceutical applications, supercritical fluids are useful as solvents in the production of particulate drugs, the extraction and separation of active ingredients, and the preparation of microemulsions and sustained drug delivery systems (2). A major disadvantage, insofar as solvent... 

While the yields of extract may be lower than that can be obtained with some liquid solvents and the use of high pressures might appear to be disadvantageous, there are nevertheless two positive features related to supercritical gas/fluid extraction of coal ... 

There are a number of online methods coupling SFE to other analytical techniques such as gas chromatography (GC), capillary or packed column supercritical fluid chromatography (SEC), and liquid chromatography (EC). The online techniques are rather sensitive because the entire portion of extracted components is transferred to the column. A disadvantage with online approaches is that coextracted fat, water, and sometimes modifier may interfere in the final analysis. Additionally overloading of the analytical column must be avoided. Therefore, only small samples are usually extracted with online methods in SFE. A comprehensive discussion of online methods can be found in Further Reading. 

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