Supercritical fluid extractors

Center in Wyndmoor Pennsylvania is developing advanced technologies for the analysis of endosulfan in meat, poultry and eggs (FEDRIP 1999). This technique will include the use of a supercritical fluid extractor in order to reduce the amount of organic solvent use and to speed up extraction times. 

Bonazzi et al. [18] reported the determination of miconazole and other imidazole antimycotics in creams by supercritical fluid extraction and derivative ultraviolet spectroscopic method. Cream based pharmaceuticals were mixed with celite and anhydrous sodium sulfate and extracted by supercritical fluid extractor (SFE) with... 

The apparatus incorporates a fibre optic interface for the spectrofluorimetric measurement on the supercritical carbon dioxide emerging from the extraction cell of a supercritical fluid extractor, prior to depressurization from up to 350 bar. Recoveries of polyaromatic hydrocarbons are between 89 and 107%, and measurements can be carried on with a relative standard deviation of less than 5%. 

A laboratory-assembled supercritical fluid extractor was designed for the efficient recovery of volatile nitrosamines from frankfurters. The nitrosamines were separated and detected using a GC-TEA-CLD. Recovery of 10 volatile aliphatic and alicylic nitrosamines from frankfurters spiked at the 20 ppb level was 84.3-104.8% with RSD 2.34-6.13%581. 

The adaptation of supercritical fluid extraction (SFE) in routine residue and metabolism analysis as well as other extraction/separation laboratories and applications has been slow. This is despite the demonstrated feasibility of using SFE for the removal of sulfonylureas, phenylmethylureas and their metabolites from soil and plant materials (1-2), as well as widespread demonstrated use of supercritical fluid extraction for other applications (3-6). The reason for this is simple. Although automated, SFE extraction apparatus typically only analyzes a single sample at a time. The technique could not compete effectively with the productivity of an experienced technician performing many sample extractions simultaneously. In essence, with a one vessel automated supercritical fluid extractor, operator attendance is high and throughput is about the same or even less than current conventional liquid-liquid and solid-liquid extraction techniques. 

Samples of sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and a standard reference soil (certified for 13 polynuclear aromatic hydrocarbons, dibenzofuran, and pentachlorophenol) were extracted with supercritical carbon dioxide in a two- or four-vessel supercritical fluid extractor to establish the efficiency of the extraction and the degree of agreement of the parallel extraction recoveries. Furthermore, the many variables that influence the extraction process (e.g., flowrate, pressure, temperature, moisture content, cell volume, sample size, extraction time, modifier type, modifier volume, static versus dynamic extraction, volume of solvent in the collection vessel, and the use of glass beads to fill the void volume) were investigated. 

Schematic of a supercritical fluid extractor. 1 = COj 2 = modifier 3 = pump 4 = oven 5 = mixing column 6 = extraction cell 7 = pressure regulator 8 = collection of the extract.

The equipment needed is much simpler, so the overall cost of leaching is much lower. This can be of interest to routine laboratories with a limited budget, unable to afford a supercritical fluid extractor. 

In summary, the problems associated with this system were related to the fact that supercritical fluids had been used previously for batch extraction processes and there had been no significant development to this time of supercritical fluid extractors for the specific application of precision cleaning. These problems were exacerbated by an unwillingness of the vendors to discuss many of the problems they had already encountered and solved, based on proprietary process concerns. 

In addition to the previous three commercial extractors, some authors have developed custom models [78,79], adapted in most cases from a supercritical fluid extractor [25,58,80]. For example, Heemken et al. altered a Suprex SF extractor for use in ASE [81] they disconnected the syringe pump from the COj cylinder and filled it with a suitable ASE solvent. The restrictor was replaced with a stainless steel capillary tube leading into the trapping vial and an additional nitrogen pipe was installed at the inlet valve of the extraction vessel for purging after extraction. In the extraction of PAHs from soil [79], a custom extractor and commercially available equipment provided equivalent results on the other hand, in the extraction of benzene and toluene from soil [78], the former provided even better results than the latter. 

LABORATORY-BUILT AND COMMERCIAL SUPERCRITICAL FLUID EXTRACTORS... 

Fig. 7.2. Schematic diagram of a straightforward supercritical fluid extractor. 1 extracting fluid source, 2 extractant propulsion unit, 3 modifier reservoir, 4 modifier propulsion unit, 5 oven, 6 equilibration coil, 7 chamber containing the thimble or sample cell, 8 back-pressure regulator, 9 collection system (A bubbling, B sorption, C cryogenic trapping).

Supercritical fluid extraction aroused the interest of commercial equipment manufacturers well before its potential was experimentally realized. However, the high expectations raised by this technique have not materialized — basically because of the strong dependence of the extraction efficiency on the properties of the sample matrix — as reflected in the declining interest of manufacturers in it. Thus, the number of manufacturers offering commercial supercritical fluid extractors fell by almost one half between the 1991 Pittsburgh Conference and 1997 — the year this number seemingly levelled off. 

Commercial supercritical fluid extractors have evolved in parallel with the consolidation of specific types of applications of the SFE technique [17]. By way of example of a commercial extractor and its changes over time, let us consider the one manufactured by Hewlett-Packard. This extractor has evolved from the oldest model (HP 7860 A, Fig. 7.3A), which was launched in 1992, to the present one (HP 7680 T, Fig. 7.3B). Essentially, however, the extractor continues to consist of the following elements ... 

Fig. 7.3. Commercially available supercritical fluid extractors from Hewlett-Packard. (A) HP 7680 A model. (B) HP 7680 T model. (Reproduced with permission of Hewlett-Packard.)...

Types of detectors used in combination with supercritical fluid extractors... 

Three approaches to the automation process can be distinguished, taking into account the criterion of the flexibility of the automation device [2], The first, denoted as flexible, is characterized by the possibility of adaptation of the instruments to new and varying demands required from the laboratory examples of these instruments are robots. The second approach, denoted as semiflexible, involves some restrictions for the tasks executed by the instrument the tasks are controlled by a computer program and its menu. As examples, autosamplers or robots of limited moves can be given. In the third approach, the instruments can execute one or two tasks, without feasibility of new requirements as examples, supercritical fluid extractors or equipment for dissolution of samples can be given. 

A typical supercritical fluid extractor includes a supercritical fluid (most often CO2 or CO2 with an organic modifier) source, a means of pressurizing the fluid, a pumping system (for the liquid CO2), an extraction thimble, a device to depressurize the supercritical fluid (flow restrictor), an analyte collection device, temperature-control systems for several zones, and an overall system controller. 

In addition to extraction from solids, supercritical fluids can be used to extract aromatic molecules from liquids. Senorans et al. have utilized carbon dioxide to extract high-quality brandy aroma using a countercurrent supercritical fluid extractor. The aroma quality is influenced by the extraction conditions. Medina and Martinez studied alcohol removal from beverages using supercritical carbon dioxide, to produce beverages with low-alcohol content but sufficient flavor, because of three key benefits 1) water and salts are not appreciably removed by the carbon dioxide 2) proteins and carbohydrates are not extracted or denatured and 3) there is a good control in the aroma recovery. The alcohol removal efficiency increases with the extraction pressure raffinate alcohol concentration can be reduced up to 3 wt.% at 250 bar and 40°C, from 6.2 wt.% in the feed. " ... 

Carbon dioxide is pumped from the bottom of the liquid tank and compressed, heated, and then the samples are extracted (Fig. 9.3). The analytes are trapped in an organic solvent as the carbon dioxide is vented from the extractor. Supercritical fluid extraction (SFE) involves some experimentation with the proper pressure to achieve the extraction of the analyte of interest. Several vendors sell the automated supercritical fluid extractors. Both the accelerated solvent extractor and the supercritical fluid extraction are expensive methods, in the price range of 50,000 each. Several general reviews of SFE include Gere and Derrico (1994), Smith (1995), and a general sample preparation review by Majors (1995). 

Finally, although it is not rigorously part of supercritical fluid extraction, a last step generally afforded hy analytical-scale supercritical fluid extractors is reconstitution of the extracted components in a solution that is appropriate for the subsequent analytical instrument. In the case of gravimetric assays, the net loss of sample weight or the net weight of extract components can be measured and reconstitution of extracted components is not necessarily employed. 

Figure 13-9. Front view of the ISCO model SFX-2-10 supercritical fluid extractor.

Figure 13-10. Fluid flow diagram of a supercritical fluid extractor.

Another reaction involving an SCF/PTC system is an esterification reaction [22] where the primary role of the SCF is to solubilize an intermediate product to prevent the overreaction to an unwanted byproduct. In this system (Scheme 4.10-3) an insoluble aromatic carboxylic acid 4 with a second reactive functional group is esterified at elevated temperature in supercritical dimethyl ether (scDME) with ethylene oxide 5, which is soluble in the fluid phase, in the presence of a thermally stable and insoluble phase-transfer catalyst. When esterification occurs, the product ester 6 is then soluble in the SCF and is pulled away from the site of reaction and trapped before the second functional group can be altered. Experimental data for this work were obtained using a modified Hewlett-Packard supercritical fluid extractor. This is an example of a PTC reaction where an intermediate product is desired, and the SCF system is designed to obtain only that intermediate. 

Lenicek J, Sekyra M, Bednarkova K, Benes I, Sipek F (2000) Short term temperature dependent air-surface exchange and atmospheric concentrations of polychlorinated naphthalenes and organochlorine pesticides. Environ Sci Technol 34 393-398 Li K, Landriault M, Fingas M, Llompart M (1998) Pressurised solvent extraction of environmental organic compounds in soils using a supercritical fluid extractor. Analusis 26 365-369... 

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