Supercritical carbon dioxide matrix

Although critical pressures are many times greater than atmospheric pressure, supercritical fluids have important commercial applications. The most important of these is the use of supercritical carbon dioxide as a solvent. Supercritical CO2 diffuses through a solid matrix rapidly, and it transports materials well because it has a lower... 

Supercritical carbon dioxide extraction (SCDE) is an ex sitn process for the treatment of low-level solid mixed and land disposal restricted (LDR) wastes. SCDE can extract hazardons solvents from waste snbstrates to prodnce land-disposable, low-level wastes. The process employs the snpercritical finid carbon dioxide as a solvent. This finid is noncombustible, nontoxic, and environmentally safe. In its supercritical state, carbon dioxide can dissolve organic contaminants allowing the fluid to quickly penetrate and facilitate transfer out of a contaminated matrix. 

Extraction of fat by supercritical carbon dioxide was investigated as an important option for minimizing the expanded use of frequently flammable and carcinogenic solvents in food analysis. Unfortunately, the presence of moisture in foods has an adverse effect on the quantitative extraction of fat by supercritical fluid extraction (SEE). Hence, samples have to be lyophilized first. The total fat content of freeze-dried meat and oilseed samples was found to be comparable to values derived from Soxhlet-extracted samples (26). Besides, only small amounts of residual lipids could be recovered by an additional extraction of the SFE-extracted matrix by the Bligh and Dyer solvent extraction procedure. As far as the minor constituents are concerned, it was found that the extraction recovery ranged from 99% for PC to 88% for PA. Hence, Snyder et al. concluded that SFE can be used as a rapid, automated method to obtain total fat, including total phospholipids, from foods (26). 

Phylloquinone has been extracted from powdered infant formula using supercritical carbon dioxide at 8000 psi and 60°C for 15 min (65). The extracted material was readily recovered by depressurization of the carbon dioxide across an adsorbent trap and then washed from the trap with a small volume of dichloromethane/acetone (1 + 1) to give a sample suitable for direct HPLC analysis. Trial experiments gave recoveries of 92% of phylloquinone from a Chro-mosorb W matrix. A similar technique was applied to the extraction of retinyl palmitate from cereal products (90). 

Many potential applications have been proposed which involve the desorption of solutes from matrix using SCF solvents at elevated pressure these include activated carbon regeneration [1,2,3,4,5] and soil remediation [6,7,8] using supercritical carbon dioxide. 

Sometimes, for solid and semisolid samples, the above-referenced methods were not suitable the extraction times were very long, with consequent chemical modifications and appearance of artifacts. Supercritical fluid extraction provides a good recovery of phenolics. The extraction behavior of phenolics has been delineated, using supercritical carbon dioxide and either sand or an inert support as sample matrix. ... 

In summary, phenol (a model toxic pollutant) was successfully extracted from both water and soil (dry and wetted) using supercritical carbon dioxide. It was found that entrainers greatly enhance the distribution coefficient of phenol for each system. However, the choice of a good entrainer is not independent of the contaminated matrix or, in the case of soil, its moisture content. Benzene was successfully extracted from both dry and wetted soil using pure carbon dioxide. Entrainers such as methanol could serve to decrease the distribution of such hydrophobic systems, indicating that further investigation in this area is warranted. 

Supercritical fluids may also be used for extraction of food. This method involves the use of supercritical carbon dioxide to remove analytes from food without dissolving the matrix (Fig. 9.3). Methanol is commonly added to the supercritical COj in order to enhance the solubility of a compound. 

Lucien FP, Foster NR. Influence of matrix composition on the solubility of hydroxybenzoic acid isomers in supercritical carbon dioxide. Ind Eng Chem Res 1996 35 4686. 

The shape of the curves indicate that the initial stage of the extraction is limited by solubility. Note the initial rise was greatest at 40 C (0.95 g/mL) at those conditions the recovery reached 82 % when only 60 mL of supercritical carbon dioxide had passed through the extraction thimble. For the same volume of the supercritical fluid (60 mL), the recovery was only 78 % for 80°C(0.81 g/mL), and 58 % for 120 C (0.67 g/mL). However, as the analyte flux begins to diminish (the easy-to-extract solutes are removed, and the more difficult solutes deeper within the matrix are now being removed), the extraction mechanism is no longer limited by saturation solubility. Rather, the extraction mechanism in the later stages of SFE is diffusion limited for... 

Furthermore, one of the advantages of carbon dioxide-based SFE is that normally extractions are carried out at low temperatures. The resolution of this dilemma can be as simple as adding a short sub-step in the beginning to remove the ethyl alcohol in the extraction thimble. Recall that the capsaicin extraction is carried out at 40 C and a density of 0.70 g/mL. The ethyl alcohol can be readily removed at a density of 0.25 g/mL at 40 C in a short three-minute dynamic extraction step. This is well below the "threshold-density" at which the capsaicin would extract or be soluble in supercritical carbon dioxide. The mechanism of using the carbon dioxide to remove ethyl alcohol from the extraction thimble under supercritical conditions is quite different than the evaporation mechanism that would be taking place in the solid trap after expansion of the carbon dioxide plus ethyl alcohol to a mixture of gas plus condensing liquid. Note that the fact that ethanol can be pre-extracted ahead of the analyte of interest demonstrates that the role of the modifier in this application is mainly matrix modification had a more polar solvent (like that afforded by a mixture of ethanol in carbon dioxide) been necessary to solvate the capsaicin, the pre-extraction step to remove the ethanol would have been detrimental to the extraction of the capsaicin. 

Each patent has somewhat different features and claims. We select one patent for more detailed discussion to highlight certain technical facets of the process. First we explain the (often misunderstood) effect of water on the extractability of caffeine by selective supercritical carbon dioxide. A number of references report that dry carbon dioxide cannot extract caffeine from dry coffee, either green or roasted, but moist carbon dioxide can. The inability of dry carbon dioxide to extract caffeine from coffee should not be misconstrued to mean that dry carbon dioxide cannot dissolve neat caffeine. This same moist-versus-dry effect is experienced if, for example, methylene chloride is used to extract caffeine from coffee. Dry methylene chloride cannot decaffein-ate dry coffee but moistened coffee can be decaffeinated. It is thought that the caffeine is chemically bound in a chlorogenic acid structure present in the coffee bean. Thus, water somehow acts as a chemical agent it frees caffeine from its bound form in the coffee matrix in both the carbon dioxide and the methylene chloride processes. 

Hills et al. (1991) applied simultaneous supercritical fluid extraction to roasted coffee beans. This technique can be used with or without a derivatizing reagent In a dissociative mechanism, the adsorbed analyte must first desorb from a matrix active site and be dissolved in the supercritical fluid and then react to form the less polar derivative, which favors solvation in the supercritical carbon dioxide. In the associative mechanism, derivatization occurs while the analyte is adsorbed on the active site of the matrix. Reaction with the adsorbed analyte results in the desorption of the non-polar derivative into the supercritical fluid . Thus 2-hexenedioic acid (E.49) was identified for the first time as a native compound (without the use of a derivatizing agent). Benzenic and furanic compounds and caffeine were also identified. 

Some triazines, including atrazine and simazine, were extracted from water by SEE after preconcentration on sohd-phase extraction disks.The freeze-dried residue or SPE disk was then introduced into the extraction cell and eluted with either pure CO2 or methanol or acetone-modified CO2. SEE has been applied in combination with online sohd-phase extraction for pesticides, including triazines. However, this technique is still used mainly on solid matrixes. The main limitation with aqueous matrixes remains the miscibility of water with supercritical carbon dioxide. Recoveries of nonpolar pesticides, extracted by SEE technique using octacedyl-bonded silica, are generally effective with CO2 elution alone. This is not the case for semipolar and polar compounds, such as triazines and their degradation products. For example, the addition of... 

IE OF THE MOST PERVASIVE PROBLEMS in chemistry is the separation of complex materials into component parts, either to characterize a mixture or natural product or to remove a particular component from a matrix. Supercritical fluids—in particular, supercritical carbon dioxide—are currently undergoing exciting developments in meeting both types of challenges while being used in extractions and in chromatography. 

Said-Galiyev, E., Nikitin, L., Vinokur, R., Gallyamov, M., Kurykin, M., Petrova, O., Lokshin, B., Volkov, I., Khokhlov, A., and Schaumbuig, K. New chelate complexes of copper and iron synthesis and impregnation into a polymer matrix from solution in supercritical carbon dioxide. Industrial and Engineering Chem. Research., 39,4891-48% (2000). 

Figure 13. Variation of ferric triflate uptake (filled circles) and conductivity (filled squares) of host polyurethane foam matrix with soak time in supercritical carbon dioxide at 45 C and 238 bar. [Adapted from ref. 50].

In an alternative approach, Morley et al. [50,51] describe a similar procedure, which is effective for the preparation of metal-SiOa nanocomposites based on the use of supercritical carbon dioxide for the impregnatimi of a preformed silica alcogel with a suitable metal complex and for the removal of the free ligand after the reduction step. Pd- and Ag-Si02 aerogels, which can be used as active catalysts, were effectively obtained by this procedure where supercritical carbon dioxide was used as a recyclable solvent, which does not alter the porous structure of the matrix. However, in addition to the many steps required, a major disadvantage of this procedure is that it requires solubility of metal complexes in sc-C02-... 

By changing the density of the supercritical fluid, different fractions may be selectively extracted from the complex mixture or sample matrix. On decompression, the extracted solutes are precipitated and may be collected for injection into a GC or SFC for analysis. Figure 8.4 shows a simple apparatus for on-line SFE/SFC solutes extracted from the sample matrix are deposited from the end of a restrictor into the internal loop of the microinjection valve of the capillary SFC. The valve loop contents are subsequently switched into the SFC column by means of liquid or supercritical carbon dioxide. 

Also, supercritical-fluid extraction (SFE) has been used for analyte enrichment and matrix separation in metal determinations [111]. The solubilities of metal dithiocarbamates in supercritical carbon dioxide have been characterized [112]. 

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