Supercritical fluids analytical applications

Although several other early studies were conducted using Supercritical Fluids (SFs), also termed at that time as dense gases , this field did not receive the attention it deserved during the first half century after its discovery. Even then, the small number of investigators dedicated to further explore the scientific and technological potential of supercritical fluids concentrated then efforts on industrial rather than analytical applications (9, 10). 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

The current state of analytical SPE was critically reviewed and no major changes of the technique have been observed. Overviews of the developments of the extraction technologies of secondary metabolites from plant materials refer to three types of conventional extraction techniques that involve the use of solvents, steam, or supercritical fluids. Each technique is described in detail with respect to typical processing parameters and recent developments. Eollowing the discussion of some technical and economic aspects of conventional and novel separation processes, a few general conclusions about the applicabilities of the different types of extraction techniques are drawn. ... 

High performance liquid chromatography (HPLC) has been by far the most important method for separating chlorophylls. Open column chromatography and thin layer chromatography are still used for clean-up procedures to isolate and separate carotenoids and other lipids from chlorophylls and for preparative applications, but both are losing importance for analytical purposes due to their low resolution and have been replaced by more effective techniques like solid phase, supercritical fluid extraction and counter current chromatography. The whole analysis should be as brief as possible, since each additional step is a potential source of epimers and allomers. 

Supercritical fluid extraction (SEE) is another modern separation technology usually employed to extract lipophilic compounds such as cranberry seed oil, lycopene, coumarins, and other seed oils. Anthocyanins generally and glycosylated anthocyanins in particular were considered unsuitable for SEE due to their hydrophilic properties, since SEE is applicable for non-polar analytes. However, a small amount of methanol was applied as co-solvent to increase CO2 polarity in anthocyanin extraction from grape pomace. New applications of SEE for anthocyanin purification have been reported for cosmetic applications from red fruits. ... 

Supercritical fluid extraction (SFE) is generally used for the extraction of selected analytes from solid sample matrices, but applications have been reported for aqueous samples. In one study, recoveries of 87-100% were obtained for simazine, propazine, and trietazine at the 0.05 ug mL concentration level using methanol-modified CO2 (10%, v/v) to extract the analytes, previously preconcentrated on a C-18 Empore extraction disk. The analysis was performed using LC/UV detection. Freeze-dried water samples were subjected to SFE for atrazine and simazine, and the optimum recoveries were obtained using the mildest conditions studied (50 °C, 20 MPa, and 30 mL of CO2). In some cases when using LEE and LC analysis, co-extracted humic substances created interference for the more polar metabolites when compared with SFE for the preparation of the same water sample. ... 

Numerous applications of SEE were published during the 1980s soon after the availability of commercial instrumentation. Supercritical fluids (SFs) have useful characteristics for the extraction of trace analytes from solid samples, most notably... 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

In analytical chemistry there is an ever-increasing demand for rapid, sensitive, low-cost, and selective detection methods. When POCL has been employed as a detection method in combination with separation techniques, it has been shown to meet many of these requirements. Since 1977, when the first application dealing with detection of fluorophores was published [60], numerous articles have appeared in the literature [6-8], However, significant problems are still encountered with derivatization reactions, as outlined earlier. Consequently, improvements in the efficiency of labeling reactions will ultimately lead to significant improvements in the detection of these analytes by the POCL reaction. A promising trend is to apply this sensitive chemistry in other techniques, e.g., in supercritical fluid chromatography [186] and capillary electrophoresis [56-59], An alter-... 

The popularity of this extraction method ebbs and flows as the years go by. SFE is typically used to extract nonpolar to moderately polar analytes from solid samples, especially in the environmental, food safety, and polymer sciences. The sample is placed in a special vessel and a supercritical gas such as CO2 is passed through the sample. The extracted analyte is then collected in solvent or on a sorbent. The advantages of this technique include better diffusivity and low viscosity of supercritical fluids, which allow more selective extractions. One recent application of SFE is the extraction of pesticide residues from honey [27]. In this research, liquid-liquid extraction with hexane/acetone was termed the conventional method. Honey was lyophilized and then mixed with acetone and acetonitrile in the SFE cell. Parameters such as temperature, pressure, and extraction time were optimized. The researchers found that SFE resulted in better precision (less than 6% RSD), less solvent consumption, less sample handling, and a faster extraction than the liquid-liquid method [27]. 

Supercritical fluid extraction (EPA 3540, for total recoverable petroleum hydrocarbons EPA 3561 for polynuclear aromatic hydrocarbons) is applicable to the extraction of semivolatile constituents. Supercritical fluid extraction involves heating and pressuring a mobile phase to supercritical conditions (where the solvent has the properties of a gas and a liquid). The supercritical fluid is passed through the soil sample, and the analytes are concentrated on a sorbent or trapped cryogenically. The analytes are eluted with a solvent and analyzed using conventional techniques. Carbon dioxide is the most popular mobile phase. 

Supercritical fluids were soon found to be highly efficient extraction media, chiefly because of their high solvating power, their low viscosities (intermediate between a gas and a liquid), and their low surface tensions that enable their penetration deep into the extraction matrix. Supercritical fluid extraction (SEE) used in isolation is generally not selective enough to separate specific solutes from the matrix without further cleanup or resolution from coextracted species prior to qualitative and quantitative analysis. Consequently, for analytical applications, SFE is usually used in combination with chromatographic techniques to improve the overall selectivity in the isolation of specific solutes. The combined use of SFE with chromatographic techniques is quite recent. 

Supercritical fluid extraction (SFE) is a fluid extraction process that has been applied on both a processing scale and for analytical chemical applications. 

Supercritical fluid chromatography (SFC) is a relatively recently developed chromatographic technique. Because of its ability to deal with compounds that are either polar or of high molecular weight, much attention has recently focused on applications of SFC to the analysis of different analytes using a variety of fluids or fluid mixtures to provide differing solvent capabilities and select vities. As a result there is a large amount of research currently underway both in SFC method development and in hardware development. 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

The first paper on supercritical fluid chromatography (SFC) by Klesper et al. in 1962 [8] considered analytical SCF applications. Only twenty years later, in 1982, a first patent on the use of SFC for production purposes was granted by Perrut [9]. During these years, various academic and industrial laboratories have demonstrated the feasibility and the applicability on a commercial scale of these chromatographic processes. 

Polycyclic aromatic hydrocarbons (PAHs) have been extracted from contaminated land samples by supercritical fluid extraction jSFE) with both pure and modified carbon dioxide. Removing an analyte from a matrix using SFE requires knowledge about die solubility of the solute, the rate of transfer of the solute from the solid to the solvent phase, and interaction of the solvent phase with the matrix. These faclors collectively control the effectiveness of the SFF process, if not of the extraction process in general. The range of samples for which SFE has been applied continues to broaden. Applications have been in the environment, food, and polymers. 

The CHEMISTRY OF SUPERCRITICAL FLUIDS has been studied extensively in the past decade. Consequently, our understanding of this field has expanded significantly. Simultaneously, the number of applications in associated analytical technologies (for example, supercritical fluid chromatography and supercritical fluid extraction) has increased. Although the areas of fundamentals and applications are clearly interrelated, they are often discussed separately. 

Supercritical fluid extraction (SFE) has the potential to change the analytical laboratory and its current extraction procedures dramatically. Predictions on the monetary exploitation of this technique are astronomical. Subsequently, the amount of research, development and applications in SFE is growing rapidly. 

Environmental applications of SFE appear to be the most widespread in the literature. A typical example is the comparison of extraction efficiency for 2,3,7,8 -tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from sediment samples using supercritical fluid extraction and five individual mobile phases with Soxhlet extraction was made (101). The mobile phases, carbon dioxide, nitrous oxide, pure and modified with 2% methanol as well as sulfur hexafluoride were examined. Pure nitrous oxide, modified carbon dioxide and modified nitrous oxide systems gave the recoveries in the acceptable range of 80 to 100%. Carbon dioxide and sulfur hexafluoride showed recoveries of less than 50% under identical conditions. Classical Soxhlet recoveries by comparison illustrated the poorest precision with average extraction efficiencies of less than 65%. Mobile phase choice, still as yet a major question in the science of supercritical fluid extraction, seems to be dependent upon several factors polarity of the solute of interest, stearic interactions, as well as those between the matrix and the mobile phase. Physical parameters of the solute of interest, as suggested by King, must also be considered. Presently, the science behind the extraction of analytes of interest from complex matrices is not completely understood. 

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