Analytical methods supercritical fluid carbon dioxide

Analytical methods for monitoring the compounds were developed or modified to permit the quantification of all 23 compounds of interest. As noted earlier, the compounds were initially studied in small-scale extractions by groups. This approach assured minimal interferences in the analyses conducted during the initial supercritical fluid carbon dioxide extractions. Table II summarizes the data on the recovery of organics from aqueous samples containing the compounds of interest at concentration levels listed in Table I when the sample preparation techniques and analytical methods described were used. For each experimental run, blank and spiked aqueous samples were carried through the sample prepration and analytical finish steps to ensure accurate and reproducible results. Analyses of sodium, calcium, and lead content were also conducted on selected samples by using standard atomic ab-... 

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 variety of modern instrumental analytical techniques have attracted considerable attention in the last decades as alternative separation and analysis methods with respect to HPLC. This includes, in particular, supercritical fluid chromatography (SFC), which utilizes condensed carbon dioxide (above or near its critical temperature of... 

Chlorinated phenolic compounds in air-dried sediments collected downstream of chlorine-bleaching mills were treated with acetic anhydride in the presence of triethylamine. The acetylated derivatives were removed from the matrix by supercritical fluid extraction (SEE) using carbon dioxide. The best overall recovery for the phenolics was obtained at 110°C and 37 MPa pressure. Two SEE steps had to be carried out on the same sample for quantitative recovery of the phenolics in weathered sediments. The SEE unit was coupled downstream with a GC for end analysis . Off-line SEE followed by capillary GC was applied in the determination of phenol in polymeric matrices . The sonication method recommended by EPA for extraction of pollutants from soil is inferior to both MAP and SEE techniques in the case of phenol, o-cresol, m-cresol and p-cresol spiked on soil containing various proportions of activated charcoal. MAP afforded the highest recoveries (>80%), except for o-cresol in a soil containing more than 5% of activated carbon. The SEE method was inefficient for the four phenols tested however, in situ derivatization of the analytes significantly improved the performance . 

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. 

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). 

In off-line collection, the effluent is depressurized, and the analyte is collected in a solvent, an open container, or an analyte trap packed with a solid support. Off-line collection is simpler, and the collected sample can be analyzed by several methods. Thus, the SFE instrument is decoupled from the analytical instruments. Other advantages of the off-line approach are accommodation of larger sample sizes, feasibility of multiple analyses from a single extraction, and accommodation of a wider range of analyte concentrations. In the analyte trap approach, the use of different solvents for elution can serve as an additional experimental parameter for increased selectivity and cleanup. In the open-container collection approach, aerosol may form because depressurization of the supercritical fluid produces a high flow rate of gaseous carbon dioxide. 

The use of supercritical fluid extraction (SEE) as an extraction technique is related to the unique properties of the supercritical fluid. These fluids have a low viscosity, high diffusion coefficients, low toxicity, and low flammability, all clearly superior to the organic solvents used in SPE extraction. The most common fluid used is carbon dioxide. SEE extractions of sediment samples have shown recoveries of >95% for all the individual PCBs. The separation of PCDDs from PCBs and chlorinated benzenes is difficult because of their similar solubility. An interesting development is the use of fat retainers. Samples, mixed in different weight ratios with, e.g., silica/silver nitrate 10% or basic alumina, can be placed in 7 ml extraction cells. The analytes are recovered by elution with 1.5-1.8 ml of hexane. With the correct fat-silica ratios and SEE conditions, no additional cleanup procedure is necessary for GC with an electron-capture detector (ECD). One drawback of SEE may be that the methods developed are valid for a specific matrix, but as soon as, e.g., the fat content of a biota sample or the type of lipids changes, the method has to be adapted. SEE is relatively complicated compared to other extraction techniques. In addition, the cell volumes are small, which limits the sample intake, and, with that, the detection limits. Einally, some reliable types of SEE equipment have recently been withdrawn from the market. This will have a substantial negative effect on the use of SEE in the near future. 

The procedure is described as follows supercritical fluid extiactions were performed with an automated ISCO SFX m 3560 instrument using 6 mL extraction vessels. The extraction vessels were filled with 100 mg of dried plant samples mixed with anhydrous sodium sulfate. The extracted analytes were collected into 10 mL of methanol. The internal standard was added to the collection vials immediately after extraction. The collection temperature was 5 °C. The SFE instrument was equipped with a 260 mL syringe pump for the addition of carbon dioxide at a flow rate of 1.5 mL/min and a manually controlled Jasco PU-980 HPLC pump for addition of the modifier (methanol) at flow rates of 0.04-0.1 mL/min (2.6-6.6 %). The restrictor temperature was set at 60 °C in all extractions [11]. In this paper, the extracts from SFE were found to be much cleaner in comparismi with those obtained by solid-liquid extractions or Soxhlet extractions. The results showed that supercritical fluid extraction is a valuable alternative technique to traditional extraction methods of Catharanthus alkaloids from dried leaves. 

In the early 1990s it appeared that supercritical-fluid extraction was going to be the future method of choice for extracting environmental soils and solid samples. SEE showed promising recoveries for many environment analytes and used very little solvent (64). As of 2001, it had not gained the widespread use that was predicted (7). SEE is very similar to the ASE technique described above, except that a supercritical fluid is used for the extraction rather than a solvent. Any pure substance that is above its critical temperature (Tc) and critical pressure (Pc) is defined as a supercritical fluid. The most frequently used extraction fluid is CO2. If CO2 is compressed to a pressure above 72.9 atm and heated to above 31.3°C, it becomes a supercritical fluid and exhibits physical properties between those of a gas and a liquid. Carbon dioxide is used most frequently in SEE as an extraction... 

Several reasons exist for why SEE has not reached widespread use. Larger sample sizes are not practical on analytical-scale instrumentation. Limitations on flow of the supercritical fluid over the sample create longer extraction times. Sample sizes of 1-2 g place analytical-scale SEE at its upper limit. Many times the detection limits required by USEPA methods can not be achieved by smaller sample sizes (e.g., 1-2 g). Carbon dioxide is nonpolar and is really the only practical... 

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