Supercritical fluid extraction variables

Supercritical fluid extraction variables can be altered with a view to improving the extraction efficiency and/or the selectivity. 

Although widely used, solvent extraction procedures have been demonstrated as sensitive to such variables as the content of humic matter and moisture within samples. Supercritical fluid extraction appears to be a more robust procedure. Thermal extraction procedures are sensitive to the size of the soil sample in some cases since the technique can result in cracking higher-molecular-weight... 

In this paper, the supercritical fluid extraction (SFE) of organic compounds from sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and from a standard reference material certified for 13 polynuclear aromatic hydrocarbons (PAH), dibenzofuran, and pentachlorophenol was examined using a two- and a four-vessel extractor. Although the results achieved by SFE for the sand and the standard reference soil samples were very encouraging, previous data obtained in our laboratory on the standard reference soil and a few other standard reference marine sediments were less favorable. It was therefore decided that an investigation of seven variables for their influence on the analyte recoveries from the standard soil sample would be useful. Two tests were conducted in which these variables were investigated. In Test 1, the seven variables selected were pressure, temperature, moisture content, cell volume, sample size, extraction time, and modifier volume. In Test 2, the seven variables were pressure, temperature, volume of toluene added to the matrix, volume of solvent in the collection vessel,... 

Supercritical fluid extraction system - Hewlett Packard Model 7680A totally automated system with unlimited-capacity reciprocating pump, specially designed extraction chamber with safety interlocks, a variable restrictor nozzle and analyte collection trap. The operation of the extractor is controlled by a personal computer which is a Microsoft Windows-based system. An animated status screen provides real-time monitoring of the extraction process. Table II gives the SFE conditions for the HP extractor. 

The relative effects of supercitical carbon dioxide density, temperature, extraction cell dimensions (I.D. Length), and cell dead volume on the supercritical fluid extraction (SFE) recoveries of polycyclic aromatic hydrocarbons and methoxychlor from octadecyl sorbents are quantitatively compared. Recoveries correlate directly with the fluid density at constant temperature whereas, the logarithms of the recoveries correlate with the inverse of the extraction temperature at constant density. Decreasing the extraction vessels internal diameter to length ratio and the incorporation of dead volume in the extraction vessel also resulted in increases in SFE recoveries for the system studied. Gas and supercritical fluid chromatographic data proved to be useful predictors of achievable SFE recoveries, but correlations are dependent on SFE experimental variables, including the cell dimensions and dead volume. 

The supercritical fluid extraction of analytes from solid sorbents is controlled by a variety of factors including the affinity of the analytes for the sorbent, the tortuosity of the sorbent bed, the vapor pressure of the analytes, and the solubility and the diffusion coefficient of the analytes in the supercritical fluid. Additionally, SFE efficiencies are affected by a complex relationship between many experimental variables, several of which are listed in Table I. Although it is well established that, to a first approximation, the solvent power of a supercritical fluid is related to its density, little is known about the relative effects of many of the other controllable variables for analytical-scale SFE. A better understanding of the relative effects of controllable SFE variables will more readily allow SFE extractions to be optimized for maximum selectivity as well as maximum overall recoveries. 

TABLE I. Some of the Variables Which May Affect Supercritical Fluid Extraction Efficiencies... 

J. T. B. Strode and L. T. Taylor, Supercritical fluid extraction employing a variable restrictor coupled to gas chromatography via a sample pre-concentration trap , J. High Resolut. Chromatogr. 19 651-654 (1996). 

Hubert et al. [101] state that accelerated solvent extraction compared to alternatives such as Soxhlet extraction, steam distillation, microwave extraction, ultrasonic extraction and, in some cases, supercritical fluid extraction is an exceptionally effective extraction technique. Hubert et al. [ 101 ] studied the effect of operating variables such as choice of solvent and temperature on the solvent extraction of a range of accelerated persistent organic pollutants in soil, including chlorobenzenes, HCH isomers, DDX, polychlorobiphenyl cogeners and polycyclic aromatic hydrocarbons. Temperatures ofbetween 20 and 180 °C were studied. The optimum extraction conditions use two extraction steps at 80 and 140 °C with static cycles (extraction time 35 minutes) using toluene as a solvent and at a pressure of 15 MPa. 

In a supercritical fluid extraction process, the analytes are transferred from their host matrix to the supercritical fluid, flushed from the extraction cell by the fluid and finally collected or sent to a detector or chromatograph for analysis. Unsurprisingly, the performance of the SFE technique is thus affected by a number of variables the most significant of which are listed in Table 7.3. 

When supercritical fluid extraction (SEE) was initially introduced, it was thought that it might be the panacea for sample extraction because it used a very innocuous solvent, CO2. The operator varied pressure, temperature, flow rate, and extraction time, with some extraction protocols requiring the use of small amounts of polar modifiers. All of these variables affected the solvating power of the carbon dioxide. In addition to the carbon dioxide, other supercritical fluids have been used. The technique continues to evolve with increasing numbers of applications being published but has not developed as once might have been predicted. 

Lopez-Avila V, Dodhiwala NS, Beckert WF. Supercritical fluid extraction and its application to environmental analysis. J Chromatogr Sci 1990 28 468—476. Tena MT, Luque de Castro MD, Valcarcel M. Systematic study of the influence of variables on the supercritical fluid extraction of pol5Tomatic hydrocarbons (PAHs). Lab Rob Autom 1993 5 255-262. 

A well-known extraction system is supercritical fluid extraction. It can be applied to matrices of different composition. This technique demonstrates the influence of matrix characteristics and common extraction variables on equilibrium analyte distribution.63 These factors assure, for supercritical fluid extrachon, the optimum conditions for efficient extraction of the matrix. 

Zhou, M., Trubey, R. K., Keil, Z. O., and Sparks, D. L., Study of the effects of environmental variables and supercritical fluid extraction parameters on the extractability of pesticide residues from soils using a multivariate optimization scheme. Environ. Sci. TechnoL, 31, 1934-1939, 1997. 

A research project (carried out under the former BCR and coordinated by the CID-CSIC in Barcelona, Spain) also enabled development of a supercritical fluid extraction procedure which was successfully applied to the certification of TBT in the CRM 462 [99]. The effect of extraction variables, such as extraction time, temperature and extraction agent composition, in supercritical fluid extraction have been optimized by using a factorial-fractional experimental design. Under the optimum conditions (T = 60 °C, P = 35 MPa, 5.1 mol methanol in CO2, t = 30 minutes), the TBT extraction efficiency was 82% with a coefficient of variation of 9.2% (after determination by GC-FPD). 

A logical extension of supercritical fluid extraction in chemical analysis is to combine the process with a chromatographic method. The variable solvating power of a supercritical fluid provides the mechanism for the selective extraction of the components of interest... 

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