On-line supercritical fluid extraction-capillary gas chromatography

Abstract

A new analytical methodology combining on-line supercritical fluid extraction with high-resolution capWary gas chromatography for automated sample preparation and analysis Is described. Analytical-scale supercritical fluid extraction utilizes the variable solvating power of a supercritical fluid to selectively extract and Isolate discrete fractions from a sample matrix. The supercritical fluid extract Is decompressed through a restrictor to deposit and concentrate the analytes at the Inlet of a standard capillary gas chromatography column for subsequent analysis. This methodology allows several modes of operation Including quantitative extraction of all analytes from a sample matrix, quantitative extraction and concentration of trace analytes, selective extractions at various solvating powers to obtain specific fractions, or multiple-step extractions at various pressures for qualitative characterizations. This Initial report describes the later two modes of operation and demonstrates the potential usefulness of this methodology for sample extraction and selective fractionation using a standard polycyclic aromatic hydrocarbon mixture and two complex sample matrices.Until recently the use of supercritical fluid extraction (SFE) has been generally confined to relatively large-scale chemical processing applications (1-3). However, the use of SFE methods for analytical applications is attracting increased attention (4-8). The potential advantages of SFE accrue from the physical properties of supercritical fluids. The compressibility of supercritical fluids is large above the critical temperature, and small changes in pressure result in large changes in the density (and solvating power) of the fluid (2).At higher densities molecular interactions increase due to shorter intermolecular distances and solvating characteristics approaching that of a liquid are imparted. However, the viscosity and solute diffusivity can remain similar to those of a gas (2), thus allowing more rapid mass transfer of solutes than feasible with liquids. Many fluids have comparatively low critical temperatures that allow extractions to be conducted at relatively mild temperatures, e.g., 31 °C for carbon dioxide. In addition to using pressure and/or temperature to control the density or solvating power, various fluids or fluid mixtures that exhibit different specific chemical interactions can be used to obtain the desired selectivity.Recent studies have shown that analytical SFE provides comparable or better extraction efficiencies than conventional Soxhlet extraction and with over an order of magnitude increase in the rate of extraction (6). Other important potential advantages of SFE include the capability of selective extraction as a function of fluid solvating power, fractionation during collection (9), and the compatibility with on-line analysis of the extraction effluent. Various modes of on-line

Solute focusing in supercritical fluid chromatography

“…Peaks were either split or broad in the beginning of the run, if the trapping temperature was -10°C to +25"C. The best peaks were obtained when the trapping temperatures were 0°C to -30°C. Good peak shapes have been obtained also at higher temperatures (5-30°C) by other research groups [2,27,32]. On-line SFE-SFC has been done by depositing the samples in a deactivated precolumn that was inside a cooled trap ( Figure 2) [5] and in an injector sample loop ( Figure 3) [2].…”

Regeneration of Adsorbents by the Use of Liquid, Subcritical and Supercritical Carbon Dioxide

“…Madras et al (1993) applied supercritical CO 2 without any modifier for the desorption of naphthalene, phenanthrene, hexachlorobenzene and pentachlorophenol from the activated carbon they studied (produced by Sigma Chemical Co.). Wright et al (1988) adsorbed 50 ppm samples of chrysene, benzanthrone, 1-nitropyrene, dibenzocarbazole and coronene on activated carbon and subjected 1 g of each sample to Soxhlet extraction with carbon disulphide for 16 h, followed by a second similar extraction using methylene chloride. Other parts of the samples were extracted with supercritical CO 2 at 398 K and 40 MPa for 1 h. No detectable levels of the adsorbed compounds were noted in the combined Soxhlet extracts, but on extraction with supercritical CO 2 the following efficiencies were achieved: chrysene -1%; benzanthrone -6%; 1-nitropyrene -10%; dibenzocarbazole -10%; and coronene -0.2%.…”

“…Most solid extraction processes, e.g. soil remediation, desorption of analytes from adsorbents (Wright et al 1988) and activated carbon regeneration, fall into this category. 3.…”

Supercritical Fluids in Environmental Remediation and Pollution Prevention

“…Supercritical C0 2 is used to remove hexachlorocyclohexane, parathion, PCBs, and PAHs from Tenax packing (24) and polyimide based adsorbents (25). Supercritical C0 2 is also the solvent of choice for extraction of PAHs from various adsorbents selectively by varying the operating conditions (26,27). A significant amount of work concentrated on activated carbon regeneration, such as desorption of phenol (28), pesticides (29), acetic acid and alachlor (30), and ethyl acetate (31).…”