Off-line supercritical fluid extraction

Kuitunen ML, Hartonen K, Riekkoa ML. 1991. Analysis of chemical warfare agents in soil samples by off-line supercritical fluid extraction and capillary gas chromatography. Journal of Microcolumn Separation 3(6) 505-512. 

Lopez-Avila, V. Dodhiwala, N.S. and Beckert, W.F. Off-line Supercritical Fluid Extraction Technique for Difficult Environmental Matrices Contaminated with Compounds of Environmental Significance. Paper presented at the Sixth Annual Waste Testing and Quality Assurance Symposium, Washington, D. C., July 1990. 

In general, supercritical fluid extractions can be performed in either an on-line extraction mode or an off-line extraction mode. Off-line supercritical fluid extraction is the most common mode and involves extracting the analytes from the matrix and collecting them in either a sorbent trap or a collection solvent [11]. Following the collection step, the analytes are determined on a separate instrument (for example, on a chromatograph or an infrared spectrometer). In the on-line supercritical fluid extraction experiment, the outlet of the supercritical fluid extraction system is connected to a second analytical... 

Vejrosta, J., P. Karasek, and J. Planeta. 1999. Analyte collection in off-line supercritical fluid extraction. Anal. Chem. 71 905-909. 

Fahmy, T.M., Pulaitis, M.E., Johnson, D.M., McNally, M.E.P., Modifier effects in the supercritical fluid extraction of solutes from clay, soil, and plant materials. Anal. Chem., 65 (10), 1462-1469,1993. Langenfeld, J.J., Hawthorne, S.B., Miller, D.J., Pawliszyn, J., Role of modifiers for analytical scale supercritical fluid extraction of environmental samples. Anal. Chem., 66(6), 909-916,1994. Hawthorne, S.B., Methodology for off-line supercritical fluid extraction. In Supercritical Fluid Extraction and Its Use in Chromatographic Sample Preparation, Westwood S.A. (Ed.), Blackie Academic and Professional, 39-64, 1993. 

Lopez-Avila V, Young R, Kin R, et al. 1993. Interlaboratory evaluation of an off-line supercritical fluid extraction/infrared spectrometric method for determination of petroleum hydrocarbons in solid matrixes. JAOAC Int 76(3) 555-562. 

Modey WK, Mulholland DA, Mahomed H, Raynor MW. Analysis of extracts from Cedrela toona (Meliaceae) by on-line and off-line supercritical fluid extraction-capillary gas chromatography. J Microcolumn Sep 1996 8 67-74. Otero ZK. Optimization of SFE by statistical methods. New Orleans Pitt-con 92, 1992. 

Off-line supercritical fluid extraction, ultrasonic supercritical fluid extraction, and on-line supercritical fluid extraction-gas chromatography methodologies that have been developed specifically for analytical sample preparation and analysis are described. These methods offer the potential for extraction rate increases of over an order of magnitude, and are compatible with online analysis which provides the basis for automated sample preparation and analysis. These methods are particularly useful for small sample sizes or trace levels of analytes, and have been demonstrated to operate quantitatively. Combined ultrasonic supercritical fluid extraction can further enhance extraction rates from macro-porous materials by inducing convection through internal pores. The apparatus and instrumentation are described in detail and several examples are presented illustrating the applicability of these methodologies. 

Sample preparation is often more difficult and time-consuming than the actual analysis procedure. Furthermore, extraction of analytes from the matrix is generally the most time-consuming step of sample preparation and it can lead to relatively inefficient analyte recoveries. Off-line supercritical fluid extraction provides an alternative to traditional Soxhlet or ultrasonic liquid extraction methods. Several recent studies have shown analytical SFE provides comparable or better extraction efficiencies than Soxhlet... 

Figure 1. Schematic diagram of the off-line supercritical fluid extraction apparatus. (Reproduced from Ref. 12. Copyright 1987 American Chemical Society.)...

Off-line supercritical fluid extraction, simultaneous ultrasonic supercritical fluid extraction, and on-line supercritical fluid extraction-gas chromatography have been described. These analytical supercritical fluid extraction methods provide the potential for very rapid extraction rates and compatibility with on-line analytical methods. Extraction rate increases of over an order of magnitude compared to Soxhlet methods have been demonstrated and even greater increases seem feasible. Optimization of fluid solvating conditions also provides the potential for selective fractionation of specific analytes. The application of ultrasound during supercritical fluid extraction provides an efficient... 

Cacho, J., Quantitative Analysis of Pesticides in Postconsumer Recycled Plastics Using Off-Line Supercritical Fluid Extraction/GC-ECD, Anal Chem. 1997 69(16) 3304-3313. 

Supercritical fluid extraction (SFE) has been extensively used for the extraction of volatile components such as essential oils, flavours and aromas from plant materials on an industrial as well as an analytical scale (61). The extract thus obtained is usually analysed by GC. Off-line SFE-GC is frequently employed, but on-line SEE-GC has also been used. The direct coupling of SEE with supercritical fluid chromatography (SEC) has also been successfully caried out. Coupling SEE with SEC provides several advantages for the separation and detection of organic substances low temperatures can be used for both SEE and SEC, so they are well suited for the analysis of natural materials that contain compounds which are temperature-sensitive, such as flavours and fragrances. 

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 most widely employed techniques for the extraction of water samples for triazine compounds include liquid-liquid extraction (LLE), solid-phase extraction (SPE), and liquid-solid extraction (LSE). Although most reports involving SPE are off-line procedures, there is increasing interest and subsequently increasing numbers of reports regarding on-line SPE, the goal of which is to improve overall productivity and safety. To a lesser extent, solid-phase microextraction (SPME), supercritical fluid extraction (SEE), semi-permeable membrane device (SPMD), and molecularly imprinted polymer (MIP) techniques have been reported. 

Figure 1. Experimental system for supercritical fluid extraction. LF = line filter PG = pressure gauge SV = shut-off valve CV = check valve PHC = preheating coil TC = thermocouple MV = micrometering valve TP = separator trap subscripts r = reactor 1,2 = trap 1,2.

Combination of static subcritical water extraction and solid-phase microextraction Comparison of CHC1F2, N2O and CO2 extractants. CHC1 F2 gave highest recovery, methanol-modified CO2 gave 90% recovery Combination of supercritical fluid extraction with off-line Fourier transform infrared spectroscopy... 

Supercritical fluid extraction (SFE) utilizes the unique properties of supercritical fluids to facilitate the extraction of organics from solid samples. Analytical scale SFE can be configured to operate on- or off-line. In the online configuration, SFE is coupled directly to an analytical instrument, such as a gas chromatograph, SFC, or high-performance liquid chromatograph. This offers the potential for automation, but the extract is limited to analysis by the dedicated instrument. Off-line SFE, as its name implies, is a stand-alone extraction method independent of the analytical technique to be used. Off-line SFE is more flexible and easier to perform than the online methods. It allows the analyst to focus on the extraction per se, and the extract is available for analysis by different methods. This chapter focuses on off-line SFE. 

Supercritical fluid extraction coupled to SFC has been used for the extraction, separation and identification of PAHs from coal. The supercritical extract was expanded with the aid of a frit restrictor accommodated in the sample cavity of a cooled micro-injector, the analytes being deposited by condensation while CO, was sent to waste through a vent valve. Subsequently, the loop contents were connected on-line to the mobile phase of the capillary chromatograph. The extracted analytes were detected by off-line FTIR spectroscopy following collection on a KBr disc and evaporation of the solvent [104]. 

Another chromatographic technique utilized for the analysis of pyrolysates is thin layer chromatography (TLC) [98, 98a]. TLC can be utilized as an off-line simple technique for the analysis of pyrolysates, with the possibility to visualize the presence of species that do not elute through the stationary phase and remain at the start line of the TLC plate. Supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC) also have been utilized for the off-line analysis of pyrolysates [99]. The procedure consists of placing off-iine the pyroiysate in the extractor, followed by capillary SFC separation and FID detection of the extract. 

Blanch, P., Reglero, G. and Herraiz, M. (1995) Analysis of wine aroma by off-line and online supercritical fluid extraction-gas chromatography,/. Agric. Food Chem., 43(5), 1251-1258. 

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 . 

The economic viability of any process, not just a supercritical fluid extraction process, requires that equipment utilization be efficient because off-line time is a capital investment penalty. In the present activated carbon... 

This paper describes various analytical supercritical fluid extraction methodologies developed in the authors laboratory and summarizes selected investigations conducted to evaluate the applicability and efficiency of these methods for a range of applications. Described methodologies include off-line... 

Figure 2.8 Supercritical fluid chromatography, with use of a miniaturized evaporative lightscattering detector, of seed extracts from (a) an autumn rapeseed (b) Camelina sativa (triacyl-glycerides containing long-chain monoenes have not been identified because reference substances are not available). SFE/SFC was off-line. Column as in Fig. 2.2. Separation conditions temperature 140°C pressure 160 atm, after 1 min programmed at -10°Cmin to 100°C and 25 atm min" to 260 atm, then programmed at -l°Cmin to 75°C and at 2 atm min to 260 atm. Extraction conditions temperature 90°C, pressure 160 atm. Abbreviations L = linoleate Ln = linolenate 0 = oleate P = palmitate S = stearate SFC = supercritical fluid chromatography SFE = supercritical fluid extraction.

Coupled systems include multidimensional and multimodal systems. Multidimensional chromatography involves two columns in series preferably two capillary columns, with different selectivity or sample capacity, to optimize the selectivity of some compounds of interest in complex profiles or to provide an enrichment of relevant fractions. In multimodal systems, two chromatographic methods or eventually a sample preparation unit and a chromatographic method are coupled in series. Coupled systems that received much interest in recent years are multidimensional CGC (MDCGC), the combination of high-performance liquid chromatography with CGC (HPLC-CGC) and the on- or off-line combination of supercritical fluid extraction with CGC (SFE-CGC). Multidimensional and multimodal techniques in chromatography arc described in detail in [65],... 

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