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Figure 7.9 Schematic of the layout of a typical supercritical fluid extraction system: BPR, back-pressure regulator (restrictor); SPE, solid-phase extraction. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission.
Why do you think that the use of a single, i.e. ‘mixed’ cylinder, is not the preferred option?
The ‘mixed cylinder’ has its shortcoming in that the CO2-to-modifier ratio varies over the lifetime of its operation. It therefore does not deliver ‘what it says on the label’!
Two pumps are required to pump the pressurized CO2 and organic modifier through the SFE system. After pumping out from the cylinder, CO2 and organic modifier are mixed with a ‘T-piece’ before introducing into the extraction cell. The combination of the two pumps allows a degree of control in terms of modifier composition (1-20 vol%) and flexibility of solvent choice. The ideal pump should deliver a constant flow rate (in the mlmin-1 range) at a suitable pressure (3500-10000 psi). Two kinds of pumps, the reciprocating type (piston pump) and syringe pump, are usually used in SFE systems, although the former is the most commonly used due to its lower cost. However, when the reciprocating pump is employed, cooling is required to prevent cavitation, i.e. gas entrapment, in the pump head. No such modification of the pump head is obviously required for the organic solvent (modifier).
The external heating, to the solvent system, needed to generate the critical temperature is carried out via an oven in which is located the sample cell or vessel. Generally, the required temperature range of the oven is up to 100°C. However, temperatures of 200-250°C may be advantageous in some circumstances. The sample vessel, which is typically made of stainless-steel, must be capable of safely withstanding high pressures (up to 10 000 psi). It should be remembered that some time will be needed to achieve the pre-set temperature for the newly prepared and inserted sample-containing vessel prior to starting the extraction process. Almost all commercial extraction vessels are of the flow-through design, which allows fresh, clean supercritical fluid to pass over the sample. Many commercial extraction vessels are capable of insertion into the system, without the need for wrenches, to allow ease of use and rapid changeover of samples. This is very helpful in preventing the pressure fittings from suffering excessive wear and tear.
Either fixed or variable (mechanical or electronically controlled) restrictors have been employed to maintain the pressure within the extraction vessel. The former is typified by the use of narrow fused-silica or metal capillary tubing, with the latter by back-pressure regulators (BPRs). However, the fixed restrictor is not a good choice, on account of its lack of robustness, although it is the cheapest
Methods for Environmental Trace Analysis
Figure 7.10 Typical procedure used for the supercritical fluid extraction of solids.
option. For the extraction of real samples, it is more appropriate to use a variable restrictor. A typical procedure used for supercritical fluid extraction is shown in Figure 7.10.
7.5.2 Example 7.3: Supercritical Fluid Extraction
of Organochlorine Pesticides from Contaminated Soil and ‘Celite’
184.108.40.206 Extraction Conditions These were as follows:
• Sample: 1 g
• SFE conditions: pressure, 250 kg cm-2; temperature, 50oC; static extraction time, 15 min, followed by 40 min dynamic extraction time; flow rate of liquid CO2, 2 mlmin-1
Comments Extracts collected in a vial containing 3-4 ml dichloromethane (DCM). Escaping CO2 and analytes vented through a C18 SPE cartridge which was back-flushed with 1 -2 ml methanol after each extraction.
220.127.116.11 Analysis by GC-MS
Separation and identification of the individual organochlorine pesticides (OCPs) was carried out on an HP 5890 Series II Plus gas chromatograph, fitted to an HP 5972A mass spectrometer. A 30 m x 0.25 mm id x 0.25 Rm film thickness DB-5 capillary column was used, with temperature programming from an initial temperature held at 85°C for 0.75 min before commencing a 16°Cmin-1 rise to 285°C, with a final time of 2 min. The split/splitless injector was held at 280°C and operated in the splitless mode with the split valve closed for 1 min following sample injection. The split flow was set at 40 mlmin-1, and the mass spectrometer transfer line was maintained at 290°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode.
18.104.22.168 Typical Results
These are shown in Figure 7.11 .
A B C D E
Figure 7.11 Results obtained for the supercritical extraction of various organochlorine pesticides from contaminated soil and ‘Celite’, showing the influence of (a) the soil matrix, and (b) the soil organic matter (SOM) content: A, lindane; B, aldrin; C, dieldrin; D, heptachlor; E, isodrin: (a) H, ‘Celite’; ?, soil: (b) H, SOM 0.2%; ?, SOM 15%; ^, SOM 35%  (cf. DQ 7.8).