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Solubility and Mass-Transfer Effects
The following three factors are considered important:
Higher temperatures increase the capacity of solvents to solubilize analytes.
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Methods for Environmental Trace Analysis
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Faster diffusion rates occur as a result of increased temperatures.
Improved mass transfer, and hence increased extraction rates, occur when fresh solvent is introduced, i.e. the concentration gradient is greater between the fresh solvent and the surface of the sample matrix.
Disruption of Surface Equilibria
As both temperature and pressure are important, both are discussed separately.
Increased temperatures can disrupt the strong solute-matrix interactions caused by van der Waals forces, hydrogen bonding, and dipole attractions of the solute molecules and active sites on the matrix.
Decreases in the viscosities and surface tensions of solvents occur at higher temperatures, thus allowing an improved penetration of the matrix, and hence improved extraction.
The utilization of elevated pressures allows solvents to remain liquified above their boiling points.
Extraction from within the matrix is possible, as the pressure allows the solvent to penetrate the sample matrix.
Since pressurized fluid extraction (PFE), also known as accelerated solvent extraction (ASE), is a relatively new technique, the commercial availability of PFE instruments is limited. A commercial PFE system (ASE 200) currently available is a fully automated sequential extractor developed by the Dionex Corporation, USA. This mainly consists of a solvent-supply system, extraction cell, oven, collection system and purge system, all of which are under computer control. A schematic diagram of a PFE system is shown in Figure 7.15. This system (ASE 200) can operate with up to 24 sample-containing extraction vessels and up to 26 collection vials, plus an additional four vial positions for rinse/waste collection.
Figure 7.15 Schematic of the layout of a typical pressurized fluid extraction system.
What advantage does automation offer?
It allows samples, once loaded into the carousel, to be sequentially extracted without the interference or requirement for laboratory personnel. This allows the analyst to perform other tasks while the extraction process is being carried out.
Five sample extraction vessel sizes are available, i.e. 1, 5, 11, 22 and 33 ml. The sample vessel has removable end-caps that allow for ease of cleaning and sample filling. Each sample vessel is fitted with two finger-tight caps with compression seals for high-pressure closure. To fill a sample vessel, one end-cap is introduced and screwed on to finger-tightness. Then, a filter paper (Whatman, Grade D28, 1.98 cm diameter) is introduced into the sample vessel, followed by the sample itself. Samples should normally be air-dried or mixed with a suitable drying agent, e.g. anhydrous sodium sulfate, and ground to 100-200 mesh size (150 ^m to 75 ^m). Then, the other end-cap is screwed on to finger-tightness and the sample vessel placed in the carousel. Through computer control, the carousel introduces selected extraction vessels consecutively. An auto-seal
Methods for Environmental Trace Analysis
actuator places the extraction vessel into the system and then returns the vessel to the carousel after extraction. The system is operated as follows. The sample cell, positioned vertically, is filled from top to bottom with the selected solvent or solvent mixture from a pump (capable of operating at up to 70 mlmin-1) and then heated to a designated temperature (40-200°C) and pressure (6.9-20.7 MPa or 1000-3000 psi). These operating conditions are maintained for a pre-specified time using static valves. After the appropriate time (typically 5 min), the static valves are released and a few ml of clean solvent (or solvent mixture) is passed through the sample cell to exclude the existing solvent(s) and extracted analytes. The flush volume is normally 0.6 times the cell volume. This rinsing is enhanced by the passage of N2 gas (45 s at 150 psi) to purge both the sample cell and the stainless-steel transfer lines. After gas purging, all extracted analytes and solvent(s) are passed through 30 cm of stainless-steel tubing which punctures a septa (solvent-resistant; coated with Teflon on the solvent side) located on top of the glass collection vials (40 or 60 ml capacity). If required, multiple extractions can be performed per extraction vessel. The arrival and level of solvent in the collection vial is monitored by using an IR sensor. In the event of system failure, an automatic shut-off procedure is instigated. A typical procedure for pressurized fluid extraction is shown in Figure 7.16.
Figure 7.16 Typical procedure used for the pressurized fluid extraction (or accelerated fluid extraction) of solids.