Laboratory Scale SFE

SFE is carried out on a laboratory scale for both sample preparation and for initial studies on possible industrial processes. A range of commercial equipment is available to carry out experimental studies conveniently. A simple system is shown schematically in Figure 1. It can be assembled in-house and shows the principles involved. The fluid, typically carbon dioxide, is supplied from a cylinder with a dip tube to a pump, which can be a pump designed for liquid chromatography capable of delivering up to 5 mL per minute at a pressure of 400 bar and displaying the pressure and the flow rate. The pump head must be accessible so that it can be cooled by circulating an ethylene glycol and water mixture from a cooler, so that the fluid substance is pumped as a liquid. An alternative method of ensuring this is to use a fluid supply with an overhead pressure of around 100 bar of helium. In this case, cooling the pump head is not necessary, but the fluid will contain a small percentage of helium. The pumped fluid substance then passes into a controlled heater, which can be an oven for gas chromatography. It first passes through a length (typically 0.3 m) of stainless steel tubing into an extraction cell, rated for 400 bar at 100°C, which is fitted with a frit at the exit end (often both ends) to keep the sample matrix to be extracted in place. The exit tube is then connected to a restrictor to maintain the pressure in the system. This can be of stainless steel or, alternatively, a quartz capillary, in which case the connector will have a graphitized ferrule. The effluent then passes through a collecting solvent to trap the extracted compounds. Because of the cooling effect as the fluid expands to atmospheric pressure, it is usually necessary to heat the restrictor and the simplest way of doing this is with a domestic hair dryer. Evaporation of the collecting solvent may occur and it will be necessary to add solvent to the vial during the extraction. This simple device, although often satisfactory, can suffer from blocking of the

Table 2 Examples of the use of SFE in analytical sample preparation

Matrix

Examples of analytes extracted

Soils, sludges, water

Food and animal tissue

Human milk and serum Polymers, food packaging Herbs, cosmetic products Plant tissue

Fly ash, engine emissions

Sedimentary rocks Fermentation broths

Agrochemicals, polychlorobiphenyls, polycyclic aromatic hydrocarbons, fuel hydrocarbons, phenols, surfactants, metals Veterinary drugs, pesticides, anabolic steroids, mycotoxins, fats Drugs

Low oligomers, polymer additives Flavours, fragrances Alkaloids, various natural products, triglycerides Polycyclic aromatic hydrocarbons, dioxins Biomarker hydrocarbons Biologically active compounds

Figure 1 Schematic diagram of a simple system for carrying out SFE on a laboratory scale.

restrictor and loss of extracted compounds because of inefficient trapping. Furthermore, it does not allow independent control of the flow rate and pressure. More sophisticated commercial methods of pressure control and trapping are available.

In a representative experiment, a 1 mL cell is loaded with 0.5 g of the material to be extracted, (previously dried and ground to particles of 0.1 mm diameter). Carbon dioxide is pumped at a rate of 0.5 mL min-1, measured as liquid at the pump. The temperature is 50°C and the pressure of 400 bar is maintained by a restrictor of 25 |im internal diameter and 12 cm length. The effluent is trapped in 3 mL of dichloromethane, ready for analysis by gas chro-matography after an internal standard had been added. The extraction is carried out for 30 min. However, conditions for SFE vary widely and the details for a particular application can be found in the many reports now in the literature.

If a modifier is required, a second liquid pump must be added to the system and the output liquid fed into a mixing chamber just before the shut-off valve in Figure 1. Modifiers are usually added in relatively small amounts, say 5% or 10% by volume. It is possible to purchase cylinders of carbon dioxide al ready containing small amounts of common modifiers, such as methanol or acetone. If a modifier is used, the trapping solvent is conveniently the same as the modifier, as modifier will precipitate in the collection vial. Trapping is usually more efficient if a modifier is used.

The experiment described above is described as dynamic extraction, as the fluid is continuously flowing through the cell. Static extraction can also be carried out in a similar system if a second shut-off valve is inserted after the extraction cell. During an experiment, the cell is pressurized with fluid and the cell isolated so that contact between the matrix and fluid can occur for a period of about 30 minutes. A short dynamic stage is then carried out to remove the fluid, containing the dissolved extract, from the cell. For a static extraction, a modifier may be added as liquid to the cell before closing it.

SFE can readily be coupled to gas chromatography by passing the restrictor through a septum into the injection port of a chromatograph. This procedure can be much more sensitive, as all the extracted material is transferred to the chromatograph, whereas in an off-line experiment, only a small fraction of the collection solvent will be injected. Thus the procedure is applicable for example to the analysis of pesticides at low levels. To carry out this procedure, the first section of the chromatographic column is cooled and the carrier gas turned off. SFE is then carried out with the carbon dioxide, or other fluid substance, passing out through the column and the extracted materials depositing at the inlet of the column. SFE is then stopped and the carrier gas passed through the column to flush out the carbon dioxide. The column is then raised to the analysis temperature and chromatography carried out.

the process of extraction can be considered to involve the three factors shown in the SFE triangle below.

Solubility

Solubility

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment