Depending on the distribution coefficient of the material between the sample matrix and the extracting solvent, solvent extraction, whether solid or liquid, can be carried out using the simple separation funnel. Even if the distribution coefficient is only unity, then 4 extraction's using the same volume of solvent as that of the sample will remove over 93% of the material, if 2, then 4 extraction's will remove over 99% of the material. Conversely, if the distribution coefficient is less than unity, then discrete solvent extraction is very inefficient. Nevertheless, providing the distribution coefficient is greater than unity and the sample volume one ml or less, then the ultimate volume of solution will probably be less than 5 ml.

Solid Phase Extraction

Extraction of liquids by solvents is a somewhat clumsy procedure and solid phase extraction is now preferred. It is commonly used in environmental chemistry and pollution studies (for example, to assess pesticide contamination of water), and for the extraction of drugs and drug metabolites from biological fluids such as blood serum and urine. It is also used in forensic chemistry and for studying the metabolic pathways of drugs and other physiologically active materials. The procedure is simple and utilizes a short tube packed with an adsorbent such as silica gel, silica bonded with a reversed phase, or macro porous polymer beads etc. The choice of the adsorbent is determined by the interactive nature of the matrix and analyte. If the solutes are dispersive and the matrix is water, then a dispersive adsorbent would be appropriate (e.g. a reversed phase). If the solutes are polar in a dispersive matrix then a polar adsorbent such as silica would be suitable.

Courtesy of Supelco Inc.

Figure 18. A Solid Phase Extraction Tube

After extraction, the solutes can be displaced from the adsorbent by a solvent that interacts more strongly with the adsorbent than the solutes themselves. The diagram of a simple solid phase extraction tube is shown in figure 18. Solid phase extraction tubes are made of inert plastic(e.g. polypropylene) and have of 1, 2, or 5 ml capacities. The tube is usually one fifth filled with adsorbent contained by plastic frit. The upper, empty part of the tube acts as a container for the sample that percolates through the adsorbent bed. The lower end of the tube can be connected to a vacuum or the top to a gas supply to increase the flow through the adsorbent bed. The extract can be concentrated by evaporation