The size of the tube depends on the sample volume, the amount of the materials to be extracted and the sample matrix. If the amount of materials to be extracted is high then a larger tube may be necessary. Before use, the packing should be preconditioned with an appropriate solvent, or solvents. Common conditioning solvents are methanol, MTBE, MEK, deionized water and potassium chloride solutions. The adsorbent activation can deteriorate if the sample is too large. The sample volume for a 3 ml tube should not exceed 250 ml.
After extraction the adsorbent bed should be washed with an appropriate solvent, one tube volume of washing solvent is usually sufficient. The extracted material is then displaced with another more strongly adsorbed solvent. Suitable displacing solvents are often recommended by the manufacturers of the extraction tubes. A volume of between 200 ml and 2 ml is usually sufficient to elute all the adsorbed solutes
Trace analysis can have problems unique to the isolation and measurement of very small quantities of material. One problem is the retention of solutes on the surfaces of the different parts of the solid phase extraction apparatus. Some substances of biological importance can also decompose, denature or thermally rearrange on such surfaces. To avoid such changes the extraction apparatus can be constructed from Teflon. A diagram an inert system, produced by Alltech, is shown in figure 19. This type of apparatus is used extensively in the biotechnology and essential oil industries where many compounds undergo molecular rearrangement and chemical change when in contact with active surfaces. An example of the use of solid phase extraction to determine trace amounts (5 ppb) of some chlorinated pesticides in drinking water is shown in figure 20. The solid phase extraction tube was the Novo-Clean C18, 47 mm tube with a membrane manifold supplied by Alltech Inc.
Figure 19 An All–Teflon Solid Phase Extraction Apparatus
The materials were removed from the water sample by the strong dispersive forces between the solutes and the C18 reversed phase in the manner already discussed. The extraction tube was preconditioned with 10 ml of methanol, 10 ml of methyl-tributylether (MTBE), 15 ml of methanol and finally 125 ml of deionized water. The water sample was pumped through the extraction tube at a rate of 100 ml/min. The solutes removed were displaced from the extraction tube with 10 ml of methanol followed by 10 ml of (MTBE) and dried over anhydrous sodium sulfate. It is seen that all the chlorinated pesticides were extracted and concentrations down to 1 ppb could be easily identified.
