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of Book 18 but an example of one of the more complex sample preparation methods will be given to illustrate some of the procedures that may be necessary.   Liquid extraction is a clumsy procedure, particularly when used on the micro scale which is often necessary in sample preparation. An alernative procedure is solid phase extraction. The procedure is relatively simple and involves the use of a short tube packed with an appropriate adsorbent such as silica, reversed phase silica or, for some applications, macro porous polymer beads. The adsorbent must be capable of removing the substances of interest from the liquid medium. Extracting trace materials from water (e.g., pollution analysis) a reversed phase would be appropriate. Then the substances could be displaced into solvents such as n-hexane, methylene dichloride etc. A diagram of a simple solid phase extraction tube is shown in figure 34

This type of extraction system is useful for biotechnology samples. 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 36. The extraction tube was designated as the Novo-Clean C18. It was 47 mm tube long which included the membrane manifold. The materials were removed from the water sample by dispersive interactions between the solutes and the C18 reversed phase. The tube was conditioned before use with 10 ml of methanol, 10 ml of methyl-tributylether (MTBE), 15 ml of methanol and

with an appropriate solvent, the sample diluted to a known volume and an aliquot used for analysis. If necessary the extract can be concentrated by evaporation and the total concentrate employed for analysis. To avoid breakdown of labile materials, a totally inert extraction apparatus can be constructed from Teflon. A diagram of such an apparatus, produced by Alltech, is shown in figure 35 which even includes a Teflon hypodermic needle. Figure 35 An All�Teflon Solid Phase Extraction Apparatus

selectively removes the material of interest. The adsorbent is then washed and the adsorbed material extract with a small amount of suitable solvent and the solvent then inject onto the column. An example of the use of this technique is in the determination of tetrahydrocannabinol carboxylic acid in urine. This substance appears in the urine of those subjects that have recently smoked marihuana. The tetrahydrocannabinol carboxylic acid can be extracted from the urine by means of a solid state extraction cartridge packed with a C18 reverse phase (a strongly dispersive packing containing bonded octyldecyldimethyl chains). The urine sample can be used direct, without pretreatment and the materials of interest are irreversibly adsorbed on the reverse phase solely by dispersive interactions. The actual procedure for extracting the tetrahydrocannabinol carboxylic acid according to Supelco is as a follows (1). 5 ml of urine was diluted with 5 ml of water and

nbsp; Courtesy of Supelco Inc. Figure 42. Chromatograms of Tetrahydrocannabinol Car-boxylic Acid from Urine   The chromatograms obtained from a urine sample and a reference standard by this procedure is shown in figure 42. This is a typical application for liquid chromatography using a solid phase extraction cartridges. It is seen that the tetrahydrocannabinol carboxylic acid is clearly and unambiguously separated from the contaminating materials with an extraction efficiency of over 90%. The chromatographic conditions were as follows. Column LC 18 Column Length 25 cm Column Diameter 4.6 mm Column Packing C18 Reverse Phase, (octadecyldimethyl chain) Mobile Phase 55% acetonitrile 45% aqueous 1% acetic acid Flow-Rate 2.5 ml/min. Detector UV adsorption at 280 nm

Another example employing a similar technique is the determination of the tricyclic antidepressant drugs in blood serum (13). The solid phase adsorbent used in this extraction is a weak ion exchanger and the material is preconditioned with a wash of 0.5 ml of 0.5M phosphoric acid followed by 1 ml of deionized water. A volume of 0.5 ml of the serum containing the tricyclic antidepressant drug standards is mixed with 0.5 ml of deionized water and allowed to percolate slowly through the packing. As in the previous example the drugs are held on the ion exchanger whereas the sample matrix materials pass through. The packing is then washed with 0.5 ml of 1.