This is a typical application for liquid chromatography using 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%.

Courtesy of Supelco Inc.

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
Sample Volume	100ml containing 500 ng tetrahydrocannabinol

Figure 42. Chromatograms of Tetrahydrocannabinol Carboxylic Acid from Urine

Tricyclic Antidepressant Drugs

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 1.0M aqueous ammonium hydroxide and then two, 1 ml aliquots of 5% methanol in water. The sample is then desorbed by two separate aliquots of 1 ml of 0.22M ammonium hydroxide in pure methanol. Finally, the sample is collected in a silanized glass tube and the solvent removed by evaporation under stream of dry nitrogen. The volume of the sample is then made up to 250 ml and 100 ml placed on the column. The separation obtained is shown in figure 43. The separation was carried out using an LC-PCN column packed with a bonded phase carrying cyanopropylmethyl moieties on the surface. Thus, in contrast to the extraction process, which appears to be based on ionic interactions with the weak ion exchange material, the LC separation appears to be based on a mixture of interactions. There will be dispersive interactions of the drugs with the hydrocarbon chains of the bonded moiety and also weakly polar interactions with the cyano group. It is seen that the extraction procedures are very efficient and all the tricyclic antidepressant drugs are eluted discretely.