Application of Solid-Phase Extraction Techniques to the Analysis of Basic and Acidic Drugs in Biological Fluids
Use of solid phase (SP) extraction techniques as an alternative to the use of organic solvents is now widely accepted, but is not well documented for assays where quantitation down to ng/ml levels is needed. In order to achieve such levels with a relatively non-specific detector such as a UV detector it is important to have a clean sample. To achieve this the use of the appropriate cartridge and the optimization of the wash procedure are essential; thus, variations in drying time between each step can significantly influence the recovery of the drug and/or endogenous plasma components. The situation is further complicated when automated sample processing techniques are used since the number of sample preparation steps may be limited. Such points have become evident to us in setting up an assay for doxepin (1 ng/ml had to be measurable) and its desmethyl metabolite in serum using the Varian ‘AASP’ with UV detection, and an assay for fenoprofen (0.3 μg/ml had to be measurable) in plasma using the ‘PREP’ (DuPont) followed by HPLC with UV detection. The operation of these automated systems has been well described [see index entry ‘Automated’ in Vols. 12, 14 & 16 — Ed.]. Serum is preferable to plasma or even plasma centrifuged prior to processing. Fibrin particles block cartridges, altering flow rates and reproducibility.