In chapter 1, an overview is given of sample preparation methods and analytical techniques in use today. Each one is discussed, and relevant examples are given. In chapter 2, the development of a method for the HPLC analysis of taurine in human plasma using acetonitrile precipitation and pre-column derivatisation with fluorescamine is presented. This procedure was found to be faster and easier to use than previous taurine assays. In chapter 3, the evaluation of novel aspirin derivatives as prodrugs for transdermal aspirin delivery was made possible by an analytical procedure developed as part of this PhD. Following in vitro application of the derivative to skin, the perfusate samples in PBS could be taken and injected directly onto the HPLC system. In addition to quantifying aspirin (ASA) and its metabolite salicylic acid (SAL), the introduction of a gradient after ASA and SAL had eluted allowed determination of the parent compounds. In chapter 4, the quantitation of ASA and SAL in human plasma using column-switching HPLC with on-line solid-phase extraction is discussed. Following a simple dilution step, the plasma was injected directly onto the system. On switching the 10-port valve, the sample containing the extracted ASA and SAL was sent to the analytical column for quantitation. The protocol was successfully applied to the investigation of levels of these compounds in healthy volunteers following an oral dose of aspirin. In chapter 5, identification and qualitative analysis of a cachectic factor and its albuminbound complex in the urine of cancer patients with cachexia is presented. This work was carried out using capillary electrophoresis.