**2.10 Analysis of biological samples**

Ideally, the biological samples collected as per the sampling procedure have to be analyzed immediately after the study but most of the times the samples are stored for several days before subjected to analysis. During storage, the drug may undergo a chemical degradation, adsorption on the walls of the container, etc., so storage of plasma samples is an important aspect of bioavailability studies. The analytical method used for the estimation of the active ingredient responsible for the therapeutic efficacy must be selective and sensitive. Drugs, that undergo the first pass effect exhibit different unchanged drug/metabolite ratio depending on the rate of absorption. In the analysis of blood and urine, the major problem is to extract quantitatively and then separate the intact drug from its major metabolites or even to separate a mixture of two or more drugs from their metabolite.
