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When a dose of drug is administered, three main phases of drug action may be distingushed. In the "pharmaceutical" phase the dosage form disintegrates, the active substance dissolves and becomes available for absorption. The second phase ("pharmaco- kinetic" phase) includes absorption, distribution, metabolism, and excretion. That fraction of the dose which finally reaches the circulation after absorption will be available for biological action in the third, or "pharmacodynamic" phase when the drug reaches the target tissues and a drug-receptor interaction takes place. The objectives in studies of drug metabolism are: (a) to identify the pathways by which drugs are transformed in the body; (b) to ascertain quantitatively the importance of each pathway and intermediate; (c) to identify and quantify endogenous constituents influenced by the drug or its metabolites which may interfere with common metabolic processes. Since metabolites usually differ from their precursors by only a single chemical group, the resulting metabolic pathways generally consist of a series of closely related compounds. Mass spectrometry is uniquely suited for the analysis of drugs and metabolites for several reasons: only a minimal amount of sample preparation is needed, closely related compounds can be analyzed in a single step, structures can often be deduced directly from the mass spectra without the need for pure reference spectra, and constituents can be quantified with relative ease even when present in fractional nanogram quantity.