Metabolites of drugs can linger in the body for considerable time after the drug, say an anabolic steroid, has been taken and been metabolized. However, in the case of alcohol, it is metabolized and for the most part out of the system in a few hours. It is conceivable, and expected, that some of the actual atoms, carbon, hydrogen and oxygen derived from the alcohol are present (as other materials) in the body and could be detected as unique isotopes or isotope ratios, but then this requires the alcohol that is imbibed to be pretty special. My understanding is that normal alcohol is gone within a relatively short period and is not detectable weeks later. Other drugs and their metabolites may take much longer to be purged from the body. The technique typically used to detect these metabolites is a device called a GC-Mass Spec, which stands for Gas Chromatograph Mass Spectrometer. This device works by taking urine, or cells from the body and burning or aspirating this into a stream of gas which blows it along a long tube containing an adsorbant. The various constituents in the urine then are separated as a function of their tendency to adsorb or stick to the stuff in the long tube. Eventually they come out the other end, and then are subjected to a high magnetic field which deflects the constituent molecules to varying amounts before they hit a target. This deflection is a function of the mass of the various molecular compounds. By analyzing a combination of the time the various components of the urine spend in the GC tube and the angle of deflection for each molecule, you can tell exactly what those molecules are, and you can even detect the amount. This is how drug metabolites are detected for the most part. GC-Mass Spec is the most commonly used method because it is relatively quick and cheap.
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