One application of instrumental methods is the determination of what drugs a person has taken twenty-four hours after the person took them. In a procedure detailed by Thomas P. Moyer at the Mayo Clinic, a 5-milliliter (0.17-ounce) sample of the patient's blood is analyzed by a technique called high performance liquid chromatography. The sample is treated and injected into a stream of water and
methanol that is called a mobile phase. The mobile phase is pumped through a column of fine sand, where the particles of sand have been coated with a thin layer of an oil-like substance (octadecane). The molecules from the blood sample, including the drug, will spend part of their time adsorbed to the modified sand (stationary phase) and part of their time in the mobile phase. The molecules that spend the majority of their time in the mobile phase will it make through the column first.
To determine when the molecules have exited the column, an ultraviolet (UV) light is placed so that it is perpendicular to the flowing stream of mobile phase. The molecules in the blood sample will absorb the UV light and create a signal at the detector. The height of the signal will be proportional to the concentration of the drug in the urine. The time between the sample injection and passage through the column is reproducible and, by comparing it to the time observed when standard samples are used, permits component identification.
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