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How are mixtures analyzed for drugs of abuse, like THC?

How is the concentration of THC in marijuana measured?

Many drugs of abuse are analyzed using chromatography. Tetrahydrocannibinol (THC) is usually determined using gas chromatography (to separate the THC from the hundreds of other compounds present) followed by mass spectroscopy (to verify that the component separated was indeed THC). Here's a sketchy outline of the procedure:

  1. Extraction. The sample is extracted into an organic solvent like pet ether or methanol.
  2. Evaporation. Some of the solvent is evaporated off to concentrate the sample.
  3. Gas Chromatography (GC). The sample is placed into a syringe an injected into a port on the GC. The port is hot enough to vaporize the sample. The vapor passes into a long tube (the "column") lined with a silicone polymer (something like Silly Putty), and an inert gas (like helium) sweeps the sample through the tube. As the sample moves along, the silicone polymer temporarily retains components in the passing stream. Some components are retained more than others; each has a characteristic "retention time". Material exiting the column is collected at the proper retention time for THC.

  4. Mass Spectroscopy (MS). The pure THC stream passes directly from the GC into an evacuated chamber. A beam of electrons ionizes the drug molecules and breaks them into fragments. The ionized fragments are accelerated through magnetic field. The field bends the trajectories of fragments with differing charge and/or mass differently, effectively separating them. The fragments strike a detector that converts the number of impacts into an electrical signal. A computer collects the signals and makes a signal vs. ion mass plot (called a "mass spectrum"). The mass spectrum for THC is unique, and can be used as a chemical fingerprint to verify the presence of THC in the sample. Further, the intensity of the signals can be related to the THC concentration.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How are mixtures analyzed for drugs of abuse, like THC?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/matter/faq/thc.shtml