Gas Chromatography/Mass Spectrometry (GC/MS)

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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Gas Chromatography/Mass Spectrometry (GC/MS)

GC/MS instruments have been used by forensics laboratories for many years to confirm positive immunoassay drug screens. GC/MS combines the separation of compounds by chromatography followed by structural identification using mass spectrometry.
Step 1: Sample preparation
A preparation step or extraction is often involved prior to analysis.
Samples for GC/MS are processed in order to:
  • Remove undesired materials in the sample matrix.
  • Make the analytes more volatile and less polar by derivatization so they are more gas-soluble
  • Convert glucuronide metabolites back to the unconjugated parent drug by hydrolysis prior to analysis.
  • Increase the sensitivity by concentrating the extract.
Samples usually need to be 'cleaned up' or purified before running them on GC/MS. The drugs in the sample are 'extracted' from the urine and put into a new mobile phase that can be injected into the GC/MS instrument. Some different extraction techniques used in sample preparation are listed below:
  • Precipitation - Proteins are precipitated out of the sample upon the addition of a salt or organic solvent, or by changing the pH of the solution.
  • Liquid/liquid extraction (LLE) - Partitioning of solutes between two immiscible liquid phases. An aqueous liquid phase containing the analytes and another liquid phase, usually an organic solvent, are mixed in a separatory funnel. The sample analytes are transferred from the aqueous phase to the organic solvent. The organic solvent is drained into a container and evaporated to concentrate the analytes.
  • Solid-phase extraction (SPE) - Partitioning of analytes between a liquid phase and a solid sorbent. The general procedure is to load a solution onto the solid phase trapping the analytes onto the sorbent, washing away undesired components, and then eluting the desired analytes with another solvent into a collection tube.
  • Solid-supported liquid extraction (SLE) - An aqueous phase containing the analytes is poured onto an inert diatomaceous earth support contained within a cartridge and allowed to drain. An organic solvent is passed through the cartridge extracting the analytes into a collection tube.