GC/MS instruments have been used by forensics laboratories for many years to confirm positive immunoassay drug screens. GC/MS combines separation of compounds by chromatography followed by structural identification using mass spectrometry.
Step 1: Sample preparation
A preparation step or extraction is usually involved prior to analysis.
Purposes for sample preparation:
- To remove undesired materials in the sample matrix.
- To make the analytes more volatile and less polar by derivatization so they are more gas-soluble
- To convert glucuronide metabolites back to the unconjugated parent drug by hydrolysis prior to analysis.
- To increase 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.
