Forensic laboratories have used gas chromatography/mass spectrometry (GC/MS) instruments 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 before analysis. Samples for GC/MS are processed to:
- Remove undesired materials in the sample matrix
- Make the analytes more volatile and less polar by derivatization (i.e., the process of chemically altering analytes) so they are more gas soluble
- Convert glucuronide metabolites back to the unconjugated parent drug by hydrolysis before analysis
- Increase the sensitivity by concentrating the extract
Samples must usually be cleaned or purified before being tested 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 adding 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 involves loading 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 within a cartridge and allowed to drain. An organic solvent is passed through the cartridge, extracting the analytes and putting them into a collection tube.
