Derivatisation
Definition
A pre-injection chemical step that converts polar, non-volatile or thermally fragile analytes into volatile, GC-friendly derivatives. Silylation with BSTFA is the most common forensic example, used for cannabinoids, opioids and anabolic steroids.
- Purpose
- Convert polar/non-volatile analytes for GC
- Common reagent
- BSTFA (silylation)
- Applies to
- Cannabinoids, opioids, anabolic steroids
- Step timing
- Performed before injection
Common questions
Why can some compounds not be run on GC without derivatisation?+
Polar or thermally fragile compounds either fail to vaporise cleanly at the injector or column temperature, or they decompose or tail badly on the column, producing poor peak shape or no signal. Converting them to a more volatile, stable derivative first restores good chromatographic behaviour.
Does derivatisation risk altering the identity of the analyte being measured?+
The derivatising reagent forms a new, larger molecule, so the mass spectrum obtained is of the derivative, not the original compound; analysts interpret the result against reference spectra of the same derivative, and a well-validated method accounts for this in identification and quantitation.
Related terms
- Capillary Column
- A fused silica tube 15 to 60 m long with an internal diameter of 0.25 to 0.53 mm and a stationary liquid...
- Carrier Gas
- The inert mobile phase that sweeps the sample through the column. Helium is traditional and gives the best peak shape on capillary...
- ECD
- Electron capture detector. A Ni-63 radioactive source produces a baseline electron current that is reduced when electron-affinic species (halogens, nitro groups, peroxides)...
- FID
- Flame ionisation detector. A hydrogen-air flame ionises C-H bond containing analytes and a collector electrode reads the ion current. Near-universal for organic...
- Headspace GC
- Gas chromatography in which the sample (blood, urine, vitreous) is sealed in a vial, equilibrated at a fixed temperature, and a portion...