Capillary Column
Definition
A fused silica tube 15 to 60 m long with an internal diameter of 0.25 to 0.53 mm and a stationary liquid film 0.1 to 3 µm thick coated on the inner wall. The default column geometry for almost all forensic GC since the 1990s.
- Material
- Fused silica tube
- Length range
- 15 to 60 m
- Internal diameter
- 0.25 to 0.53 mm
- Film thickness
- 0.1 to 3 micrometres
- Adoption
- Default GC column geometry in forensic labs since the 1990s
Common questions
Why did capillary columns replace packed columns in forensic GC?+
Their narrower bore and thinner stationary phase give far higher resolution and shorter run times, letting closely related compounds such as ignitable-liquid components or drug analogues separate more cleanly than the older, wider packed columns allowed.
How does column length affect a forensic separation?+
A longer column increases resolution between similar compounds but also lengthens run time and can broaden peaks for later-eluting analytes, so labs choose length as a tradeoff between separating a complex mixture and keeping throughput practical.
Does film thickness matter for the type of compounds being analysed?+
Yes, a thicker stationary-phase film retains volatile, low-molecular-weight compounds longer and improves their separation, while a thinner film suits less volatile, heavier analytes by reducing peak tailing and shortening their elution time.
Related terms
- Carrier Gas
- The inert mobile phase that sweeps the sample through the column. Helium is traditional and gives the best peak shape on capillary...
- Derivatisation
- 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...
- 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...