Gas chromatography

Chromatography - a general term for analytical procedures that separate a mixture into its components as the mixture passes through a column.

Principle

The speed at which the components pass through the column is inversely related to the solubility in the column solvent
--> insoluble subtance passes through first

Setup

1. A chromatographic column is packed with support material such as silica-alumina coated with polyethylene glycol or silicone oil.
2. Carrier gas (e.g. N2, Ar, He) passes through the column
3. Gas mixture to be analysed is "dissolved" in the carrier gas and then injected into the column
4. A detector at the outlet measures the appearance of sample components

Detector

• Flame ionization detector
  * For organic compounds
• Thermal conductivity detector (katharometer)
  * For respiratory gases and N2O
  * Different gases have different thermoconductivity
• Electron capture detector
  * For halogenated compounds

Advantage

• Allow detection and measurement of very low concentrations of drugs
• Used for measuring volatile anaesthetic agents, plus barbiturates, phenothiazine, benzodiazepines, steroids, catecholamines

Disadvantage

• Continuous analysis is impossible
• None of the detectors allow the absolute identification of the unknown components of a sample. Some prior knowledge of the types of substances present is necessary.