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
- A chromatographic column is packed with support material such as silica-alumina coated with polyethylene glycol or silicone oil.
- Carrier gas (e.g. N2, Ar, He) passes through the column
- Gas mixture to be analysed is "dissolved" in the carrier gas and then injected into the column
- 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.
