Chromatography

Chromatography
Phase: a physically distinctive form of a substance
Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase.
The stationary phase interacts with the components in the mixture, slowing them down.
This allows different compounds to flow over the stationary phase at different speeds, separating the components.
In TLC:
The stationary phase is a solid and the mobile phase is liquid
In Gas Chromatography:
The stationary phase is liquid or solid on a solid support, and the mobile phase is a gas.
Adsorption is the process which holds molecules of a gas or liquid to the surface of a solid.
As the mobile phase passes over the solid phase, some of the molecules are adsorbed.
TLC
Stationary Phase: Thin layer of adsorbent such as silica gel coated on a sheet of glass.
Mobile phase is a liquid solvent.
Sometime UV light is needed to see some components of the sample.
Rf value:
Distance moved by component / Distance moved by solvent front
Limitations:
  * Similar compounds have similar Rf values
  * Unknown compounds have no reference value for comparison
  * Some components may ‘hide’ behind others
Gas Chromatography
Stationary phase is a thin layer of solid or liquid coated on tubes. The tubes are heated. The mobile phase is an inert gas which moves through the column.   The sample is vaporised and carried along by the gas. The stationary phase slows some components down.
Rt is the time for a component to pass from the inlet to the detector.
  * Different compounds have different retention times
  * The area under the peak is the amount of a compound in a sample

Limitations:
  * Sample may have chemicals with the same retention time and peak shape
  * Substance may hide behind another
  * Unknown compounds have no retention times to compare to
Uses of GC-MS
  * Forensics
  * Space Probes
  * Airport security
  * Environmental analysis
NMR...