and Chromatography etc

  • Created by: ava.scott
  • Created on: 28-12-14 12:18


An analytical technique used to separate components in a mixture. We look at TLC and GC.

Phases (A physically distinctive forkm of a substance such as a solid. liquid or gas.

A mobile phases sweeps over a stationary phase, in a definite direction.

Different compnenets have different affinities for the two phases, and this is how they are separated. 

  • The stationary phase slows down components by interacting with them via ADSORPTION, on the surface of the solid.. 
  • The mobile phase dissolves the components by ABSOPRTION, and carries them along.

In TLC- the paper is the stationary phase, and the solvent is the mobile phase.

In GC the stationary phase is the liquid or solid on a supprt, and the mobile phase is a gas.

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Used to measure the extent of a chemical reaction or to check the purity of compounds.

The stationary phase is a thin layer of adsorbent, such as a silica gel (SiO2) or alumina coated ona flat sheet called the TLC plate.

The mobile phase is a liquid solvent which moves up the plate.

Producing the chromatogram

  • Mixture is dissolved
  • Small spot placed at teh bottom of the TLC plate, and allowed to dry..
  • The plate is then dipped in a small shallow layer of solvent, and it must not go above the mixture spot.
  • The jar is closed, to prevent evaporation.
  • The solvent rises, meets the spot and it is swept ipwards. However, some components in the mixture are absorbed more easily, or have less affinit with the stationary phase. This would allow them to travel furtehr with the solvent.
  • The solvent is allowed to reach the plate, and then often treated with a 'locating' agent, so that the components can be seen in different colours.

Rf values

distance moved by component/distance moved by solvent.

Conditions should always be kept the same in TLC, so that the Rf values are the same.

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Limitations of TLC

  • Similar compound can have similar Rf values
  • Unknown compunds have no reference for Rf values for comparison.
  • A solvent which dissolves all the compund may be difficult to find.
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Gas Chromatography

Used to separate volatile components in a mixture. Useful for organic compounds which evaporate easily.

The stationary phase is a thin layer of solid or liquid on the inside of a cpaillary tube , which is wound into a coil so it fits into a thermostatically controlled oven. This tube is called the column. The liquid is often a long chain alkane with a HBP.

Mobile phase is the carrier gas, often inert such as nitrogen or helium.

Producing the chromoatogram

  • Mixture is injected and vaporised.
  • carrier gas flushes it through column.
  • Components slow down as they are adsorbed to the stationary phase.
  • Each component leaves tube at a different time, and is detected by the detector, which then makes a chromotogram.

Retention time

The time it takes for a compnent to pass from the column inlet to the detector. Different components have different retention times, and can be compared to those with similar retention times.

The area beneath each peak is proportionate to the amount in the sample.

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Limitation of GC

  • Thousands of chemicals could have the same retention time, peak shape, and detector respsonse. So it cannot positively identify any components.
  • Not all substances will be separated and detected.
  • Unknown components have no retention times for comparisons.

This is why GC is often combined with mass spectrometry.

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GC can separte components in a mixture but cannot identify them conclusively.

MS can provide detailed structural info on most compounds for exact idenitifcation, but cannot separate them.

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Nmr spectroscopy uses the interaction of materials with low-energy radio-frequency radiation.

TMS (Tetramethylsilane) is used as a standard in all NMR chemical shift measurements. It gives rise to a single tall peak which is easily identifyable.

solvents must be deeuterated: deuterium is a isotope of hydrogen, with an even number of neutrons.

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