Chromatography

Question 1

Benefits of thin layer chromatography:-

Answer 1

Quick

Inexpensive

Question 2

Tlc:-

Answer 2

Solvent dissolves and carries components of mixture up the plate. They are slowed down and separated by adsorption (binding to the the surface) to the plate.

Question 3

Tlc use:-

Answer 3

Used to assess product purity and progress of reaction.

Question 4

Tlc setup:-

Answer 4

Spots of mixture on starting line on plate, suspended within solvent (mobile phase) with spots just above.
Max = solvent front.

Question 5

Tlc component speed:-

Answer 5

Speed depends on how strongly the component component is adsorbed to the stationary plate. The strong the adsorption, the slower the migration up the plate.

Question 6

Strength of adsorption depends on (2):-

Answer 6

• attractive forces between the component and the stationary phase.
• solubility of component in mobile phase. More soluble = easier to overcome adsorption and free from stationary phase.

Question 7

Rf calc:-

Answer 7

Distance travelled by the component/distance travelled by the solvent

Question 8

Rf:-

Answer 8

Measure of the distance migrated by an individual substance, relative to the distance moved by the solvent front.

Question 9

Stationary phase:-

Answer 9

Thin layer of solid e.g silica gel, SiO2 mounted on glass, metal or plastic.

Question 10

Mobile phase:-

Answer 10

Organic solvent

Question 11

Tlc limitation:-

Answer 11

Has very limited resolution- compounds w/ similar Rf values won’t be clearly separated.

Question 12

Gas chromatography stationary phase and mobile phase:-

Answer 12

Stationary= a liquid adsorbed onto an inert solid support.

Mobile=unreactive gas(often helium)

Question 13

Gc process:-

Answer 13

Small sample injected into oven machine and vaporised. Sample is carried into the column (tube) by an inert gas carrier eg helium where it then passes over the stationary phase. Detector analyses/ is input to mass spectrometer.

Question 14

Retention time:-

Answer 14

Time taken for each component of the sample to travel through the tube.

Question 15

Advs of combining gc and ms (gc-ms):-(3)

Answer 15

• very good resolution, often possible to resolve even quite complex mixtures.
• can operate w/ v. tiny amounts of sample mixture.
• a v. large database of mass spectra available = high chance of pos i.d of comps in a mixture.

Question 16

Limitations of gc-ms (3):-

Answer 16

• similar comps often have similar retention times.
• previously unknown have no reference retention times or mass spectra.
• equipment relatively sophisticated and expensive.

Question 17

Hc-ms uses:-(4)

Answer 17

• forensic science.
• testing for organic pollutants.
• in airport security for detecting explosives.
• on space missions.