Organic 16: Chromatography

Question 1

What is chromatography?

Answer 1

A family of separation techniques used to separate mixtures of substances by dissolving them in a solvent and moving it over a solid

Question 2

What is a mobile phase?

Answer 2

The phase that carries the soluble components of the mixture up the stationary phase
The solvent, or eluent, is the mobile phase

Question 3

What is a stationary phase?

Answer 3

The phase that holds back the components attracted to it in the mixture
The more affinity a component in the mixture has for the stationary phase, the slower it moves

Question 4

What is the stationary phase in thin layer chromatography?

Answer 4

A glass, metal, or plastic plate
Coated in a thin layer of silica gel (SiO2) or alumina (Al2O3)

Question 5

What is the mobile phase in thin layer chromatography?

Answer 5

Whatever solvent is used

Question 6

What are the advantages of TLC over paper chromatography?

Answer 6

TLC moves faster
Smaller amounts can be separated
Spots are more compact/specific
Plates are more robust than paper

Question 7

How do you calculate the Rf value?

Answer 7

Distance moved by spot / Distance moved by solvent

Question 8

How can you locate positions of spots in TLC?

Answer 8

Using UV light
Or chemical sprays (e.g. ninhydrin) which react with the spots to produce colours

Question 9

What is the stationary phase in column chromatography?

Answer 9

Powder such as silica, aluminium oxide, or resin
Packed into a narrow column

Question 10

What is the mobile phase in column chromatography?

Answer 10

The eluent is added at the top of the column
It runs down the column, bringing the components in the sample mixture with it
Different substances move at different speeds
And can be separated into flasks at the bottom

Question 11

Why is mineral wool used in column chromatography?

Answer 11

Acts as support for the stationary phase
Ensures uniform solvent flow, preventing mixing between the stationary phase and solvent

Question 12

What is the main advantage of column chromatography?

Answer 12

Fairly large amounts can be separated and collected
E.g. amino acids

Question 13

What is the stationary phase in gas-liquid chromatography?

Answer 13

Powder coated with oil, either packed into or coated onto the inside of a long capillary tube up to 100m long

Question 14

What is the mobile phase in gas-liquid chromatography?

Answer 14

Usually an unreactive gas such as nitrogen or helium
After injection, the sample is carried along by the gas
The mixture separates as some of the components move along with the gas and some are retained by the oil
So the components leave the column at different times after injection (have different retention times)

Question 15

What is the retention time in gas-liquid chromatography?

Answer 15

Time taken for the substance to be detected

Question 16

How are the results of gas-liquid chromatography represented?

Answer 16

On a graph
The area under each peak is proportional to the amount of each component

Question 17

How are components in gas-liquid chromatography identified?

Answer 17

By comparing retention times with that of known substances under the same conditions
Can confirm using mass spectrometer

Question 18

What are the advantages of gas-liquid chromatography?

Answer 18

Highly sensitive
Can separate minute traces of substances
Used to test blood or urine for presence of drugs

Question 19

How is a mass spectrometer used in gas-liquid chromatography?

Answer 19

GCMS
A mass spectrometer is used as the detector for a gas chromatography system
As each component of a mixture comes out of the gas chromatography column, the time taken to pass through is noted
Each component is fed automatically into the mass spectrometer

Question 20

What are the four main types of chromatography?

Answer 20

Paper chromatography
Thin layer chromatography
Column chromatography
Gas-liquid chromatography