Chromatography

Question 1

What is chromatography used to do

Answer 1

To separate and identify the components in a mixture

Question 2

What are retention times and Rf values used to do

Answer 2

Identify different substances

Question 3

Characteristics of a stationary phase

Answer 3

Does not move
Normally a solid

Question 4

Characteristics of a mobile phase

Answer 4

Does move
Normally a liquid or a gas

Question 5

What does the mobile phase do

Answer 5

It flows through the stationary phase and carries the components of the mixture with it

Question 6

Which solids are used in chromatography

Answer 6

Silicon (IV) oxide - silica
Aluminium oxide - alumina

Question 7

How is separation achieved with a solid stationary phase

Answer 7

A solid stationary phase separates by relative adsorption.
The different components in the mixture have different affinities for the adsorbent and bind with different strengths to its surface.
If a substance has a stronger adsorption to the stationary phase, it will move more slowly

Question 8

How is separation achieved with a liquid stationary phase

Answer 8

It separates by relative solubility.
The different components have different solubilities in the liquid stationary phase.
If a substance is more soluble in the stationary phase, it has a longer retention time

Question 9

Thin layer chromatography (TLC)

Answer 9

A plate is coated with a solid and a solvent moves up the plate.
Can be used to separate and identify amino acids in a hydrolysed protein

Question 10

What is the stationary and mobile phase in TLC

Answer 10

stationary phase - A solid adsorbent, usually silica (SiO2), that is highly polar

mobile phase - Liquid solvent

Question 11

Describe the process of TLC

Answer 11

Use a pencil, draw a line across the TLC plate 1 cm from the bottom of the plate. This is the base line.

Use a capillary tube to put a small spot of the mixture on the base line of the TLC place and allow to dry.

Place the TLC plate in a beaker containing a suitable solvent, making sure the solvent is below the base line otherwise the solvent will just wash the sample off the plate.

Cover the beaker with a watch glass to prevent the solvent from evaporating

The solvent will rise up the TLC plate.

Leave the TLC plate in the beaker until the solvent is about 1cm below the top of the plate.

Remove the plate from the beaker and mark the position of the solvent front with a pencil line.

Dry the plate and examine the chromatogram.

Each separated substance appears as a spot on the TLC plate.

Question 12

Rf =

Answer 12

Distance moved by a component / Distance moved by the solvent

Question 13

Describe how to see colourless compounds in TLC

Answer 13

Use a locating agent such as iodine or ninhydrin.

Question 14

How can substances in the mixture be identified

Answer 14

By comparing its Rf value with values for known reference compounds, recorded using the same solvent and adsorbent

Question 15

Examples of adsorbents

Answer 15

Silica
Aluminum oxide

Question 16

A student runs a second chromatogram on the protein sample, using a more polar solvent. Predict the effect, if any, on the Rf values of the amino acids. Explain your reasoning

Answer 16

Rf values would be larger.
Amino acids are more soluble in a polar solvent so would travel further up the plate

Question 17

What are the limitations of TLC

Answer 17

Similar compounds have similar Rf values so will be difficult to distinguish
There will be no reference Rf values for comparison for unknown compounds
May be difficult to find a solvent that separates all the components in a mixture

Question 18

What property determines whether compound A or B moves faster up the TLC plate

Answer 18

Its adsorption to the solid stationary phase

Question 19

Adsorption vs affinity

Answer 19

Adsorption - How strongly a chemical sticks to the stationary phase

Affinity - How strongly attracted the chemical is to the stationary phase

They have a positive correlation

Question 20

Gas chromatography

Answer 20

A column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature

Question 21

Stationary phase in gas chromatography

Answer 21

A liquid with a high boiling point adsorbed onto an inert solid support

Question 22

Mobile phase in GC

Answer 22

an inert gas such as helium, argon or nitrogen

Question 23

Retention time

Answer 23

The time between injection and detection of a component

Question 24

How can an unknown compound be identified using its retention time

Answer 24

By comparing its retention time with values for known reference compounds.

Question 25

What is gas chromatography used for

Answer 25

Separating and identifying volatile organic compounds present in a mixture

Question 26

How does gas chromatography tell us how many components there are in a mixture

Answer 26

Through the number of peaks recorded.
It also tells us the abundance of each substance.

Question 27

What does the area under each peak in GC represent

Answer 27

Used to determine the conc of components in the mixture as the area is proportional to the abundance

Question 28

What does retention time in GC depend on

Answer 28

The stationary phase
The temperature of the column
The length of the column
The flow rate of the carrier gas

Question 29

What happens to retention times when temperature and flow rate are greater

Answer 29

The retention times are shorter

Question 30

Limitations of GC

Answer 30

Similar compounds will have similar retention times and so will be difficult to distinguish
There will be no reference retention times for comparisons for unknown compounds

Question 31

What property determines which chemical moves faster through the capillary column in GC

Answer 31

Its solubility in the liquid stationary phase

Question 32

A mixture containing propane and chloroethane is analysed using GC. Name the compound that moves through the column more slowly and why

Answer 32

Chloroethane as it is more polar than propane. Chloroethane has a greater affinity for the stationary phase so travels through the column more slowly and has a greater retention time.

Question 33

Applications of GC

Answer 33

Measuring level of alcohol in blood samples from drivers
Detecting pesticides in river water

Question 34

How is GC commonly used

Answer 34

A mass spectrometer is combined with GC to generate a mass spectra which can be analysed or compared with a spectral database by computer for positive identification of each GC component in the mixture.

Question 35

Describe how GC-MS overcomes some of the limitations of GC

Answer 35

It produces a mass spectrum for each of the chemical separated on the GC column. These spectra are used to identify the compounds as each one has a unique mass spectrum.

Question 36

Why is nitrogen rather than oxygen used as the carrier gas in GC

Answer 36

Nitrogen is unreactive but if oxygen was used ,the sample may get oxidised

Question 37

Column chromatography

Answer 37

A column is packed with a solid and a solvent moves down the column

Question 38

Describe the process of column chromatography

Answer 38

A glass tube is filled with the stationary phase usually silica or alumina in powder form to increase the surface area.

A filter is used to retain the solid in the tube.

Solvent is added to cover all the powder.

The mixture to be analysed is dissolved in a minimum of a solvent and added to the column.

A solvent or mixture of solvents is then run through the column.

The time for each component in the mixture to reach the end of the column is recorded (retention time)

Question 39

Explain why cyclohexene has a shorter retention time than cyclohexanol

Answer 39

Cyclohexanol is more polar than than cyclohexene so cyclohexanol has a greater affinity for the stationary phase. Cyclohexene has a greater affinity for the mobile phase.