F324: Module 3: Chromatography

Question 1

I) Describe the process of chromatography using relative solubility, stating the mobile and stationary phase.

Answer 1

Stationary phase - A solid or viscous liquid adsorbed onto a solid.

Mobile phase - A gas or liquid.

The solute is more soluble in one phase than the other. The components are separated according to their different solubilities in the two phases. If a solute is relatively more soluble in the mobile phase than another solute, then it will pass through more quickly and have a short retention time.

Question 2

I) Describe the process of chromatography using Adsorption, stating the mobile and stationary phase.

Answer 2

Stationary phase - A polar solid the has to be dry.

Mobile phase - A liquid.

The components in the mixture are usually polar compounds and are attracted to the solid phase to varying degrees. The more the solute is attracted to the solid stationary phase, the more slowly it will move.

Question 3

I) Which mechanism does Thin-Layer Chromatography (TLC) work by?

Answer 3

If the solid is absolutely dry, then it works via adsorption.

Question 4

I) What are the mobile and stationary phases in TLC?

Answer 4

Stationary Phase - Dry silica or alumina coated onto a glass or plastic surface.

Mobile phase - A pure liquid (such as ethanol) or a mixture of liquids.

Question 5

I) Which mechanism does Gas Chromatography (GC) work by?

Answer 5

Relative solubility - The polarity of the stationary phase in the column can be changed and hence changes the solubility of the solutes in the stationary phase. This can be used to achieve a better separation.

Question 6

I) What are the mobile and stationary phases in GC?

Answer 6

Stationary phase - A viscous liquid such as oil adsorbed onto the surface of an inert solid an a long (1-30m) chromatography column.

Mobile phase - A pure gas, such as nitrogen or helium, or a volatile liquid. The mobile phase is called the carrier gas.

Question 7

I) What do you call the time taken for a component to move through the GC column?

Answer 7

Their Retention Time.

Question 8

I) How do you calculate a component’s Rf value in TLC?

Answer 8

Distance moved by the solute / Distance moved my solvent.

Question 9

I) How can components of a mixture be identified in TLC using their Rf value?

Answer 9

By comparing their Rf value with those of known compounds.

Or by running the unknown mixture with the known compound on the same chromatogram.

Question 10

I) What mechanism does paper chromatography work by and what is the stationary phase?

Answer 10

Relative solubility - Although paper chromatography resembles TLC, there is a small amount of water stored in the cellulose fibres meaning that the components dissolve in this water to different degrees (as they have different solubilities).

The stationary phase is Water.

Question 11

I) Why can’t you use water as the mobile phase in paper chromatography?

Answer 11

Because the stationary phase is water.

The stationary and mobile phases can’t be the same otherwise there is no difference in solubility and the components of the mixture do not separate.

Question 12

I) How can components of a mixture be identified in GC using their Retention time and what are the disadvantages to this method?

Answer 12

Their retention times are compared to known compounds in identical conditions.

Disadvantages:

• A mixture may have many components and each of these have to have reference retention times for comparison.
• Unknown compounds may have no reference times for comparison and hence identification.
• Different substances may have identical retention times.
• Substances that have high boiling points can’t be separated.

Question 13

I) How would you calculate the % of each component in a sample mixture using a GC graph?

Answer 13

% of each component = (Area of peak / Total area of peaks) x 100

Question 14

I) Why are Mass spectrometers used in GC?

Answer 14

Linking the column to a mass spectrometer mean that as the unknown leaves the column, it can be identified by comparing it’s fragmentation pattern with that of known compounds, and it can be identified almost instantly.

This technique is called Gas chromatography - Mass Spectrometry (GC-MS).

Question 15

I) Give examples of where GC-MS can be used by scientists.

Answer 15

Environmental Analysis - Looking for pollutants and pesticides.

Forensic Drug Detection

Security Operations - e.g. looking for explosives

Space probes - exploring the components of a planets atmosphere or soil.