Chromatography

Question 1

What is the aim of chromatography?

Answer 1

The aim of chromatography is to separate a mixture into it’s constituents

Question 2

What are the two key phases to chromatography?

Answer 2

The stationary phase and the mobile phase

Question 3

What is the stationary phase?

Answer 3

This is either the chromatography paper or a thin-layer chromatography (TLC) plate.

Question 4

What is the chromatography paper made of?

Answer 4

Cellulose

Question 5

What is the TLC plate made of?

Answer 5

Often a sheet of plastic , coated with a thin layer of silica gel or aluminium hydroxide.

Question 6

What are pointing out of both of the plates?

Answer 6

-OH groups

Question 7

What is the mobile phase?

Answer 7

The solvent for the biological molecules.

Question 8

What solvent is used for polar molecules?

Answer 8

Water

Question 9

What solvent is used for non-polar molecules?

Answer 9

Ethanol

Question 10

How do the different components stop at different times?

Answer 10

They travel at different speeds.

Question 11

Why are some molecules slower than others?

Answer 11

Exposed hydroxide groups make the sheet very polar, allowing it to form hydrogen bonds with the molecules. A highly polar molecule will tend to stick to the surface and hence move more slowly. A non-polar solute will travel very quickly up the plate.

Question 12

You can use the relative distance to identify molecules. How do you calculate relative distance?

Answer 12

relative distance = x(the distance from the pencil line to the centre of a spot of pigment)/y(The distance from the pencil line to the solvent front)

Question 13

If you know the relative distance of specific molecules what does it allow you to do?

Answer 13

Identify them

Question 14

Sometimes molecules are colourless. What three different methods are there to use if this is the case?

Answer 14

Ultraviolet light
Ninhydrin
Iodine

Question 15

How is UV light used to identify molecules?

Answer 15

TLC plates have a thin layer of a chemical which is fluorescent under UV light. The molecules will be exposed as they will not be glowing.

Question 16

How is Ninhydrin used to identify molecules?

Answer 16

It is used to see amino acids. You spray the ninhydrin onto the plate after it has dried. It binds to the amino acids turning them into brown or purple spots.

Question 17

How is Iodine used to identify molecules?

Answer 17

After allowing the plate to dry, place it into an enclosed container with a few iodine crystals. The iodine crystals form a gas which then binds to the molecules in each of the spots.

Question 18

What are TLC plates often used for?

Answer 18

Urine testing of athletes for illegal drugs.
Analysing drugs for purity of components.
Analysis of foods to determine the presence of contaminants.

Question 19

If two molecules travel at the exact same speed and therefore form one dot. How can you distinguish them?

Answer 19

Use a different solvent or change the pH of the solvent.