C12.2 - Analysing chromatograms

Question 1

What do scientists have that they can use to identify unknown compounds?

Answer 1

Scientists have many instruments that they can use to identify unknown compounds

Question 2

Scientists have many instruments that they can use to identify unknown compounds.
What are many of these instruments?

Answer 2

Many of these instruments are more sensitive, automated versions of the techniques that you use in school labs

Question 3

Scientists have many instruments that they can use to identify unknown compounds.
Many of these instruments are more sensitive, automated versions of the techniques, such as paper chromatography, that you use in school labs.
Example

Answer 3

For example, chromatography can be used to:
1. Separate
2. Identify
mixtures of amino acids

Question 4

You will have tried paper chromatography before.

What would you have probably used it to separate?

Answer 4

You would have probably used paper chromatography before to separate dyes in:
1. Inks
Or,
2. Food colourings

Question 5

What does chromatography always involve?

Answer 5

Chromatography always involves a:

1. Mobile phase
2. Stationary phase

Question 6

Chromatography always involves a mobile phase and a stationary phase.
What does the mobile phase do?

Answer 6

The mobile phase moves through the stationary phase

Question 7

Chromatography always involves a mobile phase and a stationary phase.
The mobile phase moves through the stationary phase, carrying the components of the mixture under investigation with it.
What will each component in the mixture have?

Answer 7

Each component in the mixture will have a different attraction for the:

1. Mobile phase
2. Stationary phase

Question 8

Chromatography always involves a mobile phase and a stationary phase.
The mobile phase moves through the stationary phase, carrying the components of the mixture under investigation with it.
Each component in the mixture will have a different attraction for the mobile phase and the stationary phase.
What will a substance with stronger forces of attraction between itself and the mobile phase then between itself and the stationary phase do?

Answer 8

A substance with stronger forces of attraction between:
1 Itself and the mobile phase
than
2. Itself and the stationary phase
will be carried a greater distance in a given time

Question 9

Chromatography always involves a mobile phase and a stationary phase.
The mobile phase moves through the stationary phase, carrying the components of the mixture under investigation with it.
Each component in the mixture will have a different attraction for the mobile phase and the stationary phase.
A substance with stronger forces of attraction between itself and the mobile phase than between itself and the stationary phase will be carried a greater distance in a given time.
What will a substance with a stronger force of attraction to the stationary phase do?

Answer 9

A substance with a stronger force of attraction to the stationary phase will not travel as far in the same time

Question 10

In paper chromatography, what is the mobile phase?

Answer 10

In paper chromatography, the mobile phase is the solvent chosen

Question 11

In paper chromatography, the mobile phase is the solvent chosen.
What can the stationary phase be thought of as?

Answer 11

The stationary phase can be thought of as the paper

Question 12

Given an unknown organic solution, what can chromatography usually tell you?

Answer 12

Given an unknown organic solution, chromatography can usually tell you if it is a:
1. Single compound
Or,
2. Mixture

Question 13

Given an unknown organic solution, chromatography can usually tell you if it is a single compound or a mixture.
What will there probably be if the unknown sample is a mixture of compounds?

Answer 13

If the unknown sample is a mixture of compounds, there will probably be more than 1 spot formed on the chromatogram

Question 14

Given an unknown organic solution, chromatography can usually tell you if it is a single compound or a mixture.
If the unknown sample is a mixture of compounds, there will probably be more than 1 spot formed on the chromatogram.
What happens on the other hand?

Answer 14

On the other hand, a single spot indicates the possibility of a pure substance

Question 15

What can happen once the compounds in a mixture have been separated using chromatography?

Answer 15

Once the compounds in a mixture have been separated using chromatography, they can be identified

Question 16

Once the compounds in a mixture have been separated using chromatography, they can be identified.
What can you compare?

Answer 16

You can compare spots on the chromatogram with others obtained from known substances

Question 17

What does a scientist making a chromatogram often not know?

Answer 17

A scientist making a chromatogram often does not know which pure compounds to include in their experiment to make a positive identification

Question 18

A scientist making a chromatogram often does not know which pure compounds to include in their experiment to make a positive identification.
What is it also not practical to do?

Answer 18

It is also not practical to store:
1. Actual chromatograms
Or,
2. Their images
,even on a computer

Question 19

A scientist making a chromatogram often does not know which pure compounds to include in their experiment to make a positive identification.
It is also not practical to store actual chromatograms or their images, even on a computer.
To make valid comparisons, what would need to be exactly the same in all the chromatograms?

Answer 19

To make valid comparisons, every variable that affects a chromatogram would need to be exactly the same in all the chromatograms

Question 20

To make valid comparisons, every variable that affects a chromatogram would need to be exactly the same in all the chromatograms.
What is it far more effective to do?

Answer 20

It is far more effective to:

1. Measure data taken from any chromatogram of the unknown sample
2. Then match it against a database

Question 21

To make valid comparisons, every variable that affects a chromatogram would need to be exactly the same in all the chromatograms.
It is far more effective to measure data taken from any chromatogram of the unknown sample, then match it against a database.
So how is the data presented?

Answer 21

The data is presented as a Rf (retention factor) value

Question 22

Rf (retention factor) value

Answer 22

A Rf (retention factor) value is a ratio

Question 23

How to calculate Rf

Answer 23

To calculate Rf:

Rf = Distance moved by substance ÷ distance moved by solvent

Question 24

What happens as the number generated in the calculation is a ratio?

Answer 24

As the number generated in the calculation is a ratio, it does not matter:
1. How long you run your chromatography experiment
Or,
2. What quantities you use

Question 25

As the number generated in the calculation is a ratio, it does not matter how long you run your chromatography experiment or what quantities you use. What do you have to do, for comparisons against an Rf database to be valid?

Answer 25

For comparisons against an Rf database to be valid, you just have to ensure that the: 1. Solvent 2. The temperature used are the same as those quoted in the databook or database

Question 26

Scientists have many instruments that they can use to identify unknown compounds. Many of these instruments are more sensitive, automated versions of the techniques that you use in school labs, such as what?

Answer 26

Many of these instruments are more sensitive, automated versions of the techniques that you use in school labs, such as paper chromatography

Question 27

Scientists have many instruments that they can use to identify unknown compounds. Many of these instruments are more sensitive, automated versions of the techniques, such as paper chromatography, that you use in school labs. For example, chromatography can be used to separate and identify mixtures of amino acids. The amino acids are what colour?

Answer 27

The amino acids are colourless

Question 28

Scientists have many instruments that they can use to identify unknown compounds. Many of these instruments are more sensitive, automated versions of the techniques, such as paper chromatography, that you use in school labs. For example, chromatography can be used to separate and identify mixtures of amino acids The amino acids are colourless, but appear as what when sprayed with a locating agent and dried?

Answer 28

The amino acids are colourless, but appear as purple spots on the paper when: 1. Sprayed with a locating agent 2. Dried

Question 29

Chromatography always involves a mobile phase and a stationary phase. The mobile phase moves through the stationary phase, carrying what with it?

Answer 29

The mobile phase moves through the stationary phase, carrying the components of the mixture under investigation with it