3.15 NMR + 3.16 Chromatography

Question 1

What does NMR stand for?

Answer 1

Nuclear Magnetic Resonance Spectroscopy

Question 2

What values can spin angular momentum be?

Answer 2

0, 0.5, 1, 1.5, 2, 2.5, etc.

Question 3

What determines the spin angular momentum?

Answer 3

The number of unpaired protons and neutrons in a nucleus of an atom

Question 4

What do protons and nucleons have a spin angular momentum (spin number) of?

Answer 4

0.5

Question 5

Nuclei with spin can do either one of two things (with respect to the magnetic field)?

Answer 5

Line up with the magnetic field or oppose the magnetic field

Question 6

Why is TMS used?

Answer 6

To calibrate the spectrum as this produces a signal providing an internal standard to which other peaks are compared.

Question 7

What makes TMS suitable for use?

Answer 7

• It only gives one proton peak
• It is non-toxic
• It is inert
• It has a low boiling point and so can easily be removed from the sample
• It gives a signal that is further right than most of the signals from organic compounds.

Question 8

What is the area of the signal in proton NMR proportional to?

Answer 8

The number of H atoms it represents

Question 9

Which atoms on proton NMR cause splitting?

Answer 9

Usually only H atoms on the neighbouring C atom

Question 10

How can you calculate the number of inequivalent H atoms on an adjacent C atom in proton NMR?

Answer 10

The number of lines = 1 + the number of inequivalent H atoms on adjacent carbon

Question 11

What is a mobile phase?

Answer 11

• The mobile phase is a substance that allows molecules to move over or through the stationary phase.
• It can be in the form of a liquid or gas depending on the type of chromatography.
• More soluble products move further with the mobile phase.

Question 12

What is the stationary phase?

Answer 12

• The stationary phase is a substance that has affinity to molecules in the mixture being analysed.
• The greater the affinity of a molecule to the stationary phase, the shorter the distance it moves with the mobile phase.

Question 13

Give the calculation for the Rf value.

Answer 13

Rf = distance moved by molecule / distance moved by solvent

Question 14

Describe thin-layer chromatography.

Answer 14

• A metal plate is coated with a thin layer of silica and solvent moves up the plate.
• The plate is then dried in a fume cupboard to reduce toxic fumes
• The chemical traces can then be viewed using a UV lamp or using a developing agent (eg iodine) to allow the traces to be seen by the naked eye.

Question 15

Draw thin-layer chromatography

Answer 15

Question 16

Describe column chromatography.

Answer 16

• A vertical column is packed with a solid, powdered substance which acts as the stationary phase. A solvent containing the mixture being analysed is then added and moves down the column as the mobile phase.
• The varying affinities of the molecules means that they drain out at different times, allowing them to be collected as separate samples.
• The time taken to drain out of the column like this is measured as the retention time.

Question 17

Draw column chromatography.

Answer 17

Question 18

Describe gas chromatography.

Answer 18

• A thin tube is packed with a solid, powdered substance which acts as the stationary phase. Instead of a solvent, a high pressure gas is passed through the tube as the mobile phase.
• This method is used to separate mixtures of volatile liquids which are fed into the gas chromatography machine as vapours
• The analysis machine records a retention time for each molecule.

Question 19

Draw gas chromatography.

Answer 19

Question 20

Describe GC-MS

Answer 20

• This is a combination of analytical techniques, gas chromatography and mass spectrometry, allowing for a more advanced level of molecular analysis.
• The molecules present are first separated using the gas chromatography technique. Then each molecule present is fed directly into a mass spectrometer so it can be accurately identified.