Chemistry - Rings, Polymers and Analysis - F324 - Separation by Chromatography (3)

Question 1

What is the advantage of chromatography?

Answer 1

Chromatography can separate out components with similar physical properties in very complex mixtures.

Question 2

Define a phase in chromatography?

Answer 2

A phase is a physically distinctive form of a substance, such as the solid, liquid and gaseous states of ordinary matter.

Question 3

Define the mobile phase.

Answer 3

The mobile phase is the phase that moves in chromatography.

Question 4

Define the stationary phase.

Answer 4

The stationary phase is the phase that does not move in chromatography.

Question 5

On which basis does chromatography work?

Answer 5

Chromatography works on the basis that different components have different affinities for a stationary phase and for a mobile phase.

Question 6

What is the stationary phase in thin layer chromatography?

Answer 6

In thin layer chromatography, the stationary phase is a solid.

Question 7

What is the mobile phase in thin layer chromatography?

Answer 7

In thin layer chromatography, the mobile phase is a liquid.

Question 8

What is the stationary phase in gas chromatography?

Answer 8

In gas chromatography, the stationary phase is a solid or a liquid mounted on a solid support.

Question 9

What is the mobile phase in gas chromatography?

Answer 9

The mobile phase in gas chromatography is a gas.

Question 10

What does a solid stationary phase separate via?

Answer 10

A solid stationary phase separates via adsorption.

Question 11

What does a liquid stationary phase separate via?

Answer 11

A liquid stationary phase separates via relative solubility.

Question 12

Define adsorption.

Answer 12

Adsorption is the process by which a solid holds more molecules of a gas or liquid or solute as a thin film on the surface of a solid or, more rarely, a liquid.

Question 13

What is usually used as the stationary phase of thin layer chromatography?

Answer 13

The stationary phase of TLC is usually a thin layer of an adorbent such as silica gel (SiO2) or alumina (AlO3) coated on a flat, inert support, usually a sheet of glass.

Question 14

How is the Rf value calculated?

Answer 14

Rf = distance moved by component / distance moved by the solvent front

Question 15

What are some of the negatives of chromatography?

Answer 15

• similar compounds will have similar Rf values
• unknown compounds have no reference Rf value
• it may be difficult to find a solvent that separates all the components in a mixture

Question 16

What needs to be considered when choosing a suitable solvent for chromatography?

Answer 16

• if the components are very soluble then they will be washed all the way up the plate
• if the components are not very soluble then there will be very little movement
• trial and error may be necessary to find a suitable solvent.

Question 17

What type of compounds is gas chromatography most useful for separating?

Answer 17

Gas chromatography is particularly useful for organic compounds that have a low boiling point and evaporate easily

Question 18

What is the stationary phase in gas chromatography?

Answer 18

The liquid for the stationary phase id often a long chain alkane with a high boiling point. e.g silicone polymers.

Question 19

What is the tubing in gas chromatography referred to as?

Answer 19

The tubing in gas chromatography is referred to as the chromatography column.

Question 20

What is the mobile phase in gas chromatography?

Answer 20

The mobile phase is an inert or unreactive carrier gas such as helium or nitrogen.

Question 21

How can the separation in gas chromatography be improved?

Answer 21

The separation can be improved by using different oven temperatures and different flow rates for the carrier gas.

Question 22

What are some of the negatives of gas chromatography?

Answer 22

• potentially thousands of chemicals have the same retention time, peak shape and detector response
• not all parts of the mixture will necessarily be separated or detected.
• unknown compounds have no reference retention time.

Question 23

What are the isotopes most commonly used in NMR spectroscopy?

Answer 23

1H, 13C, 19F and 31P

Question 24

What creates the magnetic field for NMR spectroscopy?

Answer 24

Unpaired nucleons and their residual spin creates the magnetic field.

Question 25

What is resonance?

Answer 25

The excitation and relaxation of the nucleus is called resonance.

Question 26

What is used as the reference peak in NMR?

Answer 26

Tetramethylsilane (TMS) (CH3)4Si is used as the reference peak.

Question 27

Why is TMS used as the reference?

Answer 27

TMS is used as the reference because it is chemically unreactive and volatile so it can easily be removed from the sample after running an NMR spectrum.

Question 28

Why are NMR spectra run with deuterated solvents?

Answer 28

Because you cannot use a regular organic solvent because the carbon and hydrogen within them also produce a signal. Deuterium has an even number of nucleons so produces no signal.

Question 29

What does the number of peaks in carbon-13 NMR tell you?

Answer 29

The number of peaks in carbon-13 NMR tells you the number of different carbon environments.

Question 30

What do the chemical shifts in carbon-13 NMR spectra tell you?

Answer 30

The chemical shifts in carbon-13 NMR tell you the types of carbon environment.

Question 31

What does the size of the peaks in carbon-13 NMR tell you?

Answer 31

The size of the peaks in carbon-13 NMR tells us nothing.

Question 32

Why does proton NMR require far less material than carbon NMR?

Answer 32

1H is the commonest isotope of hydrogen with a relative abundance of 99.9% compared with 13C which has a relative abundance of 1.1%.

Question 33

What does the number of peaks in proton NMR tell us?

Answer 33

The number of peaks in proton NMR tells us the number of proton environments.

Question 34

What does the chemical shift in proton NMR tell us?

Answer 34

The chemical shift in proton NMR gives the type of proton environment of each peak.

Question 35

What do the relative peak areas in proton NMR tell us?

Answer 35

The relative peak areas in proton NMR give the proportions of protons in each environment.

Question 36

What does spin-spin coupling tell us?

Answer 36

Spin-spin coupling gives us information about adjacent protons.

Question 37

What is the rule for predicting the splitting pattern due to spin-spin coupling?

Answer 37

We predict the splitting pattern using the n+1 rule.

Question 38

Why can it be difficult to identify the protons of OH and NH peaks?

Answer 38

It can be difficult to identify the OH and NH peaks as they can appear over a wide range of different chemical shift values depending on the solvent and concentration of the sample.

Question 39

What is the common name for deuterium oxide?

Answer 39

The common name for deuterium oxide is heavy water.

Question 40

How can OH and NH peaks be specifically identified?

Answer 40

A proton NMR is run, D2O is then added and shaken, then a second NMR is run which makes any peaks due to OH or NH disappear.

Question 41

How does an OH or NH peak usually show on a spectrum?

Answer 41

An OH or NH peak usually shows as a singlet which may be broad.

Question 42

What are the advantages of MRI scanners?

Answer 42

MRI scanners are harmless and non-invasive.

Question 43

Patients with what should not have MRI scans?

Answer 43

Patients with ferromagnetic implants should not undergo MRI scanning.

Question 44

What key information does mass spectrometry tell us?

Answer 44

Mass spectrometry tells us the percentage by mass of each element.

Question 45

What key information does infrared spectroscopy tell us?

Answer 45

Infrared spectroscopy gives information about bonds and functional groups that are likely to be present.

Question 46

What key information does C13 NMR tell us?

Answer 46

C13 NMR gives information about the number and types of carbon environments in each molecule.

Question 47

What key information does proton NMR tell us?

Answer 47

Proton NMR gives information about numbers and types of proton environment and the number of protons on adjacent carbon atoms.