Topic 7 + 19: Analytical Techniques I, II

Question 1

What kind of information can you find through 13C NMR spectroscopy?

Answer 1

Number of carbon environments
Chemical shift

Question 2

What is the reference molecule used in NMR

Answer 2

TMS (tetramethylsilane)

Question 3

Why is TMS used as a reference molecule

Answer 3

• Inert so won’t react
• All H and C environments are identical so produces one sharp/ intense peak
• Peak at low chemical shift
• Easy to remove (low boiling point)

Question 4

Why is a larger, more concentrated sample needed for 13C NMR rather than H NMR?

Answer 4

13C has a lower abundance than 1H

Question 5

What information can you find through 1H NMR spectroscopy?

Answer 5

• Number of H environments
• Splitting patterns (n+1 rule)
• Area (ratio of Hs in each environment)
• Chemical shift

Question 6

What does the presence of complex multiplets indicate?

Answer 6

Presence of a benzene ring

Question 7

What could be happening if there is a complex/unexplained multiplet

Answer 7

2 overlapping signals from two environments which are very similar but slightly different

Question 8

What is the stationary phase used in thin-layer chromatography (TLC)

Answer 8

Solid silica on a glass plate

Question 9

What is the stationary phase used in column chromatography (CC)

Answer 9

Solid Silica

Question 10

What is the stationary phase used in gas-liquid chromatography (GLC)

Answer 10

Microscopic liquid film on a solid support

Question 11

What is the mobile phase used in thin layer chromatography (TLC)

Answer 11

Liquid solvent, e.g. water or organic

Question 12

What is the mobile phase used in column chromatography?

Answer 12

Liquid solvent, e.g. water or organic

Question 13

What is the mobile phase used in gas-liquid chromatography?

Answer 13

Inert carrier gas, e.g. nitrogen

Question 14

What is the purpose of chromatography?

Answer 14

Separation of compounds in a mixture (analysis and identification)

Question 15

What is the purpose of the mobile phase?

Answer 15

The more soluble (the stronger the interactions between the molecule and the mobile phase) a molecule is in the mobile phase AND the less soluble in the stationary phase (retention), the higher the retention factor is.

Question 16

What is the purpose of the stationary phase?

Answer 16

To slow the molecules moving through the mobile phase - to retain them. Usually is a different type of interaction between the molecules in the stationary phase vs in the mobile phase.

Question 17

Formula for Rf value

Rf = Retention factor

Answer 17

Rf = (distance travelled by substance)/(distance travelled by solvent front)

Question 18

Explain high performance liquid chromatography (HPLC) and how it works.

Answer 18

• Essentially column chromatography
• Sample pumped through by solvent
• Stationary phase particles are smaller -> greater separation of compounds
• Detector at the end of the column
• Retention time = time taken from injection to detection

Question 19

Explain what is gas chromatography and how does it work?

Answer 19

• Used for analysing gases, volatile liquids, solids in vapour form
• Stationary phase: long coiled column
• Mobile phase: inert carrier gas moves sample molecules through stationary phase
• In gas-liquid, a non-volatile liquid is the stationary phase