Analytical techniques

Question 1

What does the no. of peaks in a Carbon-13 NMR spectrum correspond to?

Answer 1

No. of carbon environments

Question 2

What does the no. of peaks in a Proton NMR spectrum correspond to?

Answer 2

No. of proton environments

Question 3

Why is TMS added to the solution during NMR spectroscopy?

Answer 3

As a standard to measure chemical shifts against

Question 4

How do we remove O-H and N-H peaks?

Answer 4

Add D2O.

Question 5

Why does deuterium remove O-H and N-H peaks?

Answer 5

Deuterium will replace Hydrogen in O-H and N-H bonds (proton exhange). Thus those peaks will disappear as deuterium cannot show up on proton NMR spectra.

Question 6

What do we add to remove O-H and N-H peaks?

Answer 6

D2O or CDCl3

Question 7

How can IR spectra be used to determine the difference between chemicals with the same functional groups?

Answer 7

Look at the fingerprint region of the spectrum and compare it with recorded data. These are unique to each molecule.

Question 8

What is the stationary phase in chromatography?

Answer 8

The substance a fluid is moving in during chromatography (I.E. Aluminum trioxide, or silica in TLC, an inert gas like He or Ne in GC)

Question 9

What is the mobile phase in chromatography?

Answer 9

This carries the dissolved compound being tested.
It is an inert carrier gas in gas chromatography, and a liquid solvent in TLC.

Question 10

What is the stationary phase in chromatography?

Answer 10

This is the material the solvent/mobile phase travels through.

Question 11

What determines the Rf value?

Answer 11

How much component molecules adsorb (stick) to the stationary phase

Question 12

Describe how a gas chromatograph is performed

Answer 12

A substance in gaseous form is mixed with an inert carrier gas (the mobile phase) which is injected into a narrow capillary tube. The time taken for different components to exit is recorded (the retention time).

Question 13

Describe what happens in the capillary tubes

Answer 13

If the lining of the column is a liquid, components may
dissolve; if the lining is a solid, components may become
adsorbed to the surface.
2. The carrier gas flushes the components along the tube.
3. The greater the solubility or adsorption, the more the
components are slowed down. Different components are
slowed down by different amounts, so they separate.
4. Separation can be improved by using different oven
temperatures and different flow rates for the carrier gas.