Spectroscopy & Chromatography

Question 1

Why is TMS used in NMR?

Answer 1

It acts as a reference for the scale
Only a trace amount is needed
It is a volatile liquid
It does not interact with the sample

Question 2

What is chromatography?

Answer 2

A method used to separate substances in a mixture to identify them. The speed a substance travels through at is measured by the Rf value.

Question 3

How do you calculate the Rf value?

Answer 3

Distance moved by substance / distance moved by eluent

Question 4

State what is used for the mobile and stationary phase in:

a) gas liquid chromatography
b) HPLC

Answer 4

a) mobile phase: inert gas (nitrogen)
Stationary phase: viscous liquid (oil)
b) mobile phase: polar liquid (methanol)
Stationary phase: solid silica particles

Question 5

Define retention time?

Answer 5

The time taken for a component to travel through the column

Question 6

What factors affect retention time?

Answer 6

Temperature
Nature of phases
Pressure
Flow rate & attraction to phases

Question 7

How does NMR work?

Answer 7

NMR is used to identify molecules using hydrogen nuclei because they have a spin.
Normally protons are spinning in random directions.
When a magnetic field is applied nuclei absorb radio waves and align against the field.
There are more protons aligned with the magnetic field.

Question 8

Explain why HPLC rather than nmr is used to show that an anaesthetic contains no trace impurities.

Answer 8

HPLC shows one peak for each compound present (only one peak if it is pure).
In nmr peaks due to impurities are likely to be hidden by the main peaks.

Question 9

How do microwaves heat things up?

Answer 9

1. Microwave radiation passed through food creates an electric field
2. Polar molecules in food line up with the electric field by rotating.
3. The molecules collide with other molecules and generate heat energy
4. Speed depends on thickness and density of food