7-Spectroscopy and Chromatography

Question 1

What are electromagnetic waves?

Answer 1

Transverse waves produced by oscillating electric and magnetic fields at right angles to each other

Question 2

How is the speed of travel of waves calculated? What are the units?

Answer 2

ms-1

Frequency (Hz) x Wavelength (m)

Question 3

What can infared spectroscopy be used for?

Answer 3

Trace the progress of reactions

For example, maturing of wine. Chemists can see how much ethanoic acid and ethanal have been produced

Question 4

How do microwaves cause substances to heat up?

Answer 4

Cause polar substances to heat up by rotating molecules to line up with electric field then reversing the electric field, this causes the energy in the microwaves to be converted into thermal energy

Question 5

How can microwaves be used to speed up reactions?

Answer 5

When using water to extract an organic compound dissolved in tetrachloromethane
Subjecting system to microwave radiation would cause the temperature of the water to heat up but not the tetrachloromethane, causing more of the compound to dissolve in water

Question 6

What is flash photolysis?

Answer 6

Involves subjecting sample to intense UV radiation for a nanosecond which excites the system and observations are made as the system relaxes

Question 7

How does nmr work?

Answer 7

Protons possess a property called spin which causes the nuclei of certain atoms to behave like magnets
When placed in magnetic field, the nuclei align themselves with the field
Energy is needed to change this alignment which can be done using energy from radiowaves

Question 8

How do chemists use nmr?

Answer 8

To find out about hydrogen atoms in a molecule because the frequency of radiation that is absorbed depends on its environment to some extent
Protons in hydrogen atoms are shielded by electrons close to it, modifying the magnetic field it experiences

Question 9

What is the sample dissolved in in nmr? Why?

Answer 9

CCl4 because it contains no hydrogen atoms

Question 10

What is used as a reference substance in nmr? Why?

Answer 10

TMS Si(CH3)4
All protons are in the same environment which is assigned chemical shift of zero
Only a trace amount is needed
Evaporated off easily if sample needs to be recovered
Doesn’t readily react with sample
Same compound allows different chemical shifts to be identified easily

Question 11

What can be identified using low-resolution nmr?

Answer 11

Number of different types of protons (number of peaks)

Relative number of H atoms in each group ( area under peak)

Question 12

Why does splitting occur in high resolution nmr?

Answer 12

The interaction between protons on neighbouring atoms, called coupling

Question 13

What is the n+1 rule?

Answer 13

The number of lines in the split pattern for a peak is equal to one more than the number of hydrogen atoms in the adjacent group in the molecule

Question 14

How can nmr be used?

Answer 14

In nmr imaging, to give information about soft tissue damage such as in the brain immediately after a stroke as it is much faster than MRI. It measures the concentration of choline in a tumour to see how fast it is growing
In pharmaceutical industry, for example the production of nicotine

Question 15

What advantages does nmr have over gas chromatography?

Answer 15

Smaller sample is needed
Reference solution must be prepared in gas chromatography
Less solvent is needed
Much faster

Question 16

How can the relative mass of an element be obtained from mass spectrometry?

Answer 16

Relative abundance of each isotope obtained from mass spectrum
Percentage relative abundance worked out and the relative mass in 100 atoms of the mixture must be obtained
Total calculated and divided by the number of atoms

Question 17

What does an even and an odd value on a mass spectrum indicate?

Answer 17

Even- contains oxygen atom

Odd- contains nitrogen atom

Question 18

How does gas chromatography work?

Answer 18

Sample is injected then vaporised and carried through by the flow of an inert gas
Column contains a liquid which is absorbed onto the surface of an inert solid
As the sample passes through the column, the different components of the sample separate because they move at different rates

Question 19

What gases are used in gas chromatography? What does it depend on?

Answer 19

Nitrogen, helium, argon and carbon dioxide

Depends on the type of detector used

Question 20

What are the different ranges of selectivity used in gas chromatography?

Answer 20

Non-selective - all compounds except carrier gas
Selective detector - Range of compounds with a common physical or chemical property
Specific detector - single chemical compound

Question 21

How does a flame ionisation detector work?

Answer 21

Gases leaving column are mixed with hydrogen and air and burnt
Organic compounds produce ions and electrons which conduct electricity through the flame
Large electrical potential is applied at the burner tip and collector electrode is located above the flame
Current is measured

Question 22

What are the advantages and disadvantages of flame ionisation detectors?

Answer 22

Measures the mass of each substance identified
Destroys the sample
Other types of gas chromatography can also measure the concentration

Question 23

What is column chromatography?

Answer 23

Column is packed with a solid stationary phase such as calcium carbonate
A solvent is added to the column to make the mobile phase and the compound is then added
Tap is constantly open
Eg green pigment is separated into blue and yellow pigments
Blue pigment is more polar and bond more strongly to the solid so pass through column more slowly
Yellow pigment collected first
If compounds are colourless, a method of tracking them is required

Question 24

How does HPLC work?

Answer 24

Solvent recevoir
Pump to produce high pressure
Sample infected
HPLC tube
Detector which produced a signal and waste

Question 25

What is normal phase HPLC?

Answer 25

Column is filled with tiny silica particles and the solvent is non-polar
Polar molecules in the sample will stick for longer to the polar silica than non-polar molecules

Question 26

What is reversed phase HPLC?

Answer 26

Long hydrocarbon chains are attached to the silica to make it non-polar and a polar solvent is used
There will be a strong attraction between the polar molecules of the solvent and the polar molecules of the sample
Polar molecules will largely move with the solvent so will travel through the column more quickly

Question 27

What is the retention time?

Answer 27

The point at which the sample is injected to the point at which the display shows a maximum peak for that compound

Question 28

What does the retention time depend on?

Answer 28

Pressure, nature of stationary phase, exact composition of the solvent, temperature
Hence careful control of conditions is required

Question 29

How can a substance be identified when it travels through the column in HPLC?

Answer 29

UV absorption
Organic compounds absorb UV at various wavelengths
Amount of light absorbed will depend on the amount of a particular substance that is passing through the beam at that time
To avoid false readings from solvents, you would have to use greater wavelengths than what the solvent absorbs

Question 30

How can the substance P be identified in a compound in HPLC?

Answer 30

Small amount of pure P must be identified and the time taken until the peak is detected must be recorded (calibration peak)
Sample injected and if a peak is formed at the same retention time, it shows P was present
Comparing the size of the peak at the same retention time enables concentration of P to be calculated

Question 31

What is the disadvantage of HPLC?

Answer 31

Not possible to compare concentrations of different components in the sample by comparing the size of peaks because it depends on how much UV light is absorbed by the different components