WM - modern analytical techniques

Question 1

Mass spectra - what does it tell us?

Answer 1

helps identify compounds 
relative atomic mass
relative molecular mass
relative isotopic abundance
molecular structure

Question 2

Mass spectra - how does it work?

Answer 2

1. vapourisation - the sample is turned into a gas by an electrical heater
2. ionisation - the gas particles are bombarded with high energy electrons to ionise them. Electrons are knocked off the particles leaving +ve ions
3. acceleration - the +ve ions are accelerated by an electric field
4. detection - the time taken for the +ve ions to reach the detector is measured. This depends on an ion’s mass and charge. Lighter, highly charged ions first.

Question 3

Mass spectrum - what does it look like?

Answer 3

y-axis gives the abundance of ions - often as a percentage.
For an element the height of each peak gives the relative isotopic abundance
x-axis units are given as mass/charge ratio - since the charge on the ions is mostly +1 can often assume the x-axis is simply the relative mass

Question 4

mass spectra - what is the M+ peak?

Answer 4

gives the relative molecular mass of a compound
the mass/charge value is the molecular mass
its the far right peak

Question 5

mass spectra - what is the M+1 peak?

Answer 5

a smaller peak to the right of the molecular ion peak

Question 6

mass spectra - what causes the M+1 peak?

Answer 6

cause by the presence of the carbon isotope 13C

Question 7

mass spectra - fragmentation

Answer 7

the bombarding electrons make some of the molecular ions break up into fragments
- the fragments the are ions show up on the mass spectrum making a fragmentation pattern

Question 8

mass spectra - what is fragmentation used for?

Answer 8

used to identify molecules and their structure

Question 9

mass spectra - how do you work out structural formula?

Answer 9

1. work out what ion could have made each peak from its m/z value
2. identify the fragments
3. piece them together to form a molecule with the correct Mr

Question 10

infrared spectra - how does it work?

Answer 10

1. a beam of infrared radiation is passes through a sample of a chemical
2. the IR radiation is absorbed by the covalent bonds in the molecules
   - this increases their vibrational energy (they vibrate more)
3. bonds between different atoms absorb different frequencies of IR radiation and bonds in different places in a molecule also absorb different frequencies
   eg. O-H bond in alcohol and O-H bond in carboxylic acids absorb different frequencies

Question 11

infrared spectra - what does it tell us?

Answer 11

An infrared spectrometer produces a spectrum that shows you what frequencies of radiation the molecules are absorbing

can use this to identify the functional groups in a molecule

also means that you can tell if a functional group has changed during a reaction.