Chemical Analysis - Mass Spectrometry (10.4)

Question 1

What is mass spectrometry MS)?

Answer 1

• finding out the molecular formula of an unknown substance

Question 2

What can mass spectrometry tell you?

Answer 2

• relative molecular mass of a molecule
• some information about the structure of the molecule
• isotope masses + isotopic abundance of an element

Question 3

What happens in a mass spectrometer?

Answer 3

• ionisation - sample = ionised by a laser/electron gun, forming ions
• acceleration - ions + fragments = accelerated by an electric field, so they have the same kinetic energy (KE)
• separation - ions pass down a flight tube where they spread out according to mass
• detection - ions hit a detector sending an electrical impulse to a computer

Question 4

What happen in a flight tube?

Answer 4

• ions set off along the flight tube
• lighter ions travel faster + start to separate out
• lightest ions reach the detector first
  (lighter ions travel faster as Ek = 1/2mv2) - imputing a lower mass has a ke

Question 5

What is a mass spectrum?

Answer 5

• a graph of the relative abundance vs. mass of the ion

Question 6

What is the molecular ion peak (M+)?

Answer 6

• the relative molecular mass of the sample

Question 7

What is m/z?

Answer 7

• mass divided by charge

- a 1= ion with a mass 12 and 2= ion with mass 24 both have an m/z value of 12

Question 8

What is the molecular ion peak?

Answer 8

• molecular ion peak = the peak furthest to the right
• 1% of all carbon atoms have an extra neutron (carbon-13)
• therefore we see a secondary peak at [M+1]+ - with an abundance of the molecular peak

Question 9

How do you find the molecular mss of an unknown substance?

Answer 9

• find the empirical formula
• find the molecular mass from the EF
• use the mass of the EF and divide it by the molecular mass to get the ratio
• use the ratio to multiply the EF to get the molecular formula

Question 10

What is fragmentation?

Answer 10

• the peaks at lower m/z values arise because the molecular ion = very unstable
• molecular ions break apart upon ionisation
• ion + fragments can rearrange to give stable ions

Question 11

What info does the fragment ion give?

Answer 11

• gives peak in MS

Question 12

What info does the radical give?

Answer 12

• does not give peak in MS

Question 13

What info do fragmentation patterns give?

Answer 13

• clues about the structure of the molecule

e. g. length of a carbon chain

Question 14

What are the values of functional groups from fragmentation?

Answer 14

• methyl, m/z = 15
• ethyl, m/z = 29
• propyl, m/z = 43
• butyl, m/z = 57

Question 15

What are some other aspects about fragmentation?

Answer 15

• presence of an ion peak does not always prove the presence of a fragment
  > chance - some fragments add up to the same values
  > ions + fragments can rearrange + refragment
• absence of an ion peak does not always prove that the functional group = absent
  > not all possible fragments always seen
  > unstable fragments may rearrange + refragement
  > more stable the ion, more likely it is to reach the detector + the higher the peak
  > 3o > 2o > 1o carbocations
• not always possible to assign every peak to a fragment
  > unstable fragments may rearrange + refragement

Question 16

What do you do if a structure of a compound is not immediately obvious?

Answer 16

• elemental analysis
• chemical test
• mass spectrum
  > can deduce after this + after looking at the physical attributes of the unknown compound e.g. less dense than water?, colourless?