AQA Chem4: Mass Spectrometry

5 stages of mass spectrometry:

• vapourisation - this turns the sample into a gas
• ionisation - electron gun knocks electron out of molecule to make an ion
• acceleration - accelerates molecule with electric field
• deflection - magnet used to deflect ions different amounts
• detection - charged plate detects ions

Definition for relative atomic mass - the average mass of an atom  compared to 1/12 of carbon 12

Ar = sum of (m/z x relative abundance) / sum of relative abundance

Definition of relative molecular mass - the average mass of an entity compared to 1/12 of carbon 12

• the molecular ion is positively charged and has one unpaired  electron shown as M+.
• the high energy ionising beam from the electron gun causes the ionised molecule to fragment
• this involves breaking covalent bonds in the ionised molecule
• this produces an ion and a free radical
• the general equation is - M+. (g) --> X+ (g)  +  Y. (g)
• further fragmentation of X+ is possible to give another ion and free radical
• this results in a characteristic fragmentation pattern
• the molecular ion (M+.) fragments at weaker bonds first
• dominant peaks are associated with the most stable fragments (best able to have a positive charge)
• the most stable ions are carbocation and acylium ions (e.g. HCO+) 

m/z = 15        possible ion: CH3+

m/z = 29        possible ion: CH3CH2+ or HCO+

m/z = 31        possible ion: CH3O+

m/z = 43        possible ion: (CH3)2CH+ or CH3CO+

m/z = 57        possible ion: CH3CH2CO+

m/z = 77        possible ion: C6H5+

• both chlorine and bromine have isotopes
• compounds containing either Cl or Br will have more than one molecular ion peak

Chlorine:

• has 2 isotopes, chlorine 35 and 37 in the ratio of 3:1
• compounds with one Cl atom will have 2 molecular ion peaks and the ratio of the heights of the peaks will be 3:1

Bromine:

• has 2 isotopes, bromine 79 and 81 which exist in equal amounts
• compounds with one Br atom will have 2 molecular ion peaks of the same height
• compounds with 2 Br atoms will have 3 molecular ion peaks