Edexcel A Level Chemistry:复习笔记1.1.4 Mass Spectrometry

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
  • These are atoms of the same elements but with different mass numbers
• Therefore, the mass of an element is given as relative atomic mass (Ar) by using the average mass of the isotopes
  • The relative atomic mass of an element can be calculated by using the relative abundance values

• • The relative abundance of an isotope is either given or can be read off the mass spectrum

Answer

• • Ar =
  • Ar = 16.0044
  • Ar = 16.00

Answer

• • Ar =

• You can also predict how a mass spectrum might appear for a given compound, e.g. ethanol, CH3CH2OH
  • The methyl, CH3+, fragment has a mass of 15.0
  • The ethyl, CH3CH2+, fragment has a mass of 29.0
  • The base ion, CH2OH+, fragment has a mass of 31.0
  • The whole molecule has a mass of 46.0
• Predicting mass spectra becomes more complex with the inclusion of halogen isotopes such as chlorine and bromine

Chlorine

Chlorine exists as two isotopes, 35Cl and 37Cl

• A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain
  • 35Cl = M+ peak
  • 37Cl = [M+2] peak
  • The ratio of the peak heights is 3:1 (as the relative abundance of 35Cl is 3x greater than that of 37Cl)
• A diatomic chlorine molecule or a compound containing two chlorine atoms will have three molecular ion peaks due to the different combinations of chlorine isotopes they can contain
  • 35Cl + 35Cl = M+ peak
  • 35Cl + 37Cl = [M+2] peak
    • There is an alternative of 37Cl + 35Cl doubling the [M+2] peak
  • 37Cl + 37Cl = [M+4] peak
  • The ratio of the peak heights is 9:6:1
    • This ratio can be deduced by using the probability of each chlorine atom being 35Cl or 37Cl
    • 35Cl + 35Cl =
    • 35Cl + 37Cl =  but this doubles for the 37Cl + 35Cl option, therefore,
    • 37Cl + 37Cl = 

• The presence of bromine or chlorine atoms in a compound gives rise to a [M+2] and possibly [M+4] peak

Mass spectrum of compounds containing one chlorine atom (1) and two chlorine atoms (2)

Bromine

• Bromine too exists as two isotopes, 79Br and 81Br
• A compound containing one bromine atom will have two molecular ion peaks
  • 79Br = M+  peak
  • 81Br = [M+2] peak
  • The ratio of the peak heights is 1:1 (they are of similar heights as their relative abundance is the same!)

   

• A diatomic molecule of bromine or a compound containing two bromine atoms will have three molecular ion peaks
  • 79Br + 79Br= M+ peak
  • 79Br+ 81Br = [M+2] peak
  • 81Br + 81Br= [M+4] peak
  • The ratio of the peak heights is 1:2:1

   

Mass spectrum of compounds containing one bromine atom

• When a compound is analysed in a mass spectrometer, vaporised molecules are bombarded with a beam of high-speed electrons
• These knock off an electron from some of the molecules, creating molecular ions:

• The relative abundances of the detected ions form a mass spectrum: a kind of molecular fingerprint that can be identified by computer using a spectral database
• The peak with the highest m/z value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
• This value of m/z is equal to the relative molecular mass of the compound

The M+1 peak

• The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13
• The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; The more carbon atoms, the larger the [M+1] peak is
  • For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion

   

Answer:

• • The mass spectrum corresponds to propanal as the molecular ion peak is at m/z = 58
  • Propanal arises from the CH3CH2CHO+ ion which has a molecular mass of 58
  • Butanal arises from the CH3CH2CH2CHO+ ion which has a molecular mass of 72