The M+1 & M+2 Peaks (CIE A Level Chemistry)

• The [M+1] peak is caused by the presence of the carbon-13 (¹³C) isotope in the molecule
• Carbon-13 makes up approximately 1.1% of all carbon atoms
• Therefore, the [M+1] peak is much smaller than the M peak as the isotope is less common
  • The ratio of ¹³C to ¹²C is approximately 1:99

• Thus, the greater the number of carbon atoms present in a molecule the greater the height of the [M+1] peak
• The number of carbon atoms, n, in a compound can be deduced using the [M+1] peak and the following formula:
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• The presence of bromine or chlorine atoms in a compound gives rise to a [M+2] and possibly [M+4] peak

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 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
  • ³⁷Cl + ³⁷Cl = [M+4] peak
  • The ratio of the peak heights is 9:6:1

Mass spectra of chlorine containing compounds

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 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 spectra of bromine containing compounds

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