The M+1 & M+2 Peaks (Cambridge International AS Chemistry)

Determine Number of Carbon Atoms Using M+1 Peak

• 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:

  • $\mathrm{n} = \frac{100 \times \text{abundance} of [ \mathrm{M} + 1 ] }{1.1 \times \text{abundace} of \mathrm{M} ^{ + } ion}$

Worked Example Determine the number of carbon atoms of compound X with the following mass spectrum:

Answer:

• The M⁺ ion peak is at m/e 58 with a relative abundance of around 85

• The [M+1] peak is at m/e 59 with a relative abundance of 3

• Therefore, the number of carbon atoms (n) is:

  • $n=\frac{100\times3}{1.1\times85}=3.21$

• There are therefore 3 carbon atoms present in compound X

Detecting Bromine & Chlorine Atoms Using M+2 Peak

• 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 spectra of four bromine-containing compounds (A–D), illustrating characteristic patterns due to the presence of two isotopes: 79 Br 79 Br and 81 Br 81 Br. Each spectrum shows distinctive peak groupings depending on the number of bromine atoms in the molecule.

Worked Example Two stable isotopes of bromine have relative masses of 79 and 81

Which is the correct pattern of peaks in the mass spectrum of molecular bromine?

Answer:

• The correct answer is D

• Bromine is a diatomic molecule there will be 5 peaks on the mass spectrum of bromine

• Bromine consists of molecules, not individual atoms

• When bromine is passed through the mass spectrometer, an electron is given off to give the molecular ion, Br₂⁺

• Some of these will fragment to make Br + Br⁺

  • Br₂⁺ → Br + Br⁺

• The Br atom passes through the machine, and the Br⁺ ions will give lines at 79 and 81

• There will also be a line for the unfragmented Br₂⁺ ion

• This will give 3 molecular ion peaks

  • Br₂⁺ ion containing the isotopes 79 + 79 = 158
  • Br₂⁺ containing the isotopes 79 + 81 = 160
  • Br₂⁺ containing the isotopes 81 + 81 = 162