The M+1 & M+2 Peaks (Cambridge (CIE) A Level Chemistry): Revision Note

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:

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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

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