Mass Spectrometry (MS) Fragmentation Patterns (HL) (DP IB Chemistry): Revision Note

Mass spectrometry (MS) fragmentation patterns

• To analyse a sample in a mass spectrometer:

  • The molecules are vaporised

  • Then bombarded with a beam of high-speed electrons

• The high speed electrons knock an electron off some of the molecules, creating molecular ions (M+):

molecule molecular ion (M) + e^-

• The detected ions produce a mass spectrum

  • This is a molecular fingerprint used to identify compounds via a spectral database

• The peak with the highest m/z value is the molecular ion (M⁺) peak

  • This gives the molecular mass (M_r) of the compound

• In addition to the molecular ion peak, many compounds show a smaller [M+1] peak due to the presence of the carbon-13 isotope (¹³C)

The M+1 peak

• The [M+1] peak is a smaller peak next to the molecular ion peak

Mass spectrum of pent-1-ene

• The height of the [M+1] peak depends on the number of carbon atoms in the molecule:

  • More carbon atoms means a higher [M+1] peak

  • For example, hexane (6 carbon atoms) has a larger [M+1] peak than ethane (2 carbon atoms)

Fragmentation patterns

• The molecular ion peak gives the molecular mass (Mᵣ) of the compound

• However, different compounds can have the same molecular mass

• To confirm the structure, fragmentation analysis is used

• Fragments arise due to:

  • The formation of characteristic fragments

    • CH₃⁺ at m/z = 15

    • C₂H₅⁺ at m/z = 29

  • The loss of small molecules

    • H₂O at m/z = 18

    • CO at m/z = 28

    • CO₂ at m/z = 44

• The IB Chemistry data booklet (Section 22) contains a list of common neutral fragments lost:

Alkanes and alkyl cations

• In alkanes, fragmentation typically occurs by breaking C–C bonds

• The resulting fragments are alkyl cations with identifiable m/z values:

Fragmentation in a mass spectrum

Halogenoalkanes

• Halogenoalkanes often show multiple peaks around the molecular ion

  • This is caused by the presence of different isotopes of the halogen

Mass spectrum of a bromine-containing compound

• Bromine exists as two major isotopes: ⁷⁹Br and ⁸¹Br

  • These are present in roughly equal abundance

• So compounds containing bromine show two molecular ion peaks of similar height:

  • M (with ₇₉Br)

  • M + 2 (with ⁸¹Br)

Alcohols

• Alcohols often undergo dehydration

  • This produces a peak 18 units below the molecular ion

• The CH₂OH⁺ fragment is also commonly observed at m/z = 31

Fragmentation in propan-1-ol

• The following fragmentation patterns are commonly observed in propan-1-ol: