2.8 Mass Spectroscopy

Mass Spectroscopy

Definitions:

Diagram of mass spectrometerœ

Vaporisation: The samples of particles are heated until they are a gas state.

Ionisation: Particles are bombarded with a stream of electrons, electrons are knocked out of sample particles.

Acceleration: An electric field accelerated the positive ions at high speed.

Deflection: Different ions are deflected by the magnetic field depending on mass/charge

Detection: The beam of ions are passed through the machine, only ions with 1+ charge will make it through the machine.

Uses of mass spectral data:

  • Identifying molecular mass of unknown compounds.

  • Gain info about structure of chemical properties.

Summary:

  • Positive ions are formed when atoms or molecules are bombarded with electrons, knocking out an electron to form M⁺.

  • Ions are separated based on their mass-to-charge ratio in the magnetic field, with lighter or more highly charged ions deflected more.

  • The mass spectrum generated shows the m/z ratio and relative abundance of the ions, providing information about the sample's composition.

Mass Spectroscopy of Organic Compounds:

Mass spectrometry of organic compounds provides the Mr from the molecular ion peak.

M → [M]+ + e-

Fragmentation: Molecular ions fragments form due to covalent bonds breaking:

[M]+ → X+ + Y•

Fragmentation Patterns:

  • The pattern of smaller peaks corresponds to fragment ions.

  • These fragments can provide information about the structure of the molecule.

  • The more stable the ion, the greater the peak intensity.

Example of ethanol:

Mass Spectroscopy graph of ethanol.

Interpreting the Mass Spectrum

  1. Molecular Ion Peak (M⁺):

    • The peak with the highest m/z value (mass/charge) represents the molecular ion, giving the relative molecular mass (Mr) of the compound.

  2. Base Peak:

    • The tallest peak in the spectrum is the base peak. It corresponds to the most abundant ion and is often a fragment ion. -(broken up from whole molecule)

  3. Isotopic Peaks:

    • Peaks with slightly higher m/z values than the molecular ion arise due to isotopes (e.g., 13C^{13}C13C or 2H^2H2H). (not needed for exam).

Ethanol (C2H5OHC_2H_5OHC2​H5​OH)

  • Molecular ion peak (M+) at m/z = 46, representing the molecular ion [CH3CH2OH]+

  • Fragmentation may produce peaks such as:

    • [CH3]+​ at m/z = 15

    • [CH3CH2]+​ at m/z = 29

    • [CH2OH]+ at m/z = 31

    • [CH3CH2O]+ at m/z = 45