Mass Spectrometry (Oxford AQA International A Level Chemistry)

Mass Spectrometry

• Mass spectrometry (MS) is an analytical technique used to identify unknown compounds

• When a molecule is analysed in a mass spectrometer, the vaporised sample is ionised which creates molecular ions, MOLECULE⁺•:

MOLECULE MOLECULE⁺• + e^–

• The peak with the highest mass to charge ratio or m/z value is the molecular ion (M⁺) peak

• This gives information about the molecular mass of the compound

  • The m/z value of the molecular ion peak is the relative molecular mass of the compound

The M+1 peak

• The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13

• The height of the [M+1] peak for a particular ion depends on the number of atoms in the molecule

  • More carbon atoms = a larger [M+1] peak

  • For example, the [M+1] peak for a hexane ion will be higher / bigger than the [M+1] peak for an ethane ion

Fragmentation

• The molecular ion can further fragment to form new ions, molecules, and radicals

Fragmentation of a molecule in mass spectroscopy

• The relative abundances of the detected ions form a mass spectrum

Mass spectrum of pent-1-ene

• The fragments in a mass spectrum are like a molecular fingerprint that can be used by a computer to identify the compound from a database

• Fragments appear due to the formation of characteristic fragments or the loss of small molecules

Table of common fragments and small molecules

Low resolution mass spectrometry

• Low resolution mass spectrometry typically reports mass to 2 decimal places

  • This uses more accurate values for atomic mass than are shown on the periodic table

  • For example:

    • Hydrogen = 1.00

    • Carbon = 12.00

    • Oxygen = 16.00

• Low resolution mass spectrometry is suitable for general analysis or identifying simple compounds

• However, it does not work for molecules with similar molecular masses

High resolution mass spectrometry

• High resolution mass spectrometry typically reports mass to 4 or 5 decimal places

  • The values for atomic mass are far more precise than the periodic table

  • For example:

    • Hydrogen = 1.00783

    • Carbon = 12.0000

    • Oxygen = 15.99491

• Therefore, a more precise molecular mass can be calculated

• This means that it is possible to:

  • Determine the molecular formula of a compound

  • Distinguish between molecules with similar molecular masses

• However, it does not work for isomers because they have an identical molecular mass

Comparing low and high resolution mass spectrometry