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First exams 2025

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Interpreting Mass Spectra (HL) (HL IB Chemistry)

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Mass Spectra

  • The percentage abundance of the isotopes in an element can be found by the use of a mass spectrometer
  • The basic processes of mass spectrometry are:
    • The sample is vapourised
    • The sample is ionised to form positive ions
    • The ions are accelerated
    • Each ion produces a signal which is detected as a mass-to-charge ratio, written as m / e

Examiner Tip

Specific details of the processes involved in mass spectrometry are not assessed.

  • The mass spectra produced can be used to calculate the relative atomic mass of an element and its isotopes:

Worked example

Calculate the relative atomic mass of boron using its mass spectrum, to 2 dp:Mass spectrum showing the percentage abundance of two boron isotopes

 

Answer:

  • Total mass of 100 atoms = (19.9 x 10) + (80.1 x 11) = 1080.1
  • Mass of 1 atom = begin mathsize 14px style fraction numerator 1080.1 over denominator 100 end fraction end style= 10.801
  • So, the relative atomic mass of boron, rounded to 2 decimal places, is 10.80 

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    Richard

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