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Interpreting Mass Spectra (HL) (DP IB Chemistry)

Revision Note

Richard Boole

Last updated

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 Tips and Tricks

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 Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.