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Interpreting Mass Spectra (HL) (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

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 / z

  • 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 graph with two peaks at m/e 10 and 11. The m/e 10 peak shows 19.9% abundance and the m/e 11 peak shows 80.1% abundance.

 

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

Examiner Tips and Tricks

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

Revision notes on fragmentation patterns can be found here

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry Content Creator

    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.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener