High Resolution Mass Spectrometry (Edexcel International A Level Chemistry)

Revision Note

Richard

Author

Richard

Last updated

High Resolution Mass Spectrometry

  • At AS level, you have used mass spectrometry to determine:
    • Relative atomic mass from isotopic abundance
    • Possible molecular formulae from molecular ion peaks
    • Organic structures from fragmentation patterns
  • High resolution mass spectrometry can be used to deduce / confirm molecular formulae 
  • High resolution mass spectrometers provide data accurate to four, or sometimes more, decimal places
  • This means that a compounds molecular formula can sometimes be deduced by using atomic masses accurate to four decimal places

Table of selected accurate relative atomic masses

Element Hydrogen Carbon Nitrogen Oxygen
Symbol H C N O
Accurate Ar 1.0078 12.0000 14.0031 15.9949

Worked example

A compound is found to have an Mr value of 58.0417.

Determine if the compound is:

  1. C4H10 
  2. C2H6N2 
  3. C3H6O

   Answer   

Element Hydrogen Carbon Nitrogen Oxygen
Symbol H C N O
Accurate Ar 1.0078 12.0000 14.0031 15.9949

  1. C4H10 = (4 x 12.0000) + (10 x 1.0078) = 58.0780
  2. C2H6N2 = (2 x 12.0000) + (6 x 1.0078) + (2 x 14.0031) = 58.0530
  3. C3H6O =(3 x 12.0000) + (6 x 1.0078) + (15.9949) = 58.0417

  • Therefore, the correct formula of the compound is C3H6O

Examiner Tip

  • High resolution mass spectrometry can inform you about the molecular formula, it cannot tell you about the actual structure
    • In the worked example above, the final answer was C3H6O but you cannot say whether this is propanal or propanone
    • You would have to use the fragments on the mass spectrum for further detail that might help deduce more information about the structure
  • Sometimes you may calculate the molecular mass of a compound and it won't match the value given in the question exactly
    • In this case, you should:
      • Firstly, double check your calculation(s)
      • Secondly, choose the compound that is the closest 

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

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.