High Resolution Mass Spectrometry (Edexcel IAL Chemistry)

Spec point — 15.17

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

Worked Example A compound is found to have an M_r 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

Exam Hint

• 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