Using Mass Spectra
- 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:
- C4H10
- C2H6N2
- C3H6O
Answer
| Element | Hydrogen | Carbon | Nitrogen | Oxygen |
| Symbol | H | C | N | O |
| Accurate Ar | 1.0078 | 12.0000 | 14.0031 | 15.9949 |
- C4H10 = (4 x 12.0000) + (10 x 1.0078) = 58.0780
- C2H6N2 = (2 x 12.0000) + (6 x 1.0078) + (2 x 14.0031) = 58.0530
- 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
- In this case, you should:
