Mass Spectrometry
Chemistry: AQA A Level, topic 5.6.3
Mass Spectrometry
Mass spectrometry is an extremely precise technique. The theory is covered in the physical chemistry section, but here we will talk about reading spectra.
,w_800.png)
Identifying compounds
- The first step is to identify the molecular ion peak.
- This gives the mass of the compound.
- The molecular ion peak will have the highest value.
- E.g. The mass of ethanedioic acid (C2O4H2) is 90 gmol-1 and so the molecular ion peak will be at 90.
High resolution mass spectrometry
- High resolution mass spectrometry is so sensitive that it can be used to measure single atoms.
- This technique can also distinguish between molecules which have only a small difference in their molecular masses.
Butane vs propanal
- A good example of this difference is butane vs. propanal.
- Both butane and propanal have a molecular mass of 58 gmol-1.
- If you look more accurately, the actual masses are
58.078 gmol-1 for butane and
58.046 gmol-1 for propanal.
- This is because the relative masses of hydrogen and oxygen are very slightly different to the rough values of 1 and 16.
- A high resolution mass spectrometer can distinguish between these small differences, whereas a low resolution spectrometer cannot.
Butane vs propanal cont.
- If the high resolution spectrometer shows a mass of 58.078 gmol-1:
- We know the molecule must be butane and NOT propanal.
- And if the high resolution spectrometer shows a mass of 58.046 gmol-1:
- We know the molecule must be propanal and NOT butane.
Test yourself on Mass Spectrometry
Butane has a molecular mass of 58.078 gmol-1 and propanal has a molecular mass of 58.046 gmol-1.
What is the value of the molecular ion peak when propanal is added to a high resolution mass spectrometer?What would the value of the molecular ion peak be for C3H8O?