Mass spectrometry can be used to:
- determine the RAM of an element by measuring the ratio of isotopes present
- determine the molecular mass (Mr) of a compound as this will be the largest mass detected - the molecular ion peak
- find the structure of an unknown compound by identifying fragments of the original molecule
- advantages:
- only requires tiny amounts of a sample
- very fast process
How does a mass spectrometer work?:
- the sample is vaporized by heating in a vacuum, then ionised by bombarding it with high energy electrons from an electron gun which knock an electron off each atom in the sample to generate a positive ion
- sometimes more than one electron is knocked off, affecting the mass/charge ratio
- the ions are highly unstable, so are very reactive
- the inside of the mass spectrometer is a vacuum, so there is nothing for them to react with, but they may fragment
- fragmentation patterns are useful as they give characteristic masses that can allow us to build up a “jigsaw” of the parent molecule
- molecules usually fragment along the weakest bond (eg. tertiary more stable than secondary, secondary more stable than primary - due to the electron donating effect of the alkyl groups to stabilise the positive charge)
Some common fragment ions:
mass to charge ratio (m/z)
|
possible fragment ion
|
17
|
OH+
|
15
|
CH3+
|
29
|
CH2CH3+ or CHO+
|
31
|
CH2OH+ or OCH3+
|
43
|
CH3CO+
|
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