Mass Spectrometer

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Mass Spectrometer
The mass spectrometer measures the masses of atoms and molecules, it can also be used to measure
the relative abundance of different isotopes and to predict complex molecular structure.
How it works: VIADD
1.Vaporisation: Sample is in vapour form and is released into ionization chamber.
2.Ionisation: Particles are bombarded by an electron gun, which knocks off an electron off each
atom, (if set too strong, if may knock more off), producing positive ions.
3.Acceleration: Positive ions attracted to negatively charged metal plate and accelerate towards it
via electric field.
4.Deflection: Ions deflected by magnetic field, depends on m/z ratio, the heavier the particle, the
less deflection.
5.Detection: Electric current measured as ions land on plate, the greater abundance of an isotope,
the larger the current.
100
80
relative 60
abundance
40
20
18 20 22 24 26
M/Z
Calculations using Mass Spectrometer
The relative atomic mass can be calculated using this formula:
(percentage abundance of each isotope x mass of each isotope)
100
Using the Ne diagram above: RAM = (90 x 20 + 10 x 22)/100 = 20.2
Sometimes, the molecules can't cope with the extreme conditions in the mass spectrometer and they
can break apart, this is called fragmentation.
The relative molecular mass of a molecule is obtained by looking at the peak in the spectrum with
the largest m/z ratio (ie the peak furthest to the right). No calculations are required.

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