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Mass Spectrometry (MS) - How it Works
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A brief document explaining the Mass Spectrometry process.
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Definitions of terms Molecular Mass – a relative measure which should only be used in reference to things which can exist
as molecules.
The weighted average of the masses of the molecules relative to 1/12 of the mass of a carbon-12
atom
Relative Formula Mass – Can be applied to any compound, regardless of bonding.
The weighted average of the masses of the formula units relative to 1/12 of the mass of a carbon-12
atom
Number of Particles – Avogadro’s constant / moles
Mass of a Single Isotope – neutron number / (1000*Avogadro’s)
Mass Spectrometry – THE BASIC PRINCIPLE
If something moving is subjected to a sideways force, it will no longer travel in a straight line – instead it will curve off
to the side as it is deflected. The amount of this deflection depends on the weight of the moving object – a large
object will be less easily deflected than a smaller and lighter one.
Atoms and molecules can be deflected by magnetic fields if they are first turned into an ion.
1) STAGE 1 – IONISATION
The atom or molecule is ionised, usually into a positive ion, by knocking off 1 or more electrons – usually
positive, even for atoms which normally would form negative ions or don’t form ions at all. Most often they
are ionised by being bombarded by a stream of electrons.
2) STAGE 2 – ACCELERATION
The ions are accelerated so they all have the same kinetic energy – the + ions are repelled away from a
positive ionisation chamber and accelerated into a finely focused beam.
3) STAGE 3 – DEFLECTION
The ions are deflected by the magnetic field, their masses impacting the size of their deflection. The number
of positive charges on the ion also impacts deflection – the stronger the charge, the more it is deflected.
These combine into the mass / charge ratio, or M/Z (M/E).
4) STAGE 4 – DETECTION
The beam of ions passing through is detected electronically – they hit a metal box, losing their charge as the
metal plate donates an e- - the flow of e- is then recorded.
E.g. if an ion had a mass of 28 and a charge of +2, its m/z ratio would be 14, as = 14. The larger ions
which are less deflected have the largest m/z ratio; the smaller ions which are more deflected have the
smallest.
Also known as ESI, this is the most popular technique.
Electrospray A high voltage is put on a flow of liquid at atm pressure
Ionisation The spray is directed to an opening into the vacuum system, and is
desolvated (solvent is removed) into gas, and then ions.
Also known as EI
Electron Impact Gas phase molecules being analysed are bombarded by a steream of e -
Ionisation This causes the gas molecules to form an ion - however this can fragment
under the stress, making this teqhnicue less ideal.
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