Mass Spectrometry

What is a mass spectrometer?

• important analytical instrument which scientists use to identify the mass of elements, ions, isotopes or molecules, and their relative abundances

• 4 stages

  • ionisation

  • acceleration

  • deflection

  • detection

Ionisation

• sample needs to be vaporised first then it is passed into the ionisation chamber where an electrically heated metal coil gives off a stream of electrons

• the atoms/molecules in the sample are bombarded by this stream of electrons and will sometimes knock an electron from the particle, resulting in a positively charged ion

• most ions formed will be +1 charge, as it is difficult to remove a second electron from an already positive ion

Acceleration

• the positively charged ions are repelled from the ionisation chamber

• then pass through negatively charged slits which focus and accelerate this into a beam

Deflection

• the stream of positively charged ions are then deflected by a magnetic field

• the amount ions are deflected by depends on:

  • the mass of the ion - lighter ions will be deflected more than heavier ones

  • the charge of the ion - ions with a greater charge than +1 are deflected more

• considered as mass/charge ratio (m/z)

Detection

• by varying the strength of the magnetic field, the different ion streams can be focused on the ion detector, in order of increasing mass/charge ratio

• when an ion hits the detector, the charge is neutralised and this generates an electrical current

• this current is proportional to the abundance of the ion and sent to computer for analysis

• shows the different m/z values of ions present and their relative abundance