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The Mass Spectrometer

Five Stages

Vapourisation - A sample is injected into the unit. There is a very good vacuum inside which turns liquids into gases.

Ionisation - High energy electrons are fired at the atoms or molecules of the sample gas. Electrons are knocked off these particles of the sample gas and positive ions are produced. This is done using an electron gun

Acceleration - The positive ions produced pass between a series of negatively charged plates. These plates attract the positive ions and accelerate them to high speeds

Separation - The positive ions are made to move in curved paths using a magnetic field. The lighter particles are easier to deflect. The heavier particles are more difficult to deflect. Hence, the lighter particles are pushed further off course than the heavier ones.

Detection - The detector is an instrument that responds to the number of ions hitting it. A mass spectrum is produced by a recorder paper. It is a series of peaks.

Uses of a Mass Spectrometer

To identify the presence of isotopes

To measure the relative abundances of isotopes

To measure relative atomic mass and relative molecular mass

To identify unknown compounds e.g gases from waste dumps.

Principle on which this technique is based:

Charged particles moving in a magnetic field experience a force and are deflected and therefore separated according to their mass

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