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How does a mass spectrometer work?

A mass spectrometer is an important analytical instrument which scientists can use to identify the amount and type of different chemicals in a substance. In this explanation I’ll go through how the mass spectrometer works.

There are four stages in a mass spectrometer which we need to consider, these are – ionisation, acceleration, deflection, and detection. Let’s go through these in order.

Ionisation

The sample needs to be vapourised first, before being passed into the ionisation chamber. Here, an electrically heated metal coil gives off a stream of electrons. The atoms or molecules in the sample are bombarded by this stream of electrons, and in some cases, the collision will knock an electron from the particle, resulting in a positively charged ion. Most of the ions formed have a +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 (which is positively charged), and 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)

We can consider these properties as a mass/charge ratio (m/z), where the mass of the ion is divided by its positive charge.

Detection

By varying the strength of the magnetic field, the different ion streams (after deflection) can be focused on the ion detector, in order of increasing mass/charge ratio (as the lightest ions would need to be deflected the less). 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, these are sent to a computer for analysis.

A mass spectrum is generated, which shows the different m/z values of ions present, and their relative abundance

Summary

Now you know has a mass spectrometer works – just remember the four stages in order – ionisation, acceleration, deflection, and detection. You should make sure to study a diagram of a mass spectrometer – a frequent question can be asking you to sketch a diagram, or describe how a particular step works, before then going on to interpret a mass spectrum. 

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