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How do mass spectrometers work?

A mass spectrometer is a piece of lab equipment used to measure the relative atomic masses of atoms and molecules. It is widely used to identify substances in the lab.

The process is often automated and requires a number of sequential steps that take places in a specially built vacuum chamber. 

1. The sample is injected either as a gas or vapour

2. An electron gun creates a beam of electrons which collide with the sample atoms and, by removing electrons, ionise them. This creates cations which generally have a 1+ charge; about 5% gain a 2+ charge. Molecules often fragment producing a range of molecules with different masses.

3. A series of negatively charged plates with a small gap between accelerate the cations. This creates a beam of cations in which the lighter cations move faster than the heavier ones.

4. A magnetic field applied perpendicular to the direction of travel causes the beam to be deflected. The amount of deflection is dependent on the charge-to-mass ratio (m/z); heavier ions are deflected less than lighter ones. In this way the sample beam is separated by mass. 

5. The magnetic field is gradually adjusted so that ions of increasing mass strike a detector one after another. As the cations strike they remove electrons from the detector, this results in an electric current proportional to the abundance of the specific ion. 

This data is fed into a computer which creates a mass spectrum; a graph of m/z against relative abundance. By comparing the peaks to known sample graphs the identity of the original sample can be ascertained. 

George B. A Level Chemistry tutor, GCSE Science tutor, GCSE Biology t...

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