Using Mass Spectrometer is a Simple Method

Using a mass spectrometer is actually simple, although the practical construction of the equipment is far more complex. First a mass spectrum is produced then it is interpreted. The spectrum is produced by a three stage process.

Ionisation

First the chemical that needs to be analysed must be ionised. This is commonly done by bombarding a small sample of it with high-energy electrons from a heated filament. These electrons collide with the chemical compound and can knock off one of the electrons in the molecule that is under analysis. This produces a positively charged ion. The resulting ion also has some energy that is left over and this can cause a rearrangement within the molecule and further fragmentation. The resulting charged particles of differing size and mass are then ready for the next stage of the process.

Sorting of Fragments

The ions are accelerated by repelling them from a negatively charged plate, through small slits in other electrodes, forming a beam of particles. This beam of particles is then passed between the poles of an electromagnet. This causes the particles to be deflected away from their course. Particles with different masses are deflected by different amounts, so the particles can are sorted according to their masses.

Detection

The final stage is the detection of the particles. This is done by steadily varying the strength of the electromagnet so that a fixed detector measures the relative quantities of particles of different masses. The result is a series of lines on a graph representing the relative abundances of ions of different masses.

Analysis

This spectrum must now be analysed. The line with the highest mass is usually the original molecule, so that the molecular mass of the molecule can be determined from that line. Now detective work must be done by looking at the pattern of other peaks, representing fragments of the original molecule. Different types of compound will give different fragmentation patterns so these can be used to work out possible structures of the original molecule.

Mass spectrometry is often used in organic chemistry to work out the structure of complicated compounds. It is also often used with gas chromatography to analyse mixtures of compounds. The chromatograph separates out the mixture and the mass spectrometer analyses them one at a time.