How mass spectrometers work, to identify molecules and mixtures of compounds
The Mass Spectrometer has become an indispensable tool of many types of chemists over the course of the twentieth century, and will continue to be important into the twenty-first. This piece of equipment is used to identify chemical compounds by breaking them up into smaller charged particles and then detecting them. The first apparatus to be called a mass spectrometer was built by Francis Ashton in Cambridge, UK in 1918.
A Simple Method
The actual method is fairly simple, although the practical construction of the equipment is 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 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 under analysis. This produced a positively charged ion. The resulting ion also has some energy left over 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 can measure 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 the molecular mass of the molecule can be determined from that line. Now detective work must be dome 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.
Applications
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.
