Mass Spectrometry
Mass spectrometry can be used to identify molecules in a mixture based on their mass to charge ratio. Within a sample this can be measured to an accuracy of 0.01%, allowing for even minor changes in mass to be recognised. Mass spectrometry is based on exposing samples to 4 key events.
1) Ionisation
2) Acceleration
3) Deflection
4) Detection
The sample must first be ionised, to take on a positive charge. This is done by an ionisation chamber that knocks off one or more electrons to give a positive ion. The sample is then exposed to high acceleration so that all compounds in the mixture have the same kinetic energy when moving into the deflection chamber, this follows a curved shape with an electromagnetic field surrounding the tube. The amount of deflection a sample will undergo when travelling through the curve will be dependent upon two factors the mass of the compound and the charge.
Lighter compounds with a greater positive charge will be deflected the most, whilst heavier compounds with less charge will be deflected the least. These two factors combine to give the compound a mass to charge ratio. If too much or too little deflection occurs the compound will collide with the walls of the chamber and not flow through the curved tube. Compounds that collide with the walls take on electrons causing them to lose their charge and be neutralised. These will be removed from the chamber by a vacuum. The magnetic field of the deflection chamber can be altered to allow each compound to be detected. For compounds with a larger mass to charge ratio the magnetic field would need to be raised to increase deflection. With this the opposite for samples with a smaller mass to charge ratio. The compounds that make it through the deflection chamber will be met by a metal box. This box is surrounded by electrons and a wire for amplification. As the compound hits the box it will take on some of the electrons causing movement, this can be then be recorded as a signal. The output from the detection will be shown as a line graph with each bar representing an ion having a specific mass to charge ratio, the length of the line will correspond to the abundance of that ion.
