What is mass spectrometry and how does it work?

In Chemistry we need to know the structures of the molecules we make and work with. We use spectroscopy to work out the shapes of these molecules. Mass spectrometry is one type of spectroscopy and weighs the molecule to tell us the mass. Very accurate mass spectrometry can also be used to work out the atomic composition e.g. to tell us how many carbon, oxygen, nitrogen atoms we have. It also can tell us about the compositions of different isotopes of an element to allow us to calculate the relative atomic mass.

The technique works by vaporising and ionising the sample to give positively charged gaseous ions. There are many different ionisation techniques, two examples are electron impact (the molecule is bombarded with high energy electrons that knock an electron out of the molecule – like throwing bricks at a wall!) and electrospray ionisation (the sample is sprayed through a tiny needle and a proton (H⁺) added from the solvent using a voltage supply). Inside the instrument is a vacuum (this means it’s really empty – there are no air particles to collide with your sample and destroy the measurement) and the positively charged ions fly inside the chamber, deflected by electromagnetic coils (like magnets). The particles then hit the detector in the instrument to give a signal. Positively charged ions hit a negatively charged electric plate and pick up electrons from the plate, this movement of electrons is detected as a current. There are many different types of mass spectrometry. Time of flight (Tof) is a common technique where the particles are all accelerated so that they have the same kinetic energy (KE = ½ mv²). The time taken to travel a fixed distance is then measured to calculate the mass of each ion (lighter particles have a higher velocity so take less time to travel the same distance than heavier particles). The data from the spectrometer is fed into a computer to produce a mass spectrum for us to interpret. The spectrum shows the mass to charge ratio (m/z) (effectively the mass as usually all +1 charge) and relative abundance of each ion that was detected. The signal with the greatest m/z value usually gives the mass of the molecule. Other peaks with smaller mass are observed for fragments of the molecule where the molecule breaks up on impact during ionisation. The relative atomic mass of an element can also be calculated by reading the abundance and mass of the different isotopes from the mass spectrum.