501. How does a mass spectrometer work and why must it be evacuated before being used?
A mass spectrometer is a device that measures the masses of the atoms or molecules in a sample. There are many different types of mass spectrometers but they all work on roughly the same principle: they give each atom or molecule a single electric charge and look at how easy or hard it is to accelerate that atom or molecule by pushing on it with electric or magnetic fields. The more mass the atom or molecule has, the more slowly it will accelerate in response to a particular force. Some mass spectrometers use an electric field to push the atoms or molecules forward until they all have the same amount of kinetic energy and the more massive particles end up traveling more slowly than the less massive particles. Their masses can then be determined by timing how long it takes them to travel a certain distance or by sending them through a magnetic field that bends their flight paths. Because the force that a magnetic field exerts on a moving particle increases with that particle's speed, the paths of slow moving massive particles bend less than those of fast moving less massive particles. Since all of this mass analysis occurs while the particles are traveling through space, it's important that they not collide with any gas particles inside the mass spectrometer. That's why the mass spectrometer must be evacuated before use.