A mass spectrometer is a device that separates things and compares them by mass. To understand how this works you need to know that charged particles are pushed by a magnetic field – that is to say that the path of a charged particle will bend if it passes a magnet. How much the path bends depends on how fast it is moving, how big the charge is and how heavy the particle is.
In a mass spectrometer, a sample (for our purpose, lets assume that it is atoms of an element we wish to study) is heated, vaporized and ionized (an electron is removed, making the atoms into positively charges ions).
The sample is then subjected to an electric field. This means that it is placed between two plates – one charged positively and the other negatively. The positive ions will move away from the positive plate and toward the negative plate.
However, the negative plate has holes in it, so that some of the ions go through
On the other side of the plate the ions, now moving quite fast, pass through a magnetic field which bends their path. Assuming that all of the atoms have the same charge (not a perfect assumption, but pretty good) how much they bend will depend on their mass.
By comparing how much the paths of the particles are bent (actually looking at where they hit a detector) we can gauge relative mass (twice as heavy, 1.735 times lighter, etc). In addition, by counting the number of particles of each mass in a sample, we can determine the relative abundance of each isotope (what percent of an element is made of each different isotope).
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