In short
Mass spectrometry is a method that separates ions by their mass-to-charge ratio and measures how much of each there is. It finds the isotopes in an element and the mass of a molecule.
In a mass spectrometer, a sample is ionised, the ions are accelerated and then bent by a magnetic field. Lighter ions bend more. A detector records the abundance of each. For chlorine, the spectrum shows two peaks, at mass and mass , in the ratio about .
Chlorine is chlorine-35 and chlorine-37. The relative atomic mass is .
Also searched as: mass spec of chlorine.