 For a sample of chlorine gas, we can estimate that 75% of the atoms would be chlorine 35 and the remaining 25% would be chlorine 37. These different forms of an element are known as isotopes. All isotopes of an element have the same number of protons but a slightly different number of neutrons. This concept of isotopes was actually discovered using an analytical technique known as mass spectrometry. For chemical research, the pharmaceutical industry and in forensic science, there are many techniques and tests to identify a synthesized compound or a sample obtained from a crime scene. One crucial technique is calculating the molecular mass of a compound or a sample. This can be done using a mass spectrometer. The beauty of mass spectrometry is that only a very small sample is required. This is important when only a small sample is made or if the evidence is only present in minute amounts. A sample must first be vaporized. Once this is done, the vaporized sample is then bombarded with a beam of electrons. This beam of electrons knocks off electrons from the molecules which make up the sample, creating positively charged ions. This process is known as ionization. The ions formed usually have a positive one or a positive two charge. These ions are then accelerated in an electric field before being deflected by a magnetic field. The pattern of deflection, according to charge and mass, can then be depicted on a spectrum. The main idea here is that ions with a large mass to charge ratio, meaning a large mass and a small charge, are deflected less than ions with a smaller mass to charge ratio. When this was done using a sample of neon, the resulting pattern of deflection showed that three isotopes of neon existed, neon 20, neon 21, and neon 22. The positions of the peaks on the spectrum will tell you the mass of those ions. Besides showing the existence of these isotopes, the isotopic abundance can also be determined from the mass spectrum. The height or the intensity of a given peak tells you the relative abundance. So when a crime scene investigator gives the lab even a small fleck of a white powder, its molecular mass can help identify what it is. In summary, mass spectrometry is an analytical technique used to determine the molecular mass of a compound and indirectly helped to prove the existence of isotopes. Thanks for watching!