Mass spectrometry only works with ions, not with neutral molecules. That means a neutral molecules must become charged in order to do this experiment. It is common to generate a cation from the molecule by removing one electron. The electron is knocked off the molecule in a collision. The collision can be caused in two different ways: The molecule can be sent through a stream of high-energy electrons. This method is called electron ionization. The molecule is sent through a stream of small molecules, such as ammonia or methane. This method is called chemical ionization. Electron ionization frequently results in the molecule falling to pieces because of the high energy of the electrons. Chemical ionization results in a "softer" collision because momentum can be dissipated through various bonds in both colliding molecules. Chemical ionization results in less fragmentation of the target molecule. However, after chemical ionization, the ionizing molecule sometimes sticks to the target molecule, leading to a greater "molecular" mass. For example, if ammonia is used for ionization, an extra mass may be observed at 17 amu higher than expected. The reason the x-axis on a mass spectrum is labeled m/z (mass-to-charge ratio) is to acknowledge that there are really two factors contributing to the experiment. Contributor Chris P Schaller, Ph.D. , (College of Saint Benedict / Saint John's University)