Gas chromatography–mass spectrometry
The most common type of ionization is electron ionization (EI).  The molecules enter into the MS (the source is a quadrupole or the ion trap itself in an ion trap MS) where they are hit with free electrons emitted from a filament. This is like the filament one would find in a standard incandescent light bulb.  The electrons hit the molecules, causing the molecule to fragment in a characteristic way that can be repeated.  This "hard ionization" technique results in the creation of more fragments of low mass to charge ratio (m/z). EI has few, if any, fragments having a mass that is near the mass of the original molecule.  Chemists consider hard ionization to be shooting electrons into the sample molecules.  In contrast, "soft ionization" is placing a charge on the sample molecule by hitting it with an introduced gas.  The molecular fragmentation pattern depends on the electron energy applied to the system, typically 70 eV (electron Volts).  The use of 70 eV helps to compare the spectra generated from the test sample against known library spectra. (The library spectra can come from manufacturer-supplied software or software developed by the National Institute of Standards (NIST-USA)). The software searches the library spectra using a matching algorithm such as Probability Based Matching or dot-product matching.  Many method standardization agencies now control these algorithms and methods to assure their objectivity.