Basic Principle of Mass Spectrometry

“The basic principle of mass spectrometry (MS) is to generate ions from either inorganic or organic compounds by any suitable method, to separate these ions by their mass-to-charge ratio (m/z) and to detect them qualitatively and quantitatively by their respective m/z and abundance. The analyte may be ionized thermally, by electric fields or by impacting energetic electrons, ions or photons. The … ions can be single ionized atoms, clusters, molecules or their fragments or associates. Ion separation is effected by static or dynamic electric or magnetic fields.” Although this definition of mass spectrometry dates back to 1968 (Kienitz, 1968), it is still valid. One should add that ion separation by m/z can also be effected in field-free regions, provided the ions possess a well-defined kinetic energy at the entrance of the flight path.

It follows directly from this definition that atoms or molecules need to carry an electric charge, i.e., they need to be transformed into ions, for MS to work. The electric charge acts like a handle that allows to grab these atoms or molecules. In contrast to neutrals, ions can be accelerated and decelerated, can be shot into defined orbits or other flight paths, and can finally be collected and detected. The “race tracks” of these ions can be determined by application of electric and/or magnetic fields. While the Coulombic force is exerted on ions in electric fields, the Lorentz force influences ions moving with a component orthogonal to the magnetic field.