摘要： The ionization of analyte molecules is of vital importance to mass spectrometric analysis because mass spectrometry takes only ions as the object for analysis. The most commonly used ionization source in mass spectrometry is electron-impact (EI) ionization source with electrons impacting to analyte with an energy of 70 eV. The kinetic energy of 70 eV is much higher than the ionization energy of organic molecules, and the ionization process approaches the maximal ionization efficiency. Thus, a large number of fragment peaks are usually observed in an EI mass spectrum. These fragments act as fingerprints of molecules, which can be used for identification of unknown molecules by searching EI-MS library, for example NIST Mass Spectral Library. This statistically based identification is particularly convenient and effective when analyzing a single component, for example a preseparated analyte by GC. However, there are many cases that analytes cannot be preseparated, for example very active components in a reaction, or fast on-line observation of evolving analytes. Consequently, the extensive fragmentation by EI-MS turns to a severe disadvantage: if each compound of a mixture produces a certain amount of fragment peaks, serious interference can be made, which will bring muscular obstacles to peak assignment. The best strategy is to use a “soft” ionization source with high selectivity. Here, “soft” means fragment-free or fragment-controllable.