Chemical ionization

Chemical ionization (CI) is a soft ionization technique used in mass spectrometry. This was first introduced by Burnaby Munson and Frank H. Field in 1966. This technique is a branch of gaseous ion-molecule chemistry. Reagent gas molecules are ionized by electron ionization, which subsequently react with analyte molecules in the gas phase in order to achieve ionization. Negative chemical ionization (NCI), charge-exchange chemical ionization and atmospheric-pressure chemical ionization (APCI) are some of the common variations of this technique. CI has several important applications in identification, structure elucidation and quantitation of organic compounds. Beside the applications in analytical chemistry, the usefulness in chemical ionization extends toward biochemical, biological and medicinal fields as well. Chemical ionization requires a lower amount of energy compared to electron ionization (EI), but this depends on the reactant material used. This low-energy ionization mechanism yields less or sometimes no fragmentation, and usually a simpler spectrum. The lack of fragmentation limits the amount of structural information that can be determined about the ionized species. However, a typical CI spectrum has an easily identifiable protonated molecular ion peak +, which allows easy determination of molecular mass. This technique requires the transfer of high-mass entities from the reagent gas to the analyte, and therefore, the Franck-Condon principle does not govern the process of ionization. CI is thus quite useful in cases where the energy of the bombarding electrons in EI is high, resulting exclusively in fragmentation of the analyte, causing the molecular-ion peak to be less detectable or completely absent. CI uses a common source for ionization as EI, with some modifications. To facilitate the reactions between the ions-gases, the chamber is kept gastight with a pressure around 1 torr. Electrons are produced through a metal filament, which is made of tungsten, rhenium, or iridium and travel to a longer distance in the ionization chamber, due to high energy it owns. In contrast to EI, the magnet and the electron trap is not needed for CI, since the electron beam do not travel to the end of the chamber. The pressure inside the chamber is kept below 10−4 torr. A CI experiment involves the use of gas phase acid-base reactions in the chamber. Ions are produced through the collision of the analyte with ions of a reagent gas that are present in the ion source. Some common reagent gases include: methane, ammonia, water and isobutane. Inside the ion source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source with energy around 200-500 eV will preferentially ionize the reagent gas. Then, the ion/molecule reactions produces more stable reagent ions and the resultant collisions with other reagent gas molecules will create an ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma.