Chemical ionization

[2] Reagent gas molecules (often methane or ammonia)[4] are ionized by electron ionization to form reagent ions, which subsequently react with analyte molecules in the gas phase to create analyte ions for analysis by mass spectrometry.

CI mass spectrometry finds general application in the identification, structure elucidation and quantitation of organic compounds[5] as well as some utility in biochemical analysis.

[6] CI is thus useful as an alternative technique in cases where EI produces excessive fragmentation of the analyte, causing the molecular-ion peak to be weak or completely absent.

[7] Electrons are produced externally to the source volume (at a lower pressure of 10−4 torr[7] or below) by heating a metal filament which is made of tungsten, rhenium, or iridium.

[5] The electrons are introduced through a small aperture in the source wall at energies 200–1000 eV[7][8] so that they penetrate to at least the centre of the box.

Electrons entering the source will mainly ionize the reagent gas because it is in large excess compared to the analyte.

[10] One of the main advantages of CI over EI is the reduced fragmentation as noted above, which for more fragile molecules, results in a peak in the mass spectrum indicative of the molecular weight of the analyte.

Because not all analytes can do this, using NCI provides a certain degree of selectivity that is not available with other, more universal ionization techniques (EI, PCI).

In cases where very low limits of detection are needed, environmental toxic substances such as halogenated species, oxidizing and alkylating agents[12] are frequently analyzed using an electron capture detector coupled to a gas chromatograph.

[12] Compared to the other methods involving negative ion techniques, NCI is quite advantageous, as the reactivity of anions can be monitored in the absence of a solvent.

[12] This is also similar to CI and the difference lies in the production of a radical cation with an odd number of electrons.

An APCI source is composed of a liquid chromatography outlet, nebulizing the eluent, a heated vaporizer tube, a corona discharge needle and a pinhole entrance to 10−3 torr vacuum.

[11] The analyte is a gas or liquid spray and ionization is accomplished using an atmospheric pressure corona discharge.