Ionization and Interfaces

Analytes are introduced into mass spectrometers in gas, liquid, or solid states. In the latter two cases, volatilization must be accomplished either prior to or accompanying ionization. Many ionization techniques are available to produce charged species from analytes the most common ones are electron ionization (electron impact ionization), chemical ionization, matrix-assisted laser desoprtion ionization and atmospheric-pressure ionization (electrospray, atmospheric-pressure chemical ionization, and atmospheric-pressure photoionization). Electron ionization utilizes accelerated electrons (70 eV) colliding with gaseous analyte 

Atmospheric-pressure chemical ionization (APCI), which was introduced in the early 1970s, is an ionization technique in which analytes are ionized through gas-phase reactions between analytes and primary reactant 

Atmospheric-pressure photoionization (APPI), which was first applied as an LC-MS interface in 2000, is a relatively new ionization technique. The ionization process is initiated by an ultraviolet lamp (krypton discharge lamp), which emits 10.0 and 10.6 eV photons. Any compounds that have ionization energies below 10 or 

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For a number of years (1987-1992), thermospray LC-MS was the most frequently applied interface for LC-MS. It has demonstrated its applicability in both qualitative and quantitative analysis in various application areas. With the advent of the more robust LC-MS interfaces, based on atmospheric-pressure ionization, the use of thermospray interfacing and ionization rapidly decreased. The newer technology pointed out the limitations of the thermospray system, e.g. in the analysis of thermolabile compounds, ionic compounds, high molecular-mass compounds, as well as in robustness and user-friendliness. Therefore, thermospray as an ionization and interface technique for LC-MS is now history. Thermospray nebulization will continue to be used, e.g. in nebulization for ICP-MS. 

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