Charge-Exchange Chemical Ionization

Charge exchange (CE) or charge transfer ionization occurs when an ion-neutral reaction takes place in which the ionic charge is transferred to the neutral. [8] In principle, any of the reagent systems discussed so far is capable to effect CE because the respective reagent molecular ions X are also present in the plasma  

However, other processes, in particular proton transfer, are prevailing with methane, isobutane, and ammonia, for example. Reagent gases suitable for CE should exhibit abundant molecular ions even under the conditions of Cl, whereas potentially protonating species have to be absent or at least of minor abundance. 

Note The acronym CE is also used for capillary electrophoresis, a separation method. CE may be coupled to a mass spectrometer via an electrospray interface (Chaps. 11, 12), and thus CE-CI and CE-ESI-MS must not be confused. 

ConceptnaUy, charge-exchange (CE) ionization is similar to the normal mode of Cl, bnt with the difference that the odd-electron molecnlar ions are the product of CE ionization [5]. As with Cl, the reagent gas is first ionized via El to prodnce reagent gas ions, G+, which in tnm abstract an electron from a neutral analyte molecnle to convert it into a radical cation  

The internal energy, E, of the molecular ion is given by the difference between the recombination energy (RE) of G+ and the IE of the reagent gas molecules  

The RE of G+ is usually approximated to the negative value of the vertical IE of the neutral reagent gas molecules with the stipulation that G+ is formed in the ground electronic state. 

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Simonsick WJJ, Hites RA. 1985. Charge exchange chemical ionization mass spectrometry A tool for PAH isomer differentiation. In Cooke M, Dennis AJ, eds. Polynuclear aromatic hydrocarbons Mechanisms, methods and metabolism. Columbus, OH Battelle Press, 1227-1237. 

Insertion of impurities Superconductivity of bombarded metals Variations of thin film properties Wear, friction and lubrication of materials Wear, friction and lubrication of materials Chemical state of implanted atoms Reactivity of ion-bombarded surfaces Reactivity of ion-bombarded surfaces Ionization phenomena Charge exchange studies... 

Chemical ionization (Cl) is one of the most versatile ionization techniqnes as it relies on chemical reactions in the gas phase [6]. Cl is an important ionization techniqne in LC-MS. It is based on ion-molecule reactions between reagent-gas ions and the analyte molecules. Gas-phase ion-molecule reactions comprise proton transfer, charge exchange, electrophilic addition, and anion abstraction in positive-ion Cl and proton transfer (abstraction) in negative-ion Cl. Cl can be performed nndervarions pressnre conditions ... 

In an APCI interface the column effluent enters a heated nebulizer where the pneumatically assisted desolvation process is almost completed. While still in the spray chamber, ionization of analytes is initiated by corona discharge. The ionization mechanisms in APCI are almost identical to those in conventional medium pressure chemical ionization (7). Positive ion formation can be achieved by proton transfer, adduct formation or charge exchange reactions, while in the negative mode ions are formed due to proton abstraction, anion attachment and electron capture reactions. The APCI interface is compatible with flow rates exceeding 1 ml/min and will... 

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