Interface multiple charge

The research efforts in the late 1960 s of the group of Dole (41-43) on electrospray sample introduction found continuation in the work of the group of Perm (86-87) in 1984 and later. An LC-MS interface based on ESI introduction into an atmospheric-pressure ion (API) source was described by Whitehouse et al. [88] in 1985. The flow-rate limitations of the latter system were to some extent removed by the introduction of a pneumatically-assisted ESI interface (ionspray ) for LC-MS by Bruins et al. (89) in 1987. This system was developed for a Seiex API instrument which in those days was the only commercially available instmment equipped with an API source. A major breaktlnough in ESI, and as a result of this also in the commercial availability of API instruments, was aehieved in 1988 by the observation of multiple-charge ions from peptides and proteins [90-91]. This made the ESI interface to one of the most popular and powerful methods for LC-MS. The development of API interfacing for LC-MS is discussed in detail in Ch. 5. 

Coupling mass spectrometry (MS) to capillary electrophoresis provides detection and identihcation of amino acids, peptides, and proteins based on the accurate determination of their molecular masses [15]. The most critical part of coupling MS to CE is the interface technique employed to transfer the sample components from the CE capillary column into the vacuum of the MS. Electrospray ionization (ESI) is the dominant interface which allows a direct coupling under atmospheric pressure conditions. Another distinguishing features of this soft ionization technique when applied to the analysis of peptides and proteins is the generation of a series of multiple charged, intact ions. 

IP-RPLC with TrBA is suitable for the LC-MS analysis of the degradation products of dyes. " However, the alkylamines used for ion-pairing need to be volatile, which limits the number of carbons they can bear. Contrary to the tetraalkylammonium counter-ions, di- and tri-alkylamines do not tend to form adducts in modern API interfaces. Instead, they can act as H donors in their ammonium form and may then influence the ionization process of sulfonates in by decreasing the sensitivity of detection. " " In the same way, these amines diminish the risk of sodium adduct formation and of multiple charging. Due to their suppression effect, the concentration of alkylamines should be reduced to the lowest level acceptable for chromatographic retention. 

The FAIMS analyzer was interfaced to a PE Sciex API 300 triple-quadrupole MS. Ions were filtered through the FAIMS and introduced into the MS through a sampler cone that was placed at the end of the FAIMS analyzer at a 45° angle relative to the axis of the FAIMS cylinders. Using this instrument, the high-field mobility of conformer ions, ions of multiple charge states, and complex ions of leucine enkephalin were investigated. ... 

Mass discrimination, changes in the cone potential in the atmospheric interface, and accommodation of multiple charges by large molecules affect the reliabihty of measured molecular weights and molecular weight distributions especially for polydispersed polymers. Loss of resolution and raise in baseline may arise with the increase in number of possible types of ionic clusters for charges +4 and higher. 

An alternative, and a more elegant approach to analyzing large polymers is to couple a GPC via an electrospray ionization interface to the FT/MS. Each oligomeric fraction that elutes from the GPC would be measured directly by mass spectrometry with minimal interference from the multiple charge states of adjacent oligomers. 

The observation of multiple charging of proteins in electrospray ionization attracted much attention all major instrument manufacturers introduced an API system to be used in combination with an electrospray interface. At the same time, it was found that electrospray ionization is not only suitable for the mass analysis of large molecules, but is also a very efficient ionization technique of small polar or ionic molecules, e.g. drugs and their metabolites. In the past few years, hundreds of dedicated API-MS systems equipped with electrospray and APCI interfaces have found their way into many different laboratories, especially within pharmaceutical companies and biochemistry/biotechnology laboratories. 

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