Charge State of Ions

The ions produced in the source are separated in the analyzer according to their mass-to-charge (m/z) ratios. The Thomson unit (Th, named for a famous mass spec-trometrist) corresponds to an m/z ratio of 1 it is widely used in mass spectrometry. In a GC-MS context, we often speak too generally about mass measurement because z = 1 in electron ionization and positive chemical ionization and z = -1 in negative chemical ionization. 

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K. Chan, D. Wintergrass, K. Straub, Determination of the charge state of ions in TSP mass spectra. Rapid Commun. Mass Spectrom., 4 (1990) 139. 

The final problem that we will discuss here refers to a basic aspect of the interaction of swift ions with solids, which is the relation between the charge state of ions moving inside a solid and the effective charge values deduced from stopping power measurements. This question has been analyzed recently in Refs. [49,50], based on different non-linear models and reaching similar results. Hence, the approach provides a convenient framework to clarify some seeming contradictions which have been under discussion for many years. 

The problem of the charge state of ions penetrating matter is one of the most relevant questions for studies on the interaction of ions with solids. It is known that after some penetration distance the ions reach a state of charge equilibrium determined by the competition between capture and loss processes [2,7]. As a result of this equiUbrium the ions acquire a mean ionization charge as well as a stationary distribution of charge states around q. 

Fig. 7. Different estimation of the mean charge state of ions in carbon foils. The lines indicated ND and SG show the empirical fitting expressions for the average charge state of ions crossing solid foils given by Nikolaev-Dmitriev [55] and Schiwietz-Grande [56], respectively. The line BK is the Brandt-Kitagawa model [15] for the equilibrium charge of ions moving inside solids. The line ZBL is the equilibrium ionization value proposed by Ziegler et al. [9].

What multiple charging does not do, however, is to provide an equivalent increase in the resolution of the mass spectrometer and the resolution required to separate the individual isotopic contributions from a multiply charged species is identical to that required for the corresponding singly charged species. Figures 4.16 and 4.17 show the ions from the 7- - charge state of aprotin at resolutions of 1500 and 5000, respectively. 

High-resolution mass spectrometers have been used to obtain electrospray spectra and have the added advantage that they allow the direct determination of the charge state of the ions being observed, e.g. if the apparent separation of the and isotopic contributions is 0.1 Da, the charge state is 10, while if it is 0.05 Da, the charge state is 20, etc. 

Figure 11.6 Positive ion electrospray mass spectra of an equimolar mixture of five standard proteins, under different instrumental settings, showing cases where prominent signals for the different charge states of (A) insulin, (B) ubiquitin, (C) cytochrome c, (D) lysozyme, and (E) myoglobin were preferentially observed, and (F) where signals for all the proteins were more uniformly detected.

Gas-phase ion chemistry is a broad field which has many applications and which encompasses various branches of chemistry and physics. An application that draws together many of these branches is the synthesis of molecules in interstellar clouds (Herbst). This was part of the motivation for studies on the neutralization of ions by electrons (Johnsen and Mitchell) and on isomerization in ion-neutral associations (Adams and Fisher). The results of investigations of particular aspects of ion dynamics are presented in these association studies, in studies of the intermediates of binary ion-molecule Sn2 reactions (Hase, Wang, and Peslherbe), and in those of excited states of ions and their associated neutrals (Richard, Lu, Walker, and Weisshaar). Solvation in ion-molecule reactions is discussed (Castleman) and extended to include multiply charged ions by the application of electrospray techniques (Klassen, Ho, Blades, and Kebarle). These studies also provide a wealth of information on reaction thermodynamics which is critical in determining reaction spontaneity and availability of reaction channels. More focused studies relating to the ionization process and its nature are presented in the final chapter (Harland and Vallance). 

Isotopic displays contain a wealth of important and useful information they can reveal what types of elements are contained in a molecule their study is valuable in high resolution and in accurate mass determination and they can also reveal the charge state of an ion, which is particularly useful when ESI is used. 

The two ionization techniques can be used with all types of mass spectrometers. Here, only those that are the most commonly used in proteomics will be described. Because mass spectrometers use electric and magnetic fields to separate ions, they can only measure mass divided by charge values. In the examples used this is assumed implicitly. In most cases the charge state of an ion can be determined from the mass spectrum. 

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