Ion activation and dissociation

In a constant-neutral-loss scan, all precursors that undergo the loss of a specified common neutral are monitored. To obtain this information, both mass analyzers are scanned simultaneously, but with a mass offset that correlates with the mass of the specified neutral. Similar to the precursor-ion scan, this technique is also useful in the selective identification of closely related class of compounds in a mixture. For example, the loss of 44 Da is a common reaction of carboxylic acids. Through the constant-neutral loss scan, the identity of all carboxylic acids present in a complex mixture can be revealed. Similarly, by monitoring the 98-Da neutral loss, the presence of phosphopeptides can be detected in a complex mixture [6]. 

The fourth scan, selected-reaction monitoring (SRM), is useful in quantitative measurements of analytes present in complex mixtures (see Chapter 14). Conceptually, this scan mode is similar to the product-ion scan. However, instead of scanning the second mass spectrometer in a broad mass range, the two mass analyzers are adjusted to monitor one or more chosen precursor-product pairs of the analyte. This operation is identical to the selected-ion monitoring mode (SIM see Chapter 14) of data acquisition. Monitoring more than one reaction is termed multiple-reaction monitoring (MRM). 

All four scan modes can be implemented with magnetic sector- and quadrupole-based true or hybrid tandem instruments. Time-of-flight (TOF) and tandem-in-time devices are also suitable for product scan experiments, but they are unable to perform the other three scans. 

To gather information about structure-specific fragment ions in tandem mass spectrometry, ion activation and dissociation of the mass-selected precursor is 

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Figure 4.2. Pictorial representation of four scan modes of tand mass spectrometry. AD refers to ion activation and dissociation, and the filled and open circles stand for fixed and scanning mass analyzers, respectively.

Principles and Characteristics Analytical multistage mass spectrometry (MSn) relies on the ability to activate and dissociate ions generated in the ion source in order to identify or obtain structural information about an unknown compound and to analyse mixtures by exploiting two or more mass-separating steps. A basic instrument for the currently most used form, tandem mass spectrometry (MS/MS), consists of a combination of two mass analysers with a reaction region between them. While a variety of instrument set-ups can be used in MS/MS, there is a single basic concept involved the measurement of the m/z of ions before and after a reaction in the mass spectrometer the reaction involves a change in mass and can be represented as ... 

The energy received from multiple photon absorption may also be used to activate and dissociate otherwise stable gaseous ions [133]... 

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

On the basis of kinetic and electrolytic measurements (35-38), the actual active species in these solvating media may be twofold, that is, ion pairs and dissociated anions. In fact, more than one type of ion pair is possible (for example, contact, solvent separated, etc.). However, for the sake of simplicity, the actual propagation steps are best represented as follows ... 

Mixed acid dissociation constants are defined by = ajj+[A ]/[HA], where an+ denotes hydrogen ion activity and the bracketed concentrations are molar. ... 

Before going on to define single ion activities and activity coefficients, let s pause to reflect on the similarity between the case considered here (a completely dissociated electrolyte in water), and the olivine solid solution case considered in Chapter 12 ( 12.7). The physical systems are completely different, but the thermodynamic problem is almost identical, the only significant difference being that in the olivine case the concentrations were measured by mole fractions and ideality consisted in conforming to Raoult s Law, while here concentrations are measured in molality and ideality is represented by Henry s Law. Apart from that, the problem in both cases consists in choosing a solute component that is appropriate to the situation. 

By mass spectrometry, molecules are first activated and dissociated into ionic fragments that are then separated according to their charge-to-mass ratios. Mass spectrometers essentially consist of four parts equipment to ionize the sample, an ion accelerator, an ion separation system, and a detection system. The sample may be ionized by bombardment with electrons from a heated filament or by several other means. The ions produced are then accelerated and focused electrostatically and separated according to their atomic masses while traveling through a magnetic field. 

Analytical tandem mass spectrometry (MS/MS) relies on the ability to activate and dissociate ions in order to identify or obtain structural information about an unknown compound. The most common means of ion activation in tandem mass spectrometry is collision-induced dissociation (CID). CK) uses gas phase collisions between the ion and neutral target gas to cause internal excitation of the ion and subsequent dissociation. Surface-induced dissociation (SID) is analogous to the CID experiment except a surface is substituted for the collision gas as shown in Figure 1. SID offers several advantages as a means... 

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