Metastable ion

The narrow time range means that it is often acceptable to regard the number (per time), of metastable ions as a measure of the rate of decomposition at that time. That is to say 

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In the FFR of the sector mass spectrometer, the unimolecular decomposition fragments, and B, of tire mass selected metastable ion AB will, by the conservation of energy and momentum, have lower translational kinetic energy, T, than their precursor ... 

Cooks R G, Beynon J H, Capriolo R M and Lester G R 1973 Metastable Ions (Amsterdam Elsevier)... 

This is the seminal book on metastable ions, their chemistry and experimental observation. It is a must for anyone starting out in gas-phase ion chemistry. 

This is entirely analogous to the problem with simple chemical ionization, and the solution to it is similar. To give the quasi-molecular ions the extra energy needed for them to fragment, they can be passed through a collision gas and the resulting spectra analyzed for metastable ions or by MS/MS methods (see Chapters 20 through 23). 

An ion beam mainly comprises normal ions, all having the same kinetic energy gained on acceleration from the ion source, but there are also some ions in the beam with much less than the full kinetic energy these are called metastable ions. 

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

Perhaps it is worth emphasizing that the actual metastable ions are those that decompose, and the so-called metastable ions that are recorded are actually the products of decomposition, not the metastable ions themselves. It is more accurate to describe the recorded metastable ions as ions resulting from decomposition of metastable ions. The apparent m/z value of a metastable ion provides the link between an ion that fragments (the parent or precursor ion) and the ion that is formed by the fragmentation (the daughter or product ion). 

Field Free Zones and the Formation of Metastable Ions... 

Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

Linked Scanning and Metastable Ions in Quadrupole Mass Spectrometry... 

The steps (reactions) by which normal ions fragment are important pieces of information that are lacking in a normal mass spectrum. These fragmentation reactions can be deduced by observations on metastable ions to obtain important data on molecular structure, the complexities of mixtures, and the presence of trace impurities. 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

The study of metastable ions concerns substances that have been ionized by electrons and have undergone fragmentation. The stable molecular ions that are formed by soft ionization methods (chemical ionization. Cl field ionization, FI) need a boost of extra energy to make them fragment, but in such cases other methods of investigation than linked scanning are generally used. 

Scanning for metastable ions requires adjustment of electric and magnetic fields which can be adjusted individually or in conjunction with each other. If two fields are automatically adjusted at the same time, it is known as linked scanning. It is important to remember that metastable ions are not the product ions m2 but are the ions undergoing fragmentation (precursor ions mj). 

Metastable Ions Decomposing in the First Field-Free Region... 

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