Energy molecular ion

Reactive collisions of hyperthermal energy molecular ions with solid surfaces. Ann. Rev. Phys. Chem. 53, 379. 

When subjected to an electron bombardment whose energy level is much higher than that of hydrocarbon covalent bonds (about 10 eV), a molecule of mass A/loses an electron and forms the molecular ion, the bonds break and produce an entirely new series of ions or fragments . Taken together, the fragments relative intensities constitute a constant for the molecule and can serve to identify it this is the basis of qualitative analysis. 

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). 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

Ions formed in an electrospray or similar ion source are said to be thermolized, which is to say that their distribution of internal energies is close to that expected for their normal room-temperature ground state. Such ions have little or no excess of internal energy and exhibit no tendency to fragment. This characteristic is an enormous advantage for obtaining molecular mass information from the stable molecular ions, although there is a lack of structural information. 

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. 

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. 

The ion (M +) derived from the parent molecule by loss of an electron is called a molecular ion. Depending on the structure of substance M and the energy of the incident electron, the resulting ion (M"+) may break up (fragment) to give ions of smaller mass (A+, B, etc.). 

Usually, FAB yields molecular or quasi-molecular ions, which have little excess of internal energy and therefore do not fragment. This ionization method is mild and good for obtaining molecular mass (molecular weight) information. 

The ions that pass into the analyzer have near-ambient thermal energies and do not fragment but give excellent molecular or quasi-molecular ions. These ions can be investigated for their m/z values by almost any kind of mass analyzer. 

In the few microseconds that molecular ions (M ) spend in an ion source following electron ionization, many have sufficient energy to decompose to give fragment ions (F, . .., F +). 

High-mass resolution is needed to separate mass interferences of molecular and atom ions. Because of the mass defect of the binding energy of the nucleus, atomic ions have a slightly smaller mass than the corresponding molecular ions. To observe this typical mass resolutions between 5000 and 10000 are necessary. 

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