Molecular ions fragmentation

SIMS is, strictly speaking, a destructive teclmique, but not necessarily a damaging one. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one monolayer per several hours, implying that the surface structure does not change during the measurement (between seconds and minutes). In this case one can be sure that the molecular ion fragments are truly indicative of the chemical structure on the surface. 

Structure does not change during the measurement. In this case the molecular ion fragments are indicative of the chemical structure of the surface. 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

LD), in which flash vaporisation of the sample is induced, may be applied. Other techniques which permit detection of less-volatile chemical species are FD (with simultaneous desorption/ionisation of molecules), FAB (with the sample dissolved (dispersed) in a suitable liquid) and SIMS (based on bombardment of a solid surface with high-energy ions). LD-FUCR-MS is superior to FAB-MS for polymer/additive identification because it gives molecular ion fragmentation [83],... 

Databases Commercially molecular ions, fragment ions pos./neg. mode Only user-generated... 

FAB MS has been applied in a number of studies to characterize ILs. Since most ILs are viscous liquids with negligible vapor pressure, the measurement of FAB MS is possible without the addition of a liquid matrix. In principle, ILs can therefore also be used as matrix substances for the FAB analysis of other analytes dissolved therein [12]. Spectra could be measured both in the positive and in the negative ion modes as has been demonstrated, for example, for butylpyridinium- chloroaluminates and gallates [12,13]. Beside the molecular ions, fragments mainly formed by the loss of the substituents of the central core of the cations, for example, butyl groups, were observed. Together with the isotope patterns, these fragments provide valuable information about the structure of newly composed compounds and help also to identify unexpected by-products like oxidized or hydrolyzed compounds in the ILs (see section 14.3.2). 

The cleavage reaction of Equation 23-2 reveals other useful generalizations. Whatever its source, a parent molecular ion, M+, has one unpaired electron and is properly described as an odd-electron ion (a radical cation). When a parent molecular ion fragments, it does so homolytically, as shown in Equation 23-2, and produces a radical and an ion in which the electrons are paired—an even-electron ion. The m/e value of an even-electron ion is an even number for any elemental composition of C, H, O in combination with an odd number of nitrogens. These generalizations are summarized in Table 23-2 and can be useful in the interpretation of mass spectra, as illustrated by Exercises 23-4 and 23-5. 

The mass spectra of the cycloocta-1,5-diene complexes C5H5MC8H12 (M = Rh, Ir) both show the molecular ion as the base peak. The rhodium complex exhibits several metastable peaks and a detailed fragmentation scheme has been proposed. The molecular ion fragments by several... 

The molecular ion fragments and produces ions and neutrals that are not observed in the spectrum. The mass of the neutral product can be deduced from the difference between the mass of the parent ion and that of the observed ionic fragment. 

In the first stage of negative molecular ion fragmentation (M ) abstraction of OH, N02, (HN02) from all the compounds studied occurs. H20 and NO elimination is also typical of M ion of 6-nitro- and 4,6-nitro-2-aminobenzothiazole. As an example, we give here one of the ways of the fragmentation of the M - ion derived from 4-nitro-2-aminobenzothiazole (Scheme 3.75) [1356] ... 

Interpretation of mass spectra depends on the type of mass spectrometer and ionisation technique used. Hard ionisation methods such as El produce molecular ion fragmentation, which can be used to identify diagnostic fragmentation patterns and functional groups. Softer ionisation techniques such as ESI and MALDI provide pseudomolecular ion formation, and rules in accordance with spectral information can be used to identify corresponding molecular structure and elemental composition. Table 13.3 lists some of the types of information that can be provided by mass spectrometry, and Table 13.4 gives dehnitions of molecular masses that are highly relevant in mass spectrometry. 

The molecular ion fragments into cations, radicals, radical cations and neutral molecules of which only the positively charged species are detected. There are several possible fragmentations for each M but the base peak represents the most energetically favoured process with the mjz value of the base peak representing the mass of the most abundant (and therefore most stable) positively charged species. The fragmentation of into the base peak follows the simplified rules outlined in Box 30.2, and for a more detailed interpretation you should consult the correlation tables to be found in the specialist texts referred to at the end of the section. 

Authors have reported mass spectra as a tool for confirmation of the molecular structure based on intense molecular ions. Fragmentation of the isoselenazoles and isoselenazolones has not been studied in any more detail than that reported in CHEC(1984) <1984CHEC(6)333> and CHEC-II(1996) <1996CHEC-II(3)475>. 

The increasing of specificity of molecular ion fragmentation for estimation of structure of analytes (an example the determination of double bonds C = C position in carbon skeleton of the molecules)... 

The first prominent fragment is at mass 232.13b5 which corresponds to the loss of, CH3. A peak at m/e 218.1178 corresponds to the loss of Hc from the molecular ion. Fragments appear at masses 202 and 17U which are formed by a carbonyl cleavage with the loss of. 0CHaCH3 and 0... 

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