Molecular fragmentation patterns

Chemical ionisation, electron impact ionisation and field ionisation have been used to obtain molecular fragmentation patterns in speciation studies involving GC-MS. However, the systems used often lacked sensitivity. This area has expanded recently as inexpensive, smaller and more sensitive instruments have become available (Zufiaurre et al., 1997). Tributyl tin and triphenyl tin compounds have recendy been determined in seawater using GC negative ion chemical ionisation MS (Mizuishi et al., 1998). Of particular interest are those mass spectrometers in which the sample is broken down into elemental form. 

A chromatogram characteristic of di-ethylhexyl phthalate (DEHP), the most commonly used plasticizer for PVC. The mass spectrum on the right-hand side is a molecular fragmentation pattern which identifies this peak as DEHP. 

Since multiphoton absorption generally produces molecular fragmentation patterns quite different from those of the normal mass spectrum, this method additionally provides useful insight into the dynamics of multiphoton excitation. Multiphoton ionisation and laser mass spectrometry have been reviewed [236]. 

Separation of families by merely increasing the resolution evidently can not be used when the two chemical families have the same molecular formula. This is particularly true for naphthenes and olefins of the formula, C H2 , which also happen to have very similar fragmentation patterns. Resolution of these two molecular types is one of the problems not yet solved by mass spectrometry, despite the efforts of numerous laboratories motivated by the refiner s major interest in being able to make the distinction. Olefins are in fact abundantly present in the products from conversion processes. 

When atoms, molecules, or molecular fragments adsorb onto a single-crystal surface, they often arrange themselves into an ordered pattern. Generally, the size of the adsorbate-induced two-dimensional surface unit cell is larger than that of the clean surface. The same nomenclature is used to describe the surface unit cell of an adsorbate system as is used to describe a reconstructed surface, i.e. the synmietry is given with respect to the bulk tenninated (unreconstructed) two-dimensional surface unit cell. 

The mass spectra of more substituted thiazoles, or those with larger alkyl groups are more complex and involve other fragmentation patterns (117, 118, 374). The molecular ion is still abundant but decreases with increasing substitution past the ethyl group. 

The spectra of alkylarylthiazoles generally possess fragmentation patterns similar to those previously mentioned for alkyl- and arylthiazoles. In this case, scission of the S-Cj and C3-C4 bonds of the thiazole ring can occur in ion fragments as well as in the molecular ion (124). 

Although GGMS is the most widely used ana lytical method that combines a chromatographic sep aration with the identification power of mass spectrometry it is not the only one Chemists have coupled mass spectrometers to most of the mstru ments that are used to separate mixtures Perhaps the ultimate is mass spectrometry/mass spectrome try (MS/MS) m which one mass spectrometer gener ates and separates the molecular ions of the components of a mixture and a second mass spec trometer examines their fragmentation patterns ... 

As we have just seen interpreting the fragmentation patterns m a mass spectrum m terms of a molecule s structural units makes mass spectrometry much more than just a tool for determining molecular weights Nevertheless even the molecular weight can provide more information than you might think... 

Fragmentation pattern (Section 13 22) In mass spectrometry the ions produced by dissociation of the molecular ion... 

For fV-methylpyrazoIe (99), the molecular ion of which is less intense than pyrazole (a common feature for methyl-substituted pyrazoles (67ZOR1540)), the fragmentation pattern involves the methyl group (Scheme 2). These results were established using H, C and N labelling studies. 

Electron impact mass spectrometry has been employed to study the fragmentation patterns of isoxazolylmethyl- and bis(isoxazolylmethyl)-isoxazoles and the results are in agreement with proposed pathways (79AC(R)8l). Electron impact studies of nitrostyryl isoxazole (6) show fragmentation in a variety of ways. The standard loss of NO2 from the molecular ion... 

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

A series of 2,3-disubstituted pyrido[4,3-d]pyriinidin-4(3Zf)-ones (154) showed more complicated fragmentation patterns. Here again, however, strong molecular ions were invariably present and m/2e peaks were common, as were M-1 peaks. The nature of the group Rj... 

W as an important factor which influenced the fragmentation pattern. 2-Furylpyrido[4,3-d]pyrimidin-4(3i7)-one (154, Rj=furyl, R2 = H,) showed six different primary breakdown peaks originating from the molecular ion. These corresponded to the loss (in decreasing order of probability) of H-, C1H3O, -CN, CO, HCN, C4H3O-, and HCNO. 

No discussion of the fragmentation pattern of dithietes by mass spectroscopy has appeared. Only molecular and major fragmentation peaks... 

Mass spectral fragmentation patterns are usually complex, and the molecular ion is often not the base peak. The mass spectrum of propane in Figure 12.2, for instance, shows a molecular ion at m/z = 44 that is only about 30% as high as the base peak at m/z = 29. In addition, many other fragment ions are present. 

Because mass-spectral fragmentation patterns are usually complex, it s often difficult to assign structures to fragment ions. Most hydrocarbons fragment in many ways, as the mass spectrum of hexane shown in Figure 12.4 demonstrates. The hexane spectrum shows a moderately abundant molecular ion at m/z = 86... 

The mass spectra of l-acyl-l//-l-benzazepines have been recorded.23 The mass spectrum of 3-mesyl-3/7-3-benzazepine shows an intense base peak at m/e = 142duetothebcnzazepinylium ion and a peak (51 %) at m/e — 115 (-HCN) which is attributed to the indenium cation.26 Fragmentation patterns for 1H- and 5/7-2-benzazepines40 and for 5//-dibenz[c,e]azepine5 are available. The electron-impact induced fragmentation pattern of 5//-dibenz[6,/]azepine displays an intense molecular ion as the base peak, and a moderately intense (M + 1) peak.5 ... 

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