Fragmentation approach limitations

Since, in this section, we restrict ourselves to a qualitative approach, we assume that the gA and gB tensors are isotropic with the same principal value g. At very high temperature, when kT > J, XmT tends to the (XmT)ht limit corresponding to the superposition of the uncoupled A and B fragments. This limit is ... 

An enhancement of the simple substructure approach is the Fragment Reduced to an Environment that is Limited (FREL) method introduced by Dubois et al. [7] With the FREL method several centers of the molecule are described, including their chemical environment. By taking the elements H, C, N, O, and halogens into account and combining all bond types (single, double, triple, aromatic), the authors found descriptors for 43 different FREL centers that can be used to characterize a molecule. 

A theory that adequately explains all BLEVE phenomena has not yet been developed. Reid s (1979, 1980) theory seems to be a good approach to explain the strong blast waves that may be generated. But even when a liquid s temperature is below the superheat limit, the liquid may flash within seconds after depressurization, resulting in a blast wave, a fireball, and fragmentation. 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

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. 

Equation (51) has a clear physical interpretation. Recalling the lineshape for a single excitation route, where fragmentation takes place both directly and via an isolated resonance [68], p oc (e + q)2/( 1 + e2), we have that 8j3 is maximized at the energy where interference of the direct and resonance-mediated routes is most constructive, e = (q I c(S )j2. In the limit of a symmetric resonance, where q —> oo, Eq. (51) vanishes, in accord with Eq. (53) and indeed with physical intuition. The numerator of Eq. (51) ensures that 8]3 has the correct antisymmetry with respect to interchange of 1 and 3 and that it vanishes in the case that both direct and resonance-mediated amplitudes are equal for the one-and three-photon processes. At large detunings, e —> oo, and 8j3 of Eq. (51) approaches zero. 

---