Stereoisomeric effects on mass spectra—I. A Review

The unimolecular reactions that give rise to mass spectra are controlled by spatial relationships and energy considerations. In molecules that contain a heteroatom, elimination reactions, involving bond-making as well as breaking, are often prominent, e.g. loss of water from alcohols. The ease of such reactions depends on spatial relationships in the molecule, and the resultant ion intensities in the spectra of geometric isomers can consequently be correlated with differences in geometry and hence furnish a basis for assigning structures. Processes that do not involve bond-making do not have such rigorous geometric requirements, but depend rather on attainment of a transition state defined in terms of a minimum energy content. Common product ions from stereoisomeric hydrocarbons seem often to arise via a common transition state. When this condition holds, the difference between the enthalpies of the isomers is reflected in the relative appearance potentials and–though the cause-and-effect relationship here is less direct and more readily obscured by other factors–relative intensities of the common product derived from the isomers. In both classes of processes, the spectra of stereoisomers can be simplified and made more distinctive by lowering source temperature and ionizing voltage.     

Even circuits in planar graphs

We prove that a planar graph can be partitioned into edge-disjoint circuits of even length, if and only if every vertex has even valency and every block has an even number of edges.     

Hydrodynamic dispersion in $ \beta$ -lactoglobulin gels measured by PGSE NMR

The displacement scale dependent molecular dynamics of solvent water molecules flowing through b \beta -lactoglobulin gels are measured by pulse gradient spin echo (PGSE) nuclear magnetic resonance (NMR). Gels formed under different p H conditions generate structures which are characterized by magnetic resonance imaging (MRI) and PGSE NMR measured dynamics as homogeneous and heterogeneous. The data presented clearly demonstrate the applicability of the theoretical framework for modeling hydrodynamic dispersion to the analysis of protein gels.     

Quantum efficiency of the 2537 A. Photolysis of a mixed phenyl–methyl polysiloxane

The ultraviolet (λ = 2537 A.) photolysis of a degassed mixed phenyl and methyl polysiloxane liquid is examined in terms of gas and crosslinking yields. Results are compared to the published values obtained by ionizing irradiation of this type of molecule. It is shown that ultraviolet radiation is less efficient by two orders of magnitude in producing decomposition (i.e., gaseous products) than is ionizing radiation. The comparisons for crosslinking efficiencies are less certain, but the yields seem to have much more similar values in this case based on a spectroscopic estimation of crosslinking (i.e., analysis for substituted phenylcyclohexadiene formation). The gas quantum yields were ?H₂ = 2.6 × 10^?5, ?CH₄ = 0.63 × 10^?5, ?C₂H₆ ≈ 0.12 × 10^?5, and ?C₂H₂ ≈ 0.06 × 10^?5.     

Organic ions in the gas phase—XXI: Synthesis and mass spectra of deuteriothiophenes

Mass-spectral and n.m.r. analysis of thiophenes labeled by exchange with deuteriosulfuric acid establishes that exchange at the 2 and 5 positions is essentially complete before any deuterium is incorporated at positions 3 and 4. Thus, such exchange is a satisfactory procedure for position-specific labeling. Mass spectra of the labeled thiophenes show that about 60% of the [CHS]⁺ ion yield is derived from molecular ions that have not undergone prior rearrangement. The remaining 40% arises by a path or paths in which the four hydrogen atoms lose position identity. Other decomposition paths contributing to the mass spectrum are characterized by more nearly complete scrambling of hydrogens.     

The use of stable carbon isotope analysis to detect the abuse of testosterone in cattle.

The use of stable carbon isotope analysis to detect the administration of anabolic steroids to cattle was investigated. Samples were extracted by solid-phase extraction on C18 cartridges. Stable isotope ratios (13C:12C) were measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) of the underivatised extracts. A programmed temperature vaporiser (PTV) injector was installed in the GC-IRMS system, which conferred a number of advantages. First, it allowed large volumes of sample to be injected whilst the injector liner was cool. The solvent was subsequently vented to the atmosphere prior to transfer of the sample to the GC column. Thus a significantly greater amount of sample could be presented for analysis, thereby increasing the sensitivity. Second, by this means virtually all the solvent could be removed prior to analysis. This eliminates solvent peak tailing, which can be a major problem in GC-IRMS. Finally, the PTV allowed the use of higher initial GC oven temperatures, which in turn facilitated the analysis of underivatised steroids by reducing the GC run time and improving the chromatographic peak shape. The carbon isotope composition of 5 beta-androstane-3 alpha,17 alpha-diol, the major metabolite of testosterone found in bovine bile, was measured in bile samples from untreated cattle and from cattle injected intramuscularly with testosterone or a mixture of testosterone esters. There was considerable inter-animal variation in the values obtained and there was no significant difference between samples from treated and untreated animals. However, when the isotopic composition of the metabolite was normalised with respect to that of an endogenous reference compound (cholesterol) in the same sample, the difference between treated and untreated animals become statistically significant.     

Cationated cyclopropanes as reaction intermediates in mass spectra: an earlier incarnation of ion-neutral complexes

The appearance a decade ago of the concept of ion-neutral complexes as intermediates in unimolecular decompositions constituted a major advance in mechanistic interpretation of mass spectra. The concept, however, was not new; it goes back to the introduction of the cationated cyclopropane in the mid-1950s. Its development did not occur via a smooth progression; rather, its history bridges a sharp discontinuity. The early workers, in a curious historical quirk, failed to generalize on the idea beyond complexes in which the neutral component is cyclopropane or a closely related species. Hence it took an independent rediscovery of the concept to usher in the rapid growth and wide application that characterize its present stage of development.