Molecular modelling mass spectrometry

ABSTRACT. We describe an apparatus by which the detonation products of an explosive can be identified and whose relative concentrations can be determined quantitatively. These measurements can be made on products that have been formed in less than one microsecond after the passage of the detonation wave. The technique is based on the rapid quenching of chemical reactions by virtue of the free expansion of the products into vacuum. Of course, products that have been formed over a longer period of time and under different pressure/temperature conditions can also be studied. Time resolved molecular-beam mass spectrometry is used, so that whether detonation occurred or not in forming the products can be determined. We describe optical techniques, principally Schlieren photographs, that also confirm detonation. We report measurements made on six standard explosives, PETN, RDX, HMX, HNS, TNT and TATB, and one research explosive, nitric oxide. For none of the standard explosives do we measure product distributions that agree with model predictions based on equilibrium assumptions. A computer model of the free expansion is described briefly and its importance to the interpretation of the data is emphasized. 

Bushnell and co-workers [117] employed extensive molecular modelling to understand the nature of cis and trans isomerism in tetrahedral p-phenylene vinylene oligomers, and to aid the interpretation of time of flight mass spectrometry and ion mobility studies. Molecules such as T4R, shown in Figure 18, with four equivalent arms can be used to control the crystallinity in thin films. The authors reported the observation of a species in the mass spectrum resulting from the loss of an arm from the central carbon. This species will be referred to as P4R. 

Foret et al.98 collected fractions of model proteins and variants of human hemoglobins after fractionation by CIEF, and then analyzed them by matrix-assisted laser desorption-time-of-flight-mass spectrometry (MALDI-TOF-MS). As the authors point out, MS is an orthogonal method to CIEF because it separates according to molecular mass. 

Final ozonides (FOZ), 716, 717, 718 cis and trans isomers, 719, 720 dialkyl peroxide formation, 706 IR spectroscopy, 719 mass spectrometry, 690 microwave spectroscopy, 721-3 molecular model, 750 NMR spectroscopy, 724-5 ozone water disinfection, 606 X-ray crystallography, 726-30 Fireflies... 

Mass spectrometry methods based on soft ionization techniques, 59,61,88,89 matrix-assisted laser desorption ionization/time-of-flight (MALDI-TOF), have been successfully applied for the direct analysis of grape and wine extracts and for monitoring flavonoid reactions in model solution studies. They give access to the molecular weights of the different species present in a fraction or extract and, through fragmentation patterns, provide important information on their constitutive units. Description of the various MS techniques can be found in Chapters 1 and 2. 

Mass spectrometry (MS) is an analytical method based on the determination of atomic or molecular masses of individual species in a sample. Information acquired allows determination of the nature, composition, and even structure of the analyte. Mass spectrometers can be classified into categories based on the mass separation technique used. Some of the instruments date back to the beginning of the twentieth century and were used for the study of charged particles or ionised atoms using magnetic fields, while others of modest performance, such as bench-top models often used in conjunction with chromatography, rely on different principles for mass analysis. Continuous improvements to the instruments, miniaturisation and advances in new ionisation techniques have made MS one of the methods with the widest application range because of its flexibility and extreme sensitivity. 

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