Mass spectrum studies

The author thanks Dr. R. Suryanarayanan, University of Minnesota and Mr. William Ihm, Creighton University for the powder x-ray diffraction and mass spectrum studies respectively. The useful suggestions of Dr. Shanker L. Saha, Creighton University are greatfully acknowledged. 

During recent studies of the upper atmosphere of Titan - the largest moon of Saturn - the Cassani spacecraft detected numerous carbocationic species. In particular, mass spectrum studies detected significant quantities of cyclopropenium cation (35) and its methylated analogs. The formation of these ions has been studied and a mechanism... 

Knowing and 2 by studying reference components, it becomes possible to calculate from the measurements of ion intensities found on the mass spectrum. 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

During the course of biochemical studies (138). the mass spectrum of 2-acetamidothiazole was recorded its main peaks are the molecular ion (m/e= 142, relative intensity = 26%) and fragments 100 (100), 58 (2. 5), and 43 (39). For 2-acetamido-5-bromothiazole the main peak results again from the loss of C2H2O by the molecular ion. 2-AcetyIacet-amido-4-methylthiazole (2S) exhibits significant loss of from the... 

Schematic diagram of a mass spectrometer. After insertion of a sampie (A), it is ionized, the ions are separated according to m/z value, and the numbers of ions (abundances) at each m/z value are plotted against m/z to give the mass spectrum of A. By studying the mass spectrum, A can be identified,...

Electron impact/Fourier transform ion cyclotron resonance mass spectrometry was used to study the mass spectrum of 7 (96MI20). 

Structure of Oxy-F Compound F is extremely unstable and is difficult to obtain at a level of purity suitable for NMR studies. However, an oxidation product, Oxy-F, formed when F is left standing at — 20° C, is considerably more stable than F and can be purified to a sufficiently high level of purity. Oxy-F is nonfluorescent and shows absorption maxima at 237 nm and 275 nm (shoulder). The high-resolution FAB mass spectrum indicated the molecular formula of Oxy-F to be C33H3809N4Na2 [m/z 703.2363 (M + Na)+ and 681.2483 (M + H)"1"]. The H and 13C NMR data allowed the assignment of structure 7 to oxy-F (Fig. 3.2.6 Nakamura et al., 1988). 

Heath and Majer (H3) have recently used a mass spectrometer to study the decomposition of ammonium perchlorate. Decomposition was detected in the range from 110° to 120°C. At this temperature, there were ions in the mass spectrum caused by NH3, HC104, Cl2, HC1, nitrogen oxides, and 02. The appearance of the species NO, N02,02, and Cl2 in the decomposition products under very low pressure (i.e., in the absence of gas-phase molecular collisions) indicates that the principal decomposition reactions take place in the crystal and not in the gas phase. 

If the sample consists of atoms of one element, the mass spectrum gives the isotopic distribution of the sample. The relative molar masses of the isotopes can be determined by comparison with atoms of carbon-12. If the sample is a compound, the formula and structure of the compound can be determined by studying the fragments. For example, the + 1 ions that CH4 could produce are CH4, CH3+, CH, CFI4, C+, and H4. Some of the particles that strike the detector are those that result when the molecule simply loses an electron (for example, to produce Cl I4+ from methane) ... 

The mass spectrum of polymeric sulfur S, prepared from either liquid sulfur or by extraction of commercial flowers of sulfur , has been measured and interpreted in terms of Ss, Sy, and Ss molecules leaving the polymer on heating and depolymerization [203]. This result is in agreement with depolymerization studies in solution which also show Ss and Sy as the major thermal degradation products [174]. 

The mass spectrum produced should provide unambiguous molecular weight information from the wide range of compounds amenable to analysis by HPLC, including biomolecules with molecular weights in excess of 1000 Da. The study of these types of molecule by mass spectrometry may be subject to limitations associated with their ionization and detection and the mass range of the instrument being used. 

In the ion-trap, ionization of the sample is carried out as in conventional operation and ions of all m/z ratios take up stable trajectories within the trap. In the production of a conventional full-scan mass spectrum, ions of different m/z values are then sequentially made unstable and ejected from the trap to the detector. In MS-MS operation, ions of all m/z ratios, except that required for further study, are made unstable and ejected from the trap. The ions remaining in the trap, only those of the selected m/z ratio, are now excited to bring about their dissociation. The resulting product ions are then sequentially made unstable and sent to the detector to generate the product-ion spectrum. 

Further experimental work involving cone voltage studies may provide further confirmatory evidence but the most likely explanation is that the mass spectrum of the component with retention time 4.65 min is that shown in Figure 3.17, while the mass spectrum of the second component is that obtained by background subtraction, and is shown in Figure 3.22. 

MS-MS is a term that covers a number of techniques in which two stages of mass spectrometry are used to investigate the relationship between ions found in a mass spectrum. In particular, the product-ion scan is used to derive structural information from a molecular ion generated by a soft ionization technique such as electrospray and, as such, is an alternative to CVF. The advantage of the product-ion scan over CVF is that it allows a specific ion to be selected and its fragmentation to be studied in isolation, while CVF bring about the fragmentation of all species in the ion source and this may hinder interpretation of the data obtained. 

There are three main reasons for this choice. Firstly, it becomes more and more difficult to obtain recordable, molecular-ion signals from un-derivatized carbohydrates as their M, increases significantly above 3000. Secondly, the mass spectrometers that have been used in all high-mass-carbofiydrate studies published at the time of writing this article are not capable of very sensitive analysis above —3800 mass units (see later). Thirdly, at masses >4000, it is usually not practicable to work at the resolution necessary for adjacent peaks to appear as separate signals in the spectrum. To do so would require that the source and collector slits be narrowed to such a degree that there would be an unacceptable loss in sensitivity. Thus, spectra acquired at mass >4000 are usually composed of unresolved clusters. 

The kinetics study [38] utilized a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer to measure the pathway branching ratios. The ability to eject selected masses and the extremely high mass resolution of this technique ensured that the observed CD3CH2 was in fact a primary product of the reaction. Temporal profiles from this reaction are shown in Fig. 1. Noticeably absent from the mass spectrum are the cations C2D2H3 and... 

A mass spectrometric study was carried out to establish tbe structure of compoimd 69. Its mass spectrum contains tbe molecular ion peak m/z 252 (16.98%) and a base peak (100%) at m/z 210, corresponding to 2-(2-hydroxypbenyl)benzimidazole (70). A tendency towards decreasing the heterocycle size is characteristic of the mass spectrometric behavior of 1,5-benzodiazepin-2-ones [61] and consequently the mass spectra of these compounds contains intense peaks of the corresponding benzimidazoles. It is also known that the mass spectrometric fragmentation of 1,5-benzodiazepines is similar to their thermal or acid decomposition. In fact, refluxing compound 69 in concentrated sulfuric acid yields benzimidazole 70 as the main product. 

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