Mass Spectral Studies of Organic Ions in the Gas Phase

Silva, Maria Eugenia Santos Fronteira e (1973) Mass Spectral Studies of Organic Ions in the Gas Phase. PhD thesis, University of Glasgow.

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A single focusing mass spectrometer (Metropolitan Tickers MS2) was adapted to energy studies by adding an electrostatic sector. The electrostatic sector, spherical and symmetric was calculated to allow focusing of the ion beam received at the end of the magnetic sector and taking into consideration the limited space available in the instrument. Diaphragms to correct for the fringing field were introduced. A plate connected to an electrometer monitored the beam between the two sectors while a channel electron multiplier was used as the final collector. All the circuits were new and the potentials and currents obtained were stable at least within 0.1%. The resolution obtained was better than 1 Volt in 1000 Volts. The performance of the instrument was studied, in particular the mass discrimination effect against the lower masses noticed when using the channel electron multiplier in the continuous current mode, Nitrogen, methane and methanol were studied. The shapes of the peaks were obtained by plotting the intensity as obtained in the "channeltron" versus the energy determined from the value of the potential applied to the electrostatic sector plates. The following energetic ions were detected: 1) two groups of energetic ions for peak m/e =14 from nitrogen; ii) a particular shape for peak m/e = 14 from methane which could be caused by an energetic ion, or by the presence of some nitrogen mixed with the methane; iii) a group of energetic ions for peak m/e = 17 from methanol. All these results are in very good agreement with others previously reported in the literature, obtained using different techniques. The following metastable transitions, occurring between the monitor and the electrostatic sector, were detected: i) for Nitrogen ii) for Methane iii) for Methanol These transitions agree with published results obtained with different instruments. For methane one metastable peak was obtained that was attributed to a transition occurring in the first field free region. An attempt was made to obtain information about the kinetic energy released in the metastable transition. A criterion, based on information theory, was developed for the matching of an unknown mass spectrum with that in a computer data bank. The use of the principle of minimum divergence proved adequate in the majority of cases. However, when the same compound is run in various mass spectrometers the divergence between the different spectra of the same compound can he greater than between different compounds on the same instrument. It was also shown that this criterion, deduced theoretically, approached some of the simpler criterions in use, if the spectra were normalised by making the sum of the ion abundances equal to a fixed value instead of, as is a common practice, making the most intense peak equal to one hundred. To test the criterion the files were reorganized, to speed the search, and in one case the file on tape was transferred on to a disc. The programmes used are included in an Appendix. A method was devised to deduce the molecular weight of a compound when the molecular ion does not appear in the spectrum. It is based on the combination of the probabilities of occurrence of the fragment ions and does not involve an a priori knowledge of the compound, except for an estimate of the range of masses that should include the molecular weight.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Analytical chemistry
Date of Award: 1973
Depositing User: Enlighten Team
Unique ID: glathesis:1973-78637
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:07
Last Modified: 30 Jan 2020 15:07

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