Mass spectrometry characterisation of laser produced products.
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Date
1999
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Abstract
Mass spectrometers are analytical instruments that convert neutral atoms
and molecules into gaseous ions and separate those ions according to the
ratio of their mass to charge, m/z. The measurement is reported as a mass
spectrum: a plot of relative intensity vs. m/z that can be used to deduce the
chemical structure and composition of materials and compounds. Initially,
the use of mass spectrometers was restricted to the analysis of volatile
compounds. Recent advances in the development of ionisation techniques
to produce intact molecules directly from samples in the liquid or solid
phase, has extended the powerful use of mass spectrometry to compounds
of increasingly higher molecular mass.
The aim of this study was twofold: develop diagnostic techniques for the
in-situ measurement of isotope ratios in laser isotope separation
experiments; and to correlate it with the measured isotope ratios on the
collected product. The outcome is a thesis that can be divided into two
distinct fields of application: Firstly; the Atomic Vapour Laser Isotope
Separation (AVLIS) of lithium, and secondly the Molecular Laser Isotope
Separation (MLIS) of uranium,
In both AVLIS and MLIS pulsed laser systems were used to ionise and/or
dissociate atomic or molecular beams. The pulsed nature of the lasers is
ideally suited to in-situ time-of-flight detection of the produced ions.
Different types of inter-changeable ion sources are common to the same
TOF mass spectrometer. Each of these sources is selected according to its
application. For instance, applications vary from photo- and multiphoton
ionisation (laser ionisation) to surface analysis (laser desorption or particle
bombardment) to chromatography (electron impact ionisation). Four
different source configurations were considered in this study:
(i) Atomic Laser Isotope Separation (AVLIS) of lithium;
(ii) Multiphoton Ionisation (MPl) of UF6 gas;
(iii) Non-resonant ionisation during Laser Desorption (LDI) of solids; and
(iv) Matrix-Assisted Laser Desorption (MALD) of biopolymers.
The design of each of these sources will be discussed in detail in chapters
to follow. Bulk analysis of harvested laser-produced products needs to be in
correlation with in-situ analysis. Three different characterisation methods
were used in this study:
(i) Laser Desorption Time-of-Flight Mass Spectrometry (LD-TOF-MS)
(ii) Quadrupole-based Secondary Ion Mass Spectrometry (SIMS); and
(iii) TOF-MS-based Secondary Ion Mass Spectrometry (TOF-SIMS).
Chapter I describes the principles of time-of-flight mass spectrometry,
design parameters, as well as the instrumentation that were designed and
constructed for the purposes of this study. Chapter II describes the
principles of Secondary Ion Mass Spectrometry (SIMS). In particular,
research done on the establishment of tools to the non-expert user of SIMS
to select analyses conditions, is described. Chapter III reports on the
application of TOF-MS and SIMS during the AVLIS of lithium. Chapter
IV reports on the application of the different combinations of TOF-MS,
LD-TOF-MS, SIMS, and TOF-SIMS during the MLIS of uranium.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1999.
Keywords
Lasers in isotope separation., Time-of-flight mass spectrometry., Lithium--Isotopes., Uranium--Isotopes., Theses--Physics.