Mass spectrometry characterisation of laser produced products.

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.