Abstract:
This thesis embodies the results of work carried out under the general heading of photoacoustic spectroscopy. In this work the photoacoustic effect in mercury vapour is investigated. Chapter 1 outlines the basic principle of the photoacoustic effect as it applies to the detection of mercury vapour and traces the historical development of gas phase photoacoustic spectroscopy. Chapter 2 develops a theory of the photoacoustic effect for mercury vapour arising from the quenching of the mercury 1SO→3p1 transition and outlines the derivation of a modified Stern-Volmer equation which relates normalized photoacoustic amplitude to quench gas composition. A general equation is derived which relates fluorescence intensity, photoacoustic amplitude and quench gas composition. Chapter 3 outlines the development of apparatus to carry out experimental verification of the theories developed in chapter 2. The main components are a photoacoustic mercury analyser, modified to detect fluorescence as well as sound, and a quadrupole mass spectrometer used to determine the hydrogen-argon and hydrogen-helium quench gas composition. Chapter 4 details results of quenching experiments which show a linear relationship between fluorescence intensity and photoacoustic amplitude. These variables are both related to quench gas composition by Stern-Volmer and modified Stern-Volmer equations which are shown to fit the experimental data closely. Rate constants, obtained from both the photoacoustic and the fluorescence measurements as a function of quench gas composition, are presented for the quenching reaction which agree closely with literature values. Chapters 5 and 6 deal with applications arising from this work. Chapter 5 describes a new sensitive photoacoustic gas chromatographic detector which was developed from observations made in the quenching studies.
Quenching reactions initiate exothermic reactions in hydrocarbons as they elute from the column of a gas chromatograph. Sensitivities comparable to flame ionisation detectors were obtained. Chapter 6 describes a new photoacoustic mercury detector with a detection limit of less than 1 picogram. Appendix 1 describes the preparation of gold collectors used in this study. Appendix 2 provides lists and plots of the data obtained in this study.
