The Photoacoustic Effect in Mercury Vapour

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.