Analytical Aspects of Atomic Spectroscopy At

ANALYTICAL ASPECTS OF ATOMIC SPECTROSCOPY AT WAVELENGTHS IN THE FAR ULTRAVIOLET by ARTHUR FREDERICK WARD B.Sc.,M.Sc.,D.I.C. • A Thesis submitted for the Degree of DOCTOR OF PHILOSOPHY of the. University of London Department of Chemistry Imperial College of Science and Technology London S.W.7 July 1973' ABSTRACT The development of the radiofrequency induction coupled plasma as a spectroscopic source for the excitation of elements whose resonance lines lie in the far ultraviolet is described. The use of sample introduction systems for aqueous solutions based upon indirect nebulization is investigated. The determination of sulphur,phosphorus, iodine, carbon, mercury, arsenic and selenium is described at atomic lines of these elements, in the far ultraviolet using the plasma as an emission source. The technique is applied to the determination of phosphorus in soils using a simple extraction and ion exchange procedure. Some observations about plasmas and flames and a comparison of the two sources as excitation cells for atomic spectrscopy are made. The plasma temperature is determined and the effect of the operating parameters upon the temperature considered. ii ACKNOWLEDGEMENTS The work described in this thesis is entirely original except where due reference is made. No part of it has been previously submitted for any other degree. The work was carried out in the Chemistry Department of the Imperial College of Science and Technology, between October 1971 and July 1973. I would like to thank my supervisor, Dr.G.F.Kirkbright, and Professor T.S.West for their advice and encouragement. I should like to thank Mr. B. Bach of British.Steel Corporation for the loan of the Radyne . plasma unit used in this work and the Macaulay Institute for Soil Research [Aberdeen] for the provision of soil samples for analysis. I am also indebted to the Science Research Council for the provision of a studentship during this period. lit CONTENTS Abstract Acknowledgements ii Chapter 1- Introduction 1 Chapter 2 The Induction Coupled Radiofrequency plasma 44 Chapter 3 An investigation of some of the operating parameters of the induction coupled radiofrequency plasma 91 Chapter 4 An investigation of the emission spectroscopic properties of sulphur in an induction coupled radiofrequency plasma 136 Chapter 5 An investigation of the emission spectroscopic properties of phosphorus . in an induction coupled radiofrequency plasma 155 Chapter 6 An investigation of the emission spectroscopic properties of the halogens in an induction coupled radiofrequency plasma 177 Chapter 7 An investigation of the emission spectroscopic properties of carbon in an induction coupled radiofrequency plasma 192 Chapter 8 An investigation of the emission spectroscopic properties of mercury in an induction coupled radiofrequency plasma 204 Chapter 9 An investigation of the emission spectroscopic properties of arsenic and selenium in an induction coupled radiofrequency plasma 214 iv Chapter 10 Line profiles in the radiofrequency plasma 230 Chapter 11 A comparison of the induction coupled radiofrequency plasma and the nitrogen shielded nitrous oxide-acetylene flame as spectroscopic emission sources 255 Chapter 12 Conclusions and suggestions for further work 299 Appendix Principal symbols and abbreviations used 307 References 311 CHAPTER 1 Introduction. • 1 Although the far ultraviolet region of the electromagnetic spectrum was first explored before the turn of the century, its application has been developed far less rapidly than the other regions. Boyce [1] has suggested the sub-division of the far ultraviolet into smaller regions, fig.l.1. As oxygen is the principal absorbing gas in this region and vacuum techniques are most commonly used to remove it, Boyce suggested renaming the region 0.1-200 nm the Vacuum Ultraviolet. The region 100-200 nm is the range over which radiation can pass through windows and prisms and is often called the Schumann Ultraviolet in honour of the pioneer of far ultraviolet spectroscopy. The wavelength range 0.1-100 nm Boyce suggested calling the Extreme Ultraviolet. It can be seen that this region overlaps with the soft x-ray region of the electromagnetic spectrum and soft x-rays have, in fact, been detected with a vacuum spectrograph at wavelengths above 0.47 nm, while extreme ultraviolet radiation of wavelength 0.11 nm has been detected using x-ray optics. Although these regions overlap, the transitions which produce the radiation are quite different. Ultraviolet radiation is produced by electronic transitions in the outer orbital of atoms or molecular bonds, while x-rays are produced by • inner electron transitions. Fig.1.1 The Electromagnetic Spectrum 1021 Freq. kHz 103 tcP 109 1012 1015 ids I I II i 1 1 NP.- -v.-- Radio,TM ,Radar i UV. Ylrays. AC. § Infr.SLIstqjLi ,LC:Fal..1_-, i I 1 r 1 I , i i 1 1 i Wavelength m.105 102 10 10-1 10-2 10-4 107 109 10-11 101210-14 Far Ultraviolet Spectrum 200, 100 30 01 nm SdIumann U.V. Soft --4 X-rays Extreme U.V. Vacuum UV. Schumann [2] built the first vacuum spectrograph in 1893 using a fluorite prism as the dispersing medium. The linear dispersion of a prism, D, depends,on the prism angle, 2a, the focal length of the spectrograph, f, the wavelength of incident radiation and constants of the prism material. [X-K1 ]2[1-sin2a] D ' [1.1] 2K2f sins Thus the linear dispersion of a prism depends upon the square of the wavelength of the incident radiation which makes precise wavelength measurement difficult. When he built the first spectrograph, Schumann was unable to make wavelength measurements as the constants for fluorite were unknown. He was, however, able to show that oxygen was responsible for the atmospheric absorption below 200 nm. Rowland [3] described the concave diffraction grating in 1882 and showed theoretically that the linear dispersion depended only on the radius of curvature of the grating, Re, the. groove separation on the grating surface, d, and the angle of emergent radiation, 3. nR c 9 [1.2] D = acospxl° The great advantage of this grating is that the linear dispersion is independent of the incident radiation 4 and is uniform over the whole wavelength range. The resolution of the grating can be doubled by use of the second order spectrum and, as the radiation is diffracted by the front surface of the grating, there is no limit to the transmission of the grating as there is for a prism. The prism, however, has its greatest linear dispersion in the Schumann ultraviolet. Lyman [4] constructed a vacuum spectrograph in 1906 using a concave diffraction grating as the dispersing medium. He was able to accurately measure the absorption spectrum of molecular oxygen with this instrument and also showed that fluorite would not transmit below 125 nm. Lyman also investigated the far ultraviolet spectra of, several sources and developed a far ultraviolet source, the flash tube. Since these early developments, vacuum spectrometers and Tuantometers have replaced spectrographs for most applications of far ultraviolet spectroscopy. Diffraction gratings are almost exclusively used as the dispersing medium for the reasons outlined above. Various methods for mounting diffraction gratings have been employed and Samson [5] has given a detailed account of the various types of monochromator and diffraction grating mounting used in far ultraviolet spectroscopy. Although vacuum techniques are common, they 5 are not essential for the detection of far ultraviolet radiation, as many gases are transparent in the Schumann ultraviolet. In 1961 Kaye [6] described the modification of a Beckmann DK solution spectrophotometer to extend the working range down to 170 nm. He investigated helium, argon, nitrogen and hydrogen as possible purging gases and found that commercially available argon and helium were completely transparent over the range 170-200 nm and were the best gases with which to purge, although he recommendednitrogen containing less than 10-3: of oxygen on economic grounds. Hydrogen was not recommended because of its explosive nature although it possesses high transparency in this region. The detection of far ultraviolet radiation The earliest method of detecting far ultraviolet radiation was the photographic process. In 1892 Schumann [7] demonstrated that the gelatin used in the emulsion to support the photosensitive grains on the plate was responsible for the absorption of radiation below 220 nm. To improve the sensitivity of the photographic plates and to extend their working range down into the far ultraviolet, he removed most of the gelatin from the plates. While these Schumann plates will detect radiation in the far ultraviolet, the plates are very sensitive to 6 abrasion and are-slow to develop; this may result in a poor quality of the photographic reproduction. In an attempt to improve the sensitivity of photographic plates to far ultraviolet radiation without imparing the quality of the photographic reproduction, it has been suggested that thin films of fluorescent laquers or oils be applied to the surface of the emulsion to act as sensitizing agents. The disadvantage of such a treatment is that the photographic plates have to be washed with an organic solvent to remove the sensitizer before the plate can be developed. Sodium salicylate is the most commonly used sensitizing agent [5] because it has an almost uniform quantum efficiency over the range 85-200 nm. This is usually applied by brushing a concentrated alcoholic solution of sodium salicylate onto the surface of the emulsion and allowing the solvent to evaporate. The great advantage of this sensitizing agent is that it is water soluble and is thus removed when the plate is immersed in the developer. The main disadvantage of all these sensitizers is that their application is a delicate and skilled operation which must be performed on each photographic' plate prior to its being loaded into the camera. The main difficulty in applying these sensitizers is obtaining a film that is both thin and uniform.

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