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1. The effect of deviations from Beer's law on the precision of atomic absorption analysis has been examined from a theoretical point of view, and a function has been derived which makes it possible to evaluate quantitatively the effect of calibration curvature on the precision of analysis. The influence of incomplete sample volatilization on calibration curvature has been briefly investigated. 2. Possible error sources in atomic absorption spectrometry have been classified according to the "error function" (i.e., the dependence, upon transmittance T, of the uncertainty dT in a given transmittance measurement) with which they are associated. The magnitude of the contribution from each component function to the overall error function has been evaluated quantitatively, and it has been shown that the major component in nearly every case examined is that associated with the dynamic nature of the flame. Concentration ranges for optimum precision are suggested. 3. The effect of varying instrumental parameters on precision has been investigated, and generalized conditions for best precision have been ascertained. 4. The effect of an initial solvent extraction step on the precision of atomic absorption has been investigated for the elements copper and lead. It is shown that solvent extraction may be used to improve both the analytical sensitivity and the precision of analysis when very low concentrations of metal are determined. 5. The precision of analytical methods involving atomic absorption spectrometry has been studied, and the standard deviations compared with those obtained for the analysis of similar samples by means of a variety of other methods of analysis, both instrumental and classical.

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The use of a handheld Raman analyser for the positive identification of raw materials in a manufacturing pharmaceutical company was evaluated using paracetamol as test raw material to evaluate whether such a system would meet Aspen’s regulatory requirements. The approach involved subjecting the chosen raw material to identification tests under a variety of conditions so as to evaluate robustness, and specificity of the system. Thus, raw material provided by different suppliers, different packages of one manufacturing batch, and raw materials subjected to different storage conditions were evaluated. Specificity was evaluated by deliberately contaminating a sample of paracetamol with either acetanilide, or 4-aminophenol, or both at varying concentration levels. The results obtained from these investigations showed that the handheld Raman analyser can correctly identify the selected raw material (paracetamol) under a wide range of conditions, but could not correctly identify the presence of the selected contaminants at lower concentration levels (< 10 – 20 mass percent). Finally, a cost-benefit analysis was carried out in which a scenario of an existing FTIR-ATR system is used for the analysis of a specific number of raw material samples per year as opposed to a scenario in which a new handheld Raman analyser has to be purchased, set up, and used for the analysis of the same number of raw material samples. This comparison showed that the handheld Raman analyser had a pay-back time of approximately 6 months and gave a return on investment of approximately the same value as the actual purchase cost.

In this study, we have investigated the photo-response of gallium antimonide (GaSb) by measuring the spectral, injection level, as well as the temperature dependence of the steady-state photoconductivity of epitaxial films. By combining the various photo-response measurements with-galvanometric measurements, the effective excess carrier lifetimes could be calculated and described in terms of the various experimental variables considered. From a comparison of the measured and theoretical carrier lifetimes, a clear discrepancy was found for the injection dependent results. This inconsistency between the expected and measured behaviours was resolved using a two-layer model to describe the photo-conductive response of the thin film. In this model, the generation/recombination processes are considered for the bulk as well as for a depleted near-surface region. By combining these two contributions when calculating the total excess photocurrent, the various experimental results could be successfully described. The photo-response measurements revealed that the main contribution to the photocurrent came from the near-surface region, where the bandto- band and Auger recombination transitions are reduced. From the simulation of the injection level dependence of the excess carrier lifetime, some of the near-surface characteristics, such as the position of the surface Fermi-level and the surface recombination velocity, could be determined. In the case of p-type GaSb, the room temperature surface Fermi-level was found to be pinned (290±20) meV above the valence band maximum, whereas the n-type material had a pinning position of (150±20) meV above the valence band maximum. These pinning positions were shown to be independent of the doping density and relatively insensitive to the surface treatments considered. The presence of a near-surface depletion region, as well as the position of the surface Fermi-level, was corroborated by Raman spectroscopy. From a comparison of the phonon mode and the phonon-hole plasmon coupled mode, a surface Fermi-level position of (300 ± 30) meV was deduced for p-type GaSb. Finally, the effect various surface treatments have on the photo-response, and related surface properties, were investigated. Removal of the native oxide (HCl:H2O) followed by sulphur passivation (Na2S:9H2O) results in a slight decrease in the surface Fermi-level position. Aging studies however revealed that the surface characteristics reverted back to the untreated values following a few days in air. Coating the GaSb surfaces with a thin ZnS layer was found to have little effect on the surface potential, resulting only in a slight increase in the near-surface donor density. The sensitivity of the measured photocurrent to surface treatments and changes in the ambient with cooling, further validate the importance of considering the surface potential and the related space-charge when describing the photoconductive response of GaSb thin films.

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Triphenyl-, trimethyl- and dimethyltin(IV), and trimethylantimony(V) derivatives of a wide variety of acids, including those of very strong acids, as well as derivatives of a transition metal oxyanion, were synthesized. Their structural characteristics in the solid state, under strictly anhydrous conditions, were determined from their infrared spectra. These spectroscopic results can only be interpreted in terms of a very strong interaction between the organometal group and the corresponding anionic group, and provide strong evidence for coordination or partial covalent bonding between the organometal group and the anionic group. Contrary to earlier reports, no evidence is found for the existence of free R₃Sn⁺ , R2Sn²⁺ or R₃Sb²⁺ cations in the solid state. / Science, Faculty of / Chemistry, Department of / Graduate

Infrared absorption spectra of HC1 and HBr, suspended in solid argon, krypton and nitrogen, were recorded in order to obtain information on intermolecular forces. SO₂ in argon and nitrogen, and CO in argon were also studied. The spectra were observed in the temperature range from liquid helium temperatures up to the melting point of the matrix. The halogen acids gave more complicated spectra in the noble gas matrices than in nitrogen. This has been correlated with the different thermal properties of the matrix materials. Matrix to solute ratios from 100 to 800 to 1 were used and evidence was found for solute-solute interactions, arising from incomplete isolation of solute molecules at the lower ratios. During the warm-up period at the end of an experiment, additional peaks appeared in the spectra. It is suggested that these new peaks were due to clusters of solute molecules produced by diffusion of the solute through the lattice. Semi-empirical calculations were carried out to estimate shifts of vibrational frequencies of the trapped molecules. From these calculations it was concluded that repulsive intermolecular forces play an important part in determining the magnitude, and direction of the shifts. A first order perturbation calculation was made, using a Lennard-Jones' potential, to determine the effect of the matrix on the rotational energy levels of a trapped molecule. Spectra of the clathrate-hydrates of SO₂, H₂S and krypton were recorded at liquid nitrogen temperatures, and the SO₂ hydrate was studied in the temperature range from 4° to 120° K. The spectrum of the water skeletal vibrations exhibited several interesting features. The assignment of the 1600 cmˉ¹ and 2200 cmˉ¹ peaks to v₂ and v₂ + vR was confirmed and a new peak at 2410 cmˉ¹ was observed. A lattice mode in the spectrum of the S0₂ hydrate was observed in combination with V₃ of SO₂. / Science, Faculty of / Chemistry, Department of / Graduate