| Summary: | A new method for the determination of sulphur isotopic ratios by FTIR measuring the area or absorbance of individual lines, has been developed. The project could be divided into three parts: (i) In part one the infrared spectra of four sulphur gases SCO, H<sub>2</sub>S, SF<sub>6</sub> and SO<sub>2</sub>, were collected and studied to evaluate which one would be the most appropriate to determine sulphur isotopic ratios. The synthesis of these gases was also assessed and performed in those cases where the means to carry them out were available. This part was designed to test the viability of these compounds as a means of extracting sulphur from natural samples. (ii) In part two, the methodology for the determination of sulphur isotopic ratios using the infrared spectrum of sulphur dioxide, gas chosen for this project, was performed. Different factors affecting the precision of the method, such as resolution, number of scans, etc., were evaluated and optimized. The UV/Vis spectrum of this gas was collected and the viability of using the L(1)-bound overdetermined Gaussian elimination method for assessing sulphur isotopic ratios over data obtained from this spectrum as well as from its infrared spectrum, was also tested. Both methods, integration of individual lines in the infrared spectrum of SO<sub>2</sub>, and use of the L(1)-bound overdetermined Gaussian elimination method over data also obtained from the infrared spectrum of sulphur dioxide, proved to be applicable when determining isotopic ratios in enriched samples. (iii) In part three, levels of sulphur-34 and nitrogen-15 in the biomass of four arable Scottish soils were determined. A slight enrichment in both isotopes was detected for one of the soils, being the first time such a measurement has been carried out. Sulphur and nitrogen deficiency in this soil are given as a possible explanation although the lack of sufficient number of data do not allow us to establish with certainty its cause.
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