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• W2247374429 abstract "The analysis of light stable isotopes of organic compounds is nowadays a common tool in almost all fields of science. Geochemistry, archeology, product authenticity and ecology are only a few examples of the applicability of isotope ratio mass spectrometry. In this context it can answer questions regarding the botanical or synthetical origin of a sample constituent or help to reveal reaction mechanisms. However, due to several instrumental challenges there is still a need for analytical methods to determine compound-specific isotope ratios of thermolabile and non-volatile substances. Hence, many compound classes have not yet been studied using isotope ratio techniques. Thus, the work presented herewith focuses on the development of liquid chromatographic methods which can be used in combination with isotope ratio mass spectrometry. As analytes several sulfonamide drugs, trimethoprim and the herbicide glyphosate along with its metabolite aminomethylphosphonic acid were chosen. The developed methods were used to study environmentally relevant degradation mechanisms of these compounds. In the first part of the work, a high temperature liquid chromatography method was developed for the analysis of sulfadiazine, sulfathiazole, sulfametazine, sulfamethoxazole and trimethoprim. By the application of a high temperature gradient to a pure aqueous eluent for separation, all these compounds were fully resolved from each other on an octadecyl stationary phase. The lowest amounts necessary for a precise carbon isotopic analysis varied between 0.3 to 0.5 µg on column depending on the investigated substance. A repeatable offset of isotope ratios of 1.6 to 3.6 ‰ on the δ-scale has been observed and a careful investigation revealed an incomplete conversion of the analytes to the measured gas CO2 causing this offset. Additionally, it could be shown that the observed offsets could be corrected to obtain unbiased isotopic signatures. The applicability of the method was shown by the analysis of pharmaceutical products containing trimethoprim and sulfamethoxazole and direct photolysis of sulfamethoxazole. It was shown that the isotopic fractionation of this important degradation process is very low, slightly above the overall analytical precision, and is probably based on a secondary inverse isotope effect. The other compounds studied in this work were glyphosate, one of the most widely used herbicides world-wide and its metabolite aminomethylphosphonic acid. For the analysis of these compounds ion exchange and reversed phase chromatography on a porous graphitic carbon column were investigated. The latter separation mode yielded the best separation of glyphosate and aminomethylphosphonic acid. Fractionation factors for the chromatographic separation could be calculated and it was shown that a carbon isotope effect associated with anion exchange is not detectable, while in contrast a carbon isotope substitution produces a very small but observable secondary isotope effect in the cation exchange of an amine group. Since these fractionation factors are very small, it can be concluded, that adsorption of glyphosate to soil resulting from van der Waals interactions and ion exchange will not change its carbon isotope ratio significantly. One of the developed methods, a reversed-phase chromatography on porous graphitic carbon, was used to study the isotopic fractionation of the degradation of glyphosate by manganese dioxide. The observed fractionation factor of about -3.5 ‰ was too big to originate from adsorption to the mineral surface. It was concluded that two reaction pathways lead to the cleavage of glyphosate and the observed isotope effect is a sum of both. A comparison with the calculated maximum isotope effect (Streitwieser limit) showed that other factors than zero point energy differences, such as differences in symmetry and mass moments of inertia, govern the isotopic fractionation of this reaction. Since there was no change in δ13C values of AMPA, it was concluded that the origin of this unspecific degradation product in the environment may be traced in future. Therefore, the last chapter of this work aimed to develop a method to determine position specific carbon isotope ratios of glyphosate, as this is a requisite in tracing the origin of AMPA. Besides, such a method might be useful for product authenticity control of glyphosate herbicide formulations. Two approaches, nuclear magnetic spectroscopy and an enzymatic cleavage followed by liquid chromatography isotope ratio mass spectrometry of the fragments, were chosen. However, it was not possible to determine accurate and precise carbon isotope ratios mainly due to non-linear response in nuclear magnetic resonance spectroscopy and unfavorable reaction conditions for the enzymatic cleavage. The chromatographic methods developed in this work enabled new insights on reaction pathways of glyphosate and sulfamethoxazole and will certainly be used in further investigations on these substances. But there is still work necessary to extend the application field of LC-IRMS to other important compounds and elements." @default.
• W2247374429 created "2016-06-24" @default.
• W2247374429 creator A5006844516 @default.
• W2247374429 date "2014-12-23" @default.
• W2247374429 modified "2023-09-24" @default.
• W2247374429 title "Liquid Chromatography coupled to Isotope Ratio Mass Spectrometry of Selected Polar Compounds" @default.
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