Development of a new measurement system for 15N of ammonium in polar ice cores

Lehmann, Prisca (2021). Development of a new measurement system for 15N of ammonium in polar ice cores. (Dissertation, Abteilung Klima- und Umweltpyhsik, Physikalisches Institut, Universität Bern, Philosophisch-Naturwissenschaftliche Fakultät)

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High-resolution analysis of ammonium ( NH4+) concentrations in polar ice samples has become a routine application. Using continuous flow analysis (CFA), ice samples are continuously melted,
contamination is efficiently removed and the NH4+ concentration is determined fluorimetrically. However, questions such as the contribution of different terrestrial and marine NH4+ sources, and the dependence of the measured concentration on the efficiency of aerosol formation, transport and deposition are still under discussion in the ice core community.
Due to different isotope signatures of terrestrial and marine NH4+ sources, the analysis of δ15N- NH4+ has great potential to contribute to the interpretation and basic understanding of temporal changes in the existing NH4+ records and, at the same time, be applied to new questions, for example the quantification of biological activity in the Southern Ocean. Thus far, however, there is no analytical method available that allows to measure δ15N- NH4+ in polar ice. Within the scope of this thesis, we started developing a method of analysing δ15N- NH4 in polar ice samples by coupling CFA and isotope ratio mass spectrometry (IRMS). The basic principle of the analysis is the melting of the ice sample using the well-established CFA setup, followed by the extraction of NH4+ (c = 1 – 20 ppb) from the meltwater stream with a suitable adsorber. The pre-concentrated NH4+ is then oxidised on-line to N2, oxidation by-products are separated from the analyte N2, and δ15N is determined by mass spectrometry (IRMS). The conception as well as the extensive testing of the extraction of NH4+ in our pre-concentration unit and the subsequent oxidation to N2 in our thermal conversion unit are the main foci of this dissertation.
This thesis discusses the most important findings of the experiments performed, but also the problems that occurred, as well as their causes and solutions. For the pre-concentration unit, this includes the selection of zeolites as adsorber material and the subsequent tests to characterise adsorption and desorption properties. In the case of the thermal conversion unit, the investigations concentrated in particular on the oxidation of ammonia (NH3) with copper/copper oxide, the formation of nitrogen monoxide (NO) as an undesired by-product, and the optimisation of the temperature conditions for the conversion of NH3 to N2.
With the aforementioned experiments it was shown that by using the zeolite mordenite, dissolved NH4+ (c = 1.5 – 5.5 μM) is almost completely adsorbed within 3 minutes. At thermal conditions of 570°C, the pre-concentrated NH4+ is released in the form of NH3 and is available in the gas phase for subsequent oxidation. Using copper/copper oxide, the oxidative conversion of NH4+-N to N2 is successfully achieved at a temperature of approximately 500°C. The yield of this conversion is estimated to be 90%. The oxidation and δ15N analysis of the NH3 working standard is possible with a reproducibility of 0.4‰. For experiments on thermal desorption, the investigations of occurring fractionation effects are not yet completed, and currently no statement on the reproducibility of δ15N in ice-core samples is possible yet.

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