Molybdenum, a metal with excellent physical, chemical and mechanical properties, is an interesting material for applications e.g. in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is reduced because of the poor oxidation resistance and a brittle-to-ductile transition at room-temperature, which is influenced by the microstructure and the content of interstitial impurities. In this thesis, it was attempted to analyze grain boundary segregations in technically pure molybdenum to understand the influence of impurities on the resulting mechanical properties. Therefore, high sensitive analysis techniques such as atom probe tomography (APT), Auger electron spectroscopy (AES) and other chemical analysis methods were performed. APT is a powerful tool to study the concentration and location of segregations due to its almost atomic resolution for all elements. However, a site-specific sample preparation of grain boundaries with a dual focused ion beam/scanning electron microscope was required. Therefore, several methods such as the lift-out technique or the sharpening of electro-polished tips by focused ion beam were applied. The most applicable method was a combination of focused ion beam tip preparation with intermediate transmission electron microscope studies. Furthermore, AES analyses were carried out to compare the obtained results with the APT measurements.