In the atmosphere, liquid water can be supercooled down to -38C, since freezing does not only depend on thermodynamics but is steered by kinetics. The phase transition from liquid to solid occurs either homogenously where just water molecules are involved or heterogeneously, catalyzed by foreign particles, so-called ice nuclei (IN). IN trigger the formation of ice clouds, which are of interest due to their importance for climate and weather. The influence of biological IN on atmospheric processes is still not fully understood and many questions are still remaining. How exactly do biological IN (BIN) influence cloud formation? How does the transport process of BIN into the free troposphere work? Which chemical properties and structures do BIN have? This thesis focuses on the later of these questions. Betula pendula birch pollen, known to be ice nucleation active, were analyzed to characterize their IN. For the analyses birch pollen washing water (BP-WW) was used. ^Since many different substances are present in birch pollen, solid phase extraction was performed to reduce sample complexity. Results revealed that IN within birch pollen may have amphiphilic properties. Since Pummer et al. (2012) it is known that IN from birch pollen are within a macromolecular size range. In this thesis we used centrifugal filtration to get information about the size of IN. When using a filter with a 30 kDa cut-off, the retentate remained ice nucleation active, whereas the filtrate did not, indicating that IN are larger than 30 kDa. Results from the analysis of BB-WW with the Agilent Bioanalyzer 2100 revealed that there are different proteinaceous substances in varying sizes present in the pollen. However, ice nucleation activity of those separated substances could not be tested. For the separation of different substances within BP-WW, fractions obtained from SPE and centrifugal filtration, capillary electrophoresis (CE-UV) was used. ^Many various components were found to be present in birch pollen. However, IN could not be detected with this method. According to LC-ESI-MS and Tandem-MS (ESI-MS/MS and ESI-MS3) experiments, the flavanol quercetin linked with two hexose sugars was found to be present in birch pollen. It is thought to be responsible for the yellow color of the pollen. However, results verified that this flavonoid is not ice nucleation active and thus no IN from birch pollen.