In recent years, the energy demand has been continuously increasing. At the same time, fossil fuels are being progressively replaced by renewables. However, this shift from fossil fuels such as coal to renewable fuels like wood creates new challenges, as many industrial plants continue to rely on legacy fuels. Unlike coal, the elements present in renewable resources can vary greatly. The differences are influenced by a variety of factors. For example, waste wood can be contaminated by different additives (paints, fire retardants, and others). To understand under which boundary conditions (e.g., temperature, gasification atmosphere) the respective elements are bound in the ash/slag or released into the gas phase, experiments with a molecular beam mass spectrometer (MBMS) with an upstream electrically heated flow reactor were conducted. Pieces of clean wood were impregnated with various heavy metals and examined under several boundary conditions (temperature and gasification atmosphere). Furthermore, impregnated cellulose partly mixed with single ash components served as model fuel for detailed investigations. Additionally, thermochemical equilibrium calculations were carried out. The results of the experiments show that the release of some heavy metals (Cd, Pb, Sb, Sn, Zn) is very strong already at low temperatures, while for others (Cr, Cu) no release can be detected even at high temperatures. The corresponding thermodynamic equilibrium calculations comply with these findings. Since the process management and preparation of the fuels can be adjusted accordingly, these results form an important basis for planning gasification processes using waste wood as fuel.