Continuous improvement of modern analytical chemistry techniques has increased the possibilities of measurements in the field of nuclear forensics. Once successful, they can be expanded to more isotopic systems with increasing sensitivity. The examples in nuclear forensics considered in this thesis are investigations of the undeclared radioactive ruthenium release in 2017 and the source identification of cesium isotope ratios. Air filter stations from the informal monitoring network “Ring of Five” permanently monitor and collect airborne particle samples. In October 2017, this network detected radioruthenium in its samples.Radioruthenium was trapped in the filters, suggesting that only the radionuclides 103Ru and 106Ru were released in this undeclared event. Since no other elements were observed, a pure ruthenium release must have occurred. “Ring of Five” stations exchanged data and suggestions of the origin and background. The combination of airborne concentration data and air dispersion modeling allowed the network to trace its origin back to the Southern Urals (Russia), where the nuclear facility complex “Majak Production Association” is located.Additional information such as age estimation was obtained by very accurate radioruthenium isotope measurements to reprocessed fuel of an likely age of less than two years. The reprocessing background and purity of the released ruthenium were determined by the complex chemistry of the behavior of different oxidation state compounds. It could be concluded that the release likely occurred in the form of RuCl3 and RuO2. These indicated a release during RuO4 trapping with HCl, which is a part of nuclear reprocessing.Going deeper in the area of isotope ratios, the stable ruthenium isotopes showed a strong contribution of non-natural ruthenium isotopes. A considerable shift from natural ratios was measured and could be linked to spent fuel from Russian reactor type VVER.So far, no organization, facility or state has assumed responsibility for this release.Isotopic ratio determination can strongly increase the knowledge about materials, age, origin, or processing. Advances in mass spectrometric techniques such as ICP-QQQ-MS enables the determination of previously difficult-to-measure isotopes or ratios due to interference, e.g. 135Cs/137Cs.For this ratio determination, interfering isobars were reduced by a three step chemical protocol. Barium interferences are reduced up to several orders of magnitude, while the average cesium recovery rate yielded 78 %.Typical isotopic ratios were found for biological samples with known Chernobyl, Fukushima and global weapon fallout signatures of 135Cs/137Cs. More importantly, an anomalous ratio was found for trinitite. This inconsistency was explained by different decay chain half-lifes, whereby the 135Xe mother nuclide is predominantly blown away.In conclusion, isotope ratios are an even more powerful tool to answer analytical questions than simple concentrations and activities due to the increased gain of information depth.