On top of the characterization of bulk isotope compositions in samples, we are interested in the stable isotope composition mainly of hydrogen and carbon in different organic substance classes to trace carbon and nitrogen uptake and incorporation into individual molecules or to trace the fate of organic matter in the environment at a molecular level. This can be achieved after extraction and purification of the compound classes of interest, followed by gas chromatographic separation into individual molecules and the conversion in CO2 and H2, which is then transferred and directly measured on our second isotope ratio mass spectrometer. This instrument is also connected to a gas bench, in which we prepare samples online for carbonate measurements. After addition of phosphoric acid, the CO2 released from the samples is purified in the gas bench and then measured on the isotope ratio mass spectrometer for stable carbon and oxygen isotope compositions.