Methodical aspects of the 15N-analysis of Precambrian and Palaeozoic sediments rich in organic matter

A Precambrian and five Palaeozoic sediments from different stratigraphic units with levels of organic carbon ranging between 0.3 and 9.7 wt.% were used to test several common digestion procedures. The digestion techniques were evaluated with respect to investigating both the ammonium nitrogen and the total nitrogen in sediments rich in organic matter. Depending on the reaction conditions used, only N-NH4 was attacked when the HF/H2SO4 digestion technique was used, while on the other hand the total-N was released by the Kjeldahl digestion technique commonly applied to the N-extraction of organic material. In comparison to Kjeldahl digestion technique, an elemental analyzer coupled to a mass spectrometer (EA-MS) was used to obtain the total nitrogen content and N-isotope ratios from the test samples. The nitrogen concentrations given by this latter technique were lower and the δ15N values depleted in 15N compared to the digestion technique. This indicates that the ammonium-N in these sediments was not completely detected by the common EA-MS procedure.