Since the end of the 80s elemental analyzer-isotope ratio mass spectrometer connections have been used for the fast, automatic and highly precise determination of carbon and nitrogen content as well as their isotopic composition in one run. But for artificially enriched stable isotopes as tracer in biological processes and since these processes have a high biological variability anyway (e.g. soil processes) the use of these highly precise but also sophisticated and expensive instruments is not required. In this case the use of a quadrupole mass spectrometer connected with an elemental analyzer can offer a low cost alternative. As shown, such coupling is suitable for automatic simultaneous routine analysis of total nitrogen and carbon and their isotopic enrichment (¹⁵N, ¹³C) in plant material and soils. The relative standard deviation for ¹⁵N and ¹³C determination is 2% To meet this precision a careful sample homogenization by grinding is very important. The duration of one measurement is 6–8 min. depending on whether nitrogen alone or both nitrogen and carbon are determined. This enables a high sample throughput.