Automatic simultaneous determination of total carbon and ¹³c as well as total nitrogen and ¹⁵n in isotopically enriched samples of soil and plant material using a quadrupole mass spectrometer coupled to an elemental analyser

Abstract

Elemental analyser‐isotope ratio mass spectrometer coupling represents the state of the art for the rapid, automatic, high‐precision determination of carbon and nitrogen levels along with their isotopic composition of soil and plant samples in one run. A quadrupole mass spectrometer interfaced with an elemental analyser (EA‐QMS) provides a low‐cost, easy‐to‐use alternative for artificially enriched stable isotopes such as tracer in biological processes — especially as such processes (e.g. soil processes) are already characterised by high biological variability. A quadrupole mass spectrometer MS CUBE (Balzers AG) is connected to a VARIO EL elemental analyser (Elementar GmbH). The reproducibility attained with this set‐up is < 0.2% RSD for natural 15N abundance and better than 0.12% RSD for abundance > 1 at.% in the case of nitrogen levels > 100 ug. This performance enables reliable results up to enrichments of > 0.003 at. % excess (8 %o in δ notation) 15N. The limit of nitrogen analysed is 10 ug. The precision for the natural 13C abundance of homogeneous substances is < 0.05%, permitting reliable results up to enrichments of > 0.002 at.% excess (2%o in δ notation). The operation of this EA‐QMS system is fully computer‐based and up to 80 samples can be automatically processed in one run. Each measurement takes about 15 min.