δ13C and δ15N analyses of solid, liquid and gaseous samples
Elemental Analysis – Isotope Ratio Mass Spectrometry (EA-IRMS)
Bulk material from soils, plants and other sources can be combusted quantitatively to CO2 (and other oxides if applicable). In a second step the produced NOx is reduced to elemental N2. Thus the δ15N and δ13C composition of carbon and nitrogen containing bulk samples can be analysed. Details on the analytical principles can be found in Brooks et al. (2003) and Werner et al. (1999). The BGC-IsoLab uses an EA (NA 1110, CE Instruments, Milan, Italy) coupled to a Delta+XL IRMS (Thermo Finnigan, Bremen, Germany) via a ConFlow III. The autosampler has been modified to allow the analysis of gaseous samples (e.g. CH4 or N2O). A 10-port valve with two 1 ml sample loops can be connected to the autosampler allowing the alternating measurements of gaseous samples with solid samples. In this way such samples can be standardised against solid primary standards (Mohn et al., 2016; Sperlich et al., 2016).
Solid samples and standards are weighed into tin-capsules so that the anticipated signal of each is of similar intensity. We aim for signal intensities around 6000 mV to improve measurement precision. Samples are standardised against in-house standards Acetylanilide (Ali-j3; δ13C: -30.06 ± 0.1 ‰, δ15N: -1.51 ± 0.1 ‰) and Caffeine (Caf-j3 ; δ13C: -40.46 ± 0.1 ‰, δ15N: -15.46 ± 0.1 ‰). See Figure 2 for the long term performance of the quality control measurements of caffeine. The details on the necessary post-measurement corrections and on the structure of a sample sequence can be found in Werner and Brand (2001).