Solid - δ13C, δ15N
We use a Costech Elemental Analyzer, Conflo III, MAT253 for a continuous flow based measurement of our solid organic material δ13C and δ15N. Samples are flash combusted at 1000 °C with excess oxygen in a Costech ECS 4010 Elemental Analyzer (EA) equipped with a zero-blank autosampler. The combustion column was packed with cobaltous oxide for a combustion aid and silvered cobaltous oxide to scrub sulfur compounds. A helium carrier set to 80 mL / min moves combustion products through a 700 °C reduced copper column to ensure complete reduction of NOx compounds to N2 and also to absorb all excess oxygen. Sample gases were then carried through a magnesium perchlorate trap to absorb all water and then through a 3 m gas chromatography column to separate N2 from CO2. The EA effluent flows into a ThermoFinnigan Conflo III where the stream is split to reduce the flow rate from 80 mL / min to 2 mL / min. The reduced flow rate then flows through an open split with excess helium via fused silica capillary tubing. A pressure differential between the ThermoFinnigan MAT253 isotope ratio mass spectrometer and the Conflo III along fused silica capillary tubing allows helium and sample gas to move into the electron bombardment source for subsequent ionization, acceleration, focusing, separation, and measurement of masses 28, 29 for δ15N and 44, 45, 46 for δ13C. Internal laboratory reference materials are interspersed with samples for a two point calibration which allows for blank correction and conversion to the δ15N Air-N2 and the δ13C VPDB scales. We have an assortment of in-house standards calibrated against the international reference materials NBS19, LSVEC, IAEA-N-1, USGS32, USGS40, and USGS41.
The IsoDat software provided δ15N and δ13C values relative to the respective reference gas cylinders (along with all other selected data) are imported into a matlab script for data reduction. A linear regression is completed, independently for δ15N and δ13C, using the measured δ values and the accepted values of our internal reference materials. All data are corrected to the Air-N2 scale, for δ15N, and to the VPDB scale, for δ13C using these respective linear equations. Precision and accuracy are estimated for each run using a third internal reference material that is treated as an unknown. Reference materials are chosen to bracket the sample range in δ15N and δ13C values.
Standard Operating Procedures
- Barrie A, Davies JE, Park AJ, Workman CT. (1989). Continuous-flow stable isotope analysis for biologists. Spectroscopy 4(7), 42–52.
- Verardo DJ, Froelich PN, McIntyre A. (1990). Determination of organic carbon and nitrogen in marine sediments using the Carlo Erba NA-1500 Analyzer. Deep-Sea Research 37, 157-165. doi: 10.1016/0198-0149(90)90034-S