Determination of the δ¹⁵N and δ¹³C of total nitrogen and carbon in solids; RSIL lab code 1832

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 1832 is to determine the δ(¹⁵N/¹⁴N), abbreviated as δ¹⁵N, and the δ(¹³C/¹²C), abbreviated as δ¹³C, of total nitrogen and carbon in a solid sample. A Carlo Erba NC 2500 elemental analyzer (EA) is used to convert total nitrogen and carbon in a solid sample into N₂ and CO₂ gas. The EA is connected to a continuous flow isotope-ratio mass spectrometer (CF-IRMS), which determines the relative difference in stable nitrogen isotope-amount ratio (¹⁵N/¹⁴N) of the product N₂ gas and the relative difference in stable carbon isotope-amount ratio (¹³C/¹²C) of the product CO₂ gas. The combustion is quantitative; no isotopic fractionation is involved. Samples are placed in tin capsules and loaded into a Costech Zero Blank Autosampler on the EA. Under computer control, samples then are dropped into a heated reaction tube that contains an oxidant, where combustion takes place in a helium atmosphere containing an excess of oxygen gas. Combustion products are transported by a helium carrier through a reduction furnace to remove excess oxygen and to convert all nitrous oxides into N₂ and through a drying tube to remove water. The gas-phase products, mainly CO₂ and N₂, are separated by a gas chromatograph. The gas is then introduced into the IRMS through a Finnigan MAT (now Thermo Scientific) ConFlo II interface. The Finnigan MAT ConFlo II interface is used for introducing not only sample into the IRMS but also N₂ and CO₂ reference gases and helium for sample dilution. The flash combustion is quantitative; no isotopic fractionation is involved. The IRMS is a Thermo Scientific Delta V CF-IRMS. It has a universal triple collector, two wide cups with a narrow cup in the middle; it is capable of measuring mass/charge (m/z) 28, 29, 30 or with a magnet current change 44, 45, 46, simultaneously. The ion beams from these m/z values are as follows: m/z 28 = N₂ = ¹⁴N/¹⁴N; m/z 29 = N₂ = ¹⁴N/¹⁵N primarily; m/z 30 = NO = ¹⁴N/¹⁶O primarily, which is a sign of contamination or incomplete reduction; m/z 44 = CO₂ = ¹²C¹⁶O¹⁶O; m/z 45 = CO₂ = ¹³C¹⁶O¹⁶O primarily; and m/z 46 = CO₂ = ¹²C¹⁶O¹⁸O primarily.