Determination of the δ¹⁵N of total nitrogen in solids; RSIL lab code 2893

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 2893 is to determine the δ(¹⁵N/¹⁴N), abbreviated as δ¹⁵N , of total nitrogen in solid samples. A Carlo Erba NC 2500 elemental analyzer (EA) is used to convert total nitrogen in a solid sample into N₂ gas. The EA is connected to a continuous flow isotope-ratio mass spectrometer (CF-IRMS), which determines relative difference in the isotope-amount ratios of stable nitrogen isotopes (¹⁵N/¹⁴N)of the product N₂ gas. The combustion is quantitative; no isotopic fractionation is involved. Samples are placed in a tin capsule and loaded into the Costech Zero Blank Autosampler of the EA. Under computer control, samples are dropped into a heated reaction tube that contains an oxidant, where the combustion takes place in a helium atmosphere containing an excess of oxygen gas. Combustion products are transported by a helium carrier through a reduction tube to remove excess oxygen and 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 isotope-ratio mass spectrometer (IRMS) through a Finnigan MAT (now Thermo Scientific) ConFlo II interface, which also is used to inject N₂ reference gas and helium for sample dilution. The IRMS is a Thermo Scientific Delta V Plus CF-IRMS. It has a universal triple collector, two wide cups with a narrow cup in the middle, capable of measuring mass/charge (m/z) 28, 29, 30, simultaneously. The ion beams from N₂ are as follows: m/z 28 = N₂ = ¹⁴N¹⁴N; m/z 29 = N₂ = ¹⁴N¹⁵N primarily; m/z 30 = NO = ¹⁴N¹⁶O primarily, which is a sign of contamination or incomplete reduction.