Determination of the δ 15 N and δ 13 C of total nitrogen and carbon in solids; RSIL lab code 1832

Abstract

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 1832 is to determine the δ( 15 N/ 14 N), abbreviated as δ 15 N, and the δ( 13 C/ 12 C), abbreviated as δ 13 C, 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 N 2 and CO 2 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 ( 15 N/ 14 N) of the product N 2 gas and the relative difference in stable carbon isotope-amount ratio ( 13 C/ 12 C) of the product CO 2 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 N 2 and through a drying tube to remove water. The gas-phase products, mainly CO 2 and N 2 , 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 N 2 and CO 2 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 = N 2 = 14 N/ 14 N; m/z 29 = N 2 = 14 N/ 15 N primarily; m/z 30 = NO = 14 N/ 16 O primarily, which is a sign of contamination or incomplete reduction; m/z 44 = CO 2 = 12 C 16 O 16 O; m/z 45 = CO 2 = 13 C 16 O 16 O primarily; and m/z 46 = CO 2 = 12 C 16 O 18 O primarily.