Isotopic stability of mass reference materials and possible use as isotopic reference materials

Abstract

Natural isotope variation or fractionation depends on equilibrium and kinetic processes affecting the individual isotope. The purpose of the of this work is to determine the stability and reproducibility of historical data of δ(15N/14N) abbreviated as δ15N, and the δ(13C/12C), abbreviated as δ13C, of total N and C in three solid samples (Aspartic acid, Nicotinamide and Acetanilide). These three samples contain N and C. The primary reference material for relative N isotope-ratio measurements (δ15N) used were atmospheric nitrogen gas (N2), which is widespread and homogeneous and by convention, has a δ15N consensus value of 0‰. The primary reference material for relative C isotope-ratio measurements (δ13C) is the L-SVEC lithium carbonate which have consensus value of -46.6±0‰ on the Vienna Peedee Belemnite (VPDB scale. The secondary references used were IAEAN1 (+0.4±0.2‰), IAEAN2 (+20.3±0.2‰) for δ15N and IAEACH3 (-24.724±0.041‰), IAEACH6 (-10.449±0.033‰) for δ13C. The δ15N and δ13C data were provided for the Laboratory of Isotope Ration Mass Spectrometry (IRMS) of the Department of Geology University of Tartu. The δ15N and δ13C measurements were made with a Delta V Plus CF-IRMS, which alternately measures the isotope-amount ratios of the sample N2 and CO2 gases and one or more injections of the working reference N2 and CO2 gases. From the data and results obtained for uncertainties for the data provided for the 2014-2020, we obtained the following results: for Aspartic acid δ15N (-7.0±1.6 ‰), δ13C (2.0±28.6‰), for Nicotinamide δ15N (-2.0±1.0 ‰), δ13C (-34.5±0.7‰) and for Acetanilide δ15N (1.1±1.1 ‰), δ13C ( -27.0±0.9‰). These uncertainties probably could be due to fractionation process happening during sample preparation and transformation to gas before entering the IRMS. From the analysis of the stability we can conclude that the Aspartic acid (+35.1774 to -31.2543), Nicotinamide (-34.1254 to-34.7113) and Acetanilide (-26.5022 to -27.2740) are more stable for δ13C, but unstable for δ15N during, during 2014-2020. Lastly there is a good long-term reproducibility (SRW) only for Acetanilide and Nicotinamide with % RSD pooled of 1% for δ13C.