Keywords:stable isotopes, international reference material, carbonate, IAEA-603, SEM
The international reference material IAEA-603 prepared from Carrara marble (calcite) was newly released on 2016 to replace the reference material NBS19 (exhausted). From the reference sheet of IAEA-603 (Fajgelj and Assonov, 2016), assigned carbon and oxygen isotopic (δ13C and δ18O) values are +2.46±0.01‰ and -2.37±0.04‰, respectively at a sample size of about 120 µg. For sub-microgram scale isotopic analysis, we examined the grain-scale stable carbon and oxygen isotopic variations of IAEA-603. The individual grains of IAEA-603 (grain size: 200 to 760 µm) were measured with an IsoPrime100 isotope ratio mass spectrometer with customized continuous-flow gas preparation system (MICAL3c, Ishimura et al., 2004, 2008) at National Institute of Technology, Ibaraki College, Japan, and the individual weight of each grain estimated from the reacted CO2 gas volume was 4-90μg. The standard deviations (1σ uncertainties) of δ13C and δ18O of the individual grains of IAEA-603 are ±0.07‰ and ±0.16‰, respectively (N = 16), and the grain-scale analytical results also indicated homogeneous values as same as stable isotopes of the single grains of NBS19 (the standard deviation: ±0.10‰ in δ13C, ±0.16‰ in δ18O, N = 16, Ishimura et al., 2008). In IAEA-603, δ13C and δ18O ranged within 0.2‰ and 0.4‰, respectively. Both δ13C and δ18O of the individual grains showed non-significant grain-size dependencies. From the scanning electron microscope observations, the individual grains of IAEA-603 were dominantly polycrystalline with translucent grains composed of coarse crystals, and contained a certain amount of white opaque grains containing fine crystals. On the other hand, NBS19 showed single-crystal-like grains. Thus, as the reference sheet of IAEA-603 (Fajgelj and Assonov, 2016) mentioned, the storage in sealed ampules is important for avoiding exchange with atmospheric moisture and atmospheric CO2, especially in polycrystalline IAEA-603.