Two new organic reference materials for δ13C and δ15N measurements and a new value for the δ13C of NBS 22 oil

Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different ¹³C and ¹⁵N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a δ¹³C value of −26.24‰ relative to VPDB and a δ¹⁵N value of −4.52‰ relative to N₂ in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in ¹³C and ¹⁵N. USGS41 has a δ¹³C value of +37.76‰ and a δ¹⁵N value of +47.57‰. The δ¹³C and δ¹⁵N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (δ¹³C = +1.95‰), L-SVEC lithium carbonate (δ¹³C = −46.48‰), IAEA-N-1 ammonium sulfate (δ¹⁵N = 0.43‰), and USGS32 potassium nitrate (δ¹⁵N = 180‰) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of δ¹³C is better than 0.13‰, and that of δ¹⁵N is better than 0.13‰ in 100-μg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a δ¹³C value for NBS 22 oil of −29.91‰, in contrast to the commonly accepted value of −29.78‰ for which off-line blank corrections probably have not been quantified satisfactorily.