Tyler B. Coplen Heike Geilmann Willi A. Brand J.K. Böhlke Haiping Qi 2003 <p><span>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 </span>¹³<span>C and </span>¹⁵<span>N 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 </span><i>δ</i>¹³<span>C value of −26.24‰ relative to VPDB and a </span><i>δ</i>¹⁵<span>N value of −4.52‰ relative to N</span>₂<span> in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in </span>¹³<span>C and </span>¹⁵<span>N. USGS41 has a </span><i>δ</i>¹³<span>C value of +37.76‰ and a </span><i>δ</i>¹⁵<span>N value of +47.57‰. The </span><i>δ</i>¹³<span>C and </span><i>δ</i>¹⁵<span>N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (</span><i>δ</i>¹³<span>C = +1.95‰), L-SVEC lithium carbonate (</span><i>δ</i>¹³<span>C = −46.48‰), IAEA-N-1 ammonium sulfate (</span><i>δ</i>¹⁵<span>N = 0.43‰), and USGS32 potassium nitrate (</span><i>δ</i>¹⁵<span>N = 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 </span><i>δ</i>¹³<span>C is better than 0.13‰, and that of </span><i>δ</i>¹⁵<span>N 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 </span><i>δ</i>¹³<span>C 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.</span></p> application/pdf 10.1002/rcm.1219 en Wiley Two new organic reference materials for δ13C and δ15N measurements and a new value for the δ13C of NBS 22 oil article