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 ?? 13C value of -26.24??? relative to VPDB and a ?? 15N 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 ?? 13C value of +37.76??? and a ??15N value of +47.57???. The ??13C and ??15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (??13C = +1.95???), L-SVEC lithium carbonate (??13C = -46.48???), IAEA-N-1 ammonium sulfate (??15N=0.43???), and USGS32 potassium nitrate (?? 15N = 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 ??13C is better than 0.13???, and that of ??15N is better than 0. 13??? in 100-??g 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 ?? 13C 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. Published in 2003 by John Wiley & Sons, Ltd.
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Two new organic reference materials for ??13C and ??15N measurements and a new value for the ?? 13C of NBS 22 oil