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 C-13 and N-15 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 delta(13)C value of -26.24parts per thousand relative to VPDB and a delta(15)N value of -4.52parts per thousand relative to N-2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in C-13 and N-15. USGS41 has a delta(13)C value of +37.76parts per thousand and a delta(15)N value of +47.57parts per thousand. The delta(13)C and delta(15)N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta(13)C = +1.95parts per thousand), L-SVEC lithium carbonate (delta(13)C = -46.48parts per thousand), IAEA-N-1 ammonium sulfate (delta(15)N = 0.43parts per thousand), and USGS32 potassium nitrate (delta(15)N = 180parts per thousand) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta(13)C is better than 0.13parts per thousand, and that of delta(15)N is better than 0.13parts per thousand in 100-mug 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 delta(13)C value for NBS 22 oil of -29.91parts per thousand, in contrast to the commonly accepted value of -29.78parts per thousand for which off-line blank corrections probably have not been quantified satisfactorily. Published in 2003 by John Wiley Sons, Ltd.