Tyler B. Coplen Stanley J. Mroczkowski Willi A. Brand Lauren Brandes Heike Geilmann Arndt Schimmelmann Haiping Qi 2016 <h3>Rationale</h3><p>The widely used <span class="smallCaps">l</span>-glutamic acid isotopic reference material USGS41, enriched in both ¹³C and ¹⁵N, is nearly exhausted. A new material, USGS41a, has been prepared as a replacement for USGS41.</p><h3>Methods</h3><p>USGS41a was prepared by dissolving analytical grade <span class="smallCaps">l</span>-glutamic acid enriched in ¹³C and ¹⁵N together with <span class="smallCaps">l</span>-glutamic acid of normal isotopic composition. The <i>δ</i>¹³C and <i>δ</i>¹⁵N values of USGS41a were directly or indirectly normalized with the international reference materials NBS 19 calcium carbonate (<i>δ</i>¹³C_VPDB = +1.95 mUr, where milliurey = 0.001 = 1 ‰), LSVEC lithium carbonate (<i>δ</i>¹³C_VPDB = −46.6 mUr), and IAEA-N-1 ammonium sulfate (<i>δ</i>¹⁵N_Air = +0.43 mUr) and USGS32 potassium nitrate (<i>δ</i>¹⁵N = +180 mUr exactly) by on-line combustion, continuous-flow isotope-ratio mass spectrometry, and off-line dual-inlet isotope-ratio mass spectrometry.</p><h3>Results</h3><p>USGS41a is isotopically homogeneous; the reproducibility of <i>δ</i>¹³C and <i>δ</i>¹⁵N is better than 0.07 mUr and 0.09 mUr, respectively, in 200-μg amounts. It has a <i>δ</i>¹³C value of +36.55 mUr relative to VPDB and a <i>δ</i>¹⁵N value of +47.55 mUr relative to N₂ in air. USGS41 was found to be hydroscopic, probably due to the presence of pyroglutamic acid. Experimental results indicate that the chemical purity of USGS41a is substantially better than that of USGS41.</p><h3>Conclusions</h3><p>The new isotopic reference material USGS41a can be used with USGS40 (having a <i>δ</i>¹³C_VPDB value of −26.39 mUr and a <i>δ</i>¹⁵N_Air value of −4.52 mUr) for (i) analyzing local laboratory isotopic reference materials, and (ii) quantifying drift with time, mass-dependent isotopic fractionation, and isotope-ratio-scale contraction for isotopic analysis of biological and organic materials. Published in 2016. This article is a U.S. Government work and is in the public domain in the USA.</p> application/pdf 10.1002/rcm.7510 en Wiley A new organic reference material, L-glutamic acid, USGS41a, for δ¹³C and δ¹⁵N measurements − a replacement for USGS41 article