Rationale

The widely used l-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.

Methods

USGS41a was prepared by dissolving analytical grade l-glutamic acid enriched in ¹³C and ¹⁵N together with l-glutamic acid of normal isotopic composition. The δ¹³C and δ¹⁵N values of USGS41a were directly or indirectly normalized with the international reference materials NBS 19 calcium carbonate (δ¹³C_VPDB = +1.95 mUr, where milliurey = 0.001 = 1 ‰), LSVEC lithium carbonate (δ¹³C_VPDB = −46.6 mUr), and IAEA-N-1 ammonium sulfate (δ¹⁵N_Air = +0.43 mUr) and USGS32 potassium nitrate (δ¹⁵N = +180 mUr exactly) by on-line combustion, continuous-flow isotope-ratio mass spectrometry, and off-line dual-inlet isotope-ratio mass spectrometry.

Results

USGS41a is isotopically homogeneous; the reproducibility of δ¹³C and δ¹⁵N is better than 0.07 mUr and 0.09 mUr, respectively, in 200-μg amounts. It has a δ¹³C value of +36.55 mUr relative to VPDB and a δ¹⁵N 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.

Conclusions

The new isotopic reference material USGS41a can be used with USGS40 (having a δ¹³C_VPDB value of −26.39 mUr and a δ¹⁵N_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.