title: A new organic reference material, l-glutamic acid, USGS41a, for δ(13) C and δ(15) N measurements - a replacement for USGS41.
abstract: BACKGROUND
  The widely used l-glutamic acid isotopic reference material USGS41, enriched in both (13) C and (15) N, 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 (13) C and (15) N together with l-glutamic acid of normal isotopic composition. The δ(13) C and δ(15) N values of USGS41a were directly or indirectly normalized with the international reference materials NBS 19 calcium carbonate (δ(13) CVPDB = +1.95 mUr, where milliurey = 0.001 = 1 ‰), LSVEC lithium carbonate (δ(13) CVPDB = -46.6 mUr), and IAEA-N-1 ammonium sulfate (δ(15) NAir = +0.43 mUr) and USGS32 potassium nitrate (δ(15) N = +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 δ(13) C and δ(15) N is better than 0.07 mUr and 0.09 mUr, respectively, in 200-μg amounts. It has a δ(13) C value of +36.55 mUr relative to VPDB and a δ(15) N 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 δ(13) CVPDB value of -26.39 mUr and a δ(15) NAir 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.
authors.affiliation: U.S. Geological Survey, 431 National Center, Reston, VA, 20192, USA.
authors.forename: Haiping
authors.initials: H
authors.lastname: Qi
authors.affiliation: U.S. Geological Survey, 431 National Center, Reston, VA, 20192, USA.
authors.forename: Tyler B
authors.initials: TB
authors.lastname: Coplen
authors.affiliation: U.S. Geological Survey, 431 National Center, Reston, VA, 20192, USA.
authors.forename: Stanley J
authors.initials: SJ
authors.lastname: Mroczkowski
authors.affiliation: Max-Planck-Institute for Biogeochemistry, P.O. Box 10 01 64, 07701, Jena, Germany.
authors.forename: Willi A
authors.initials: WA
authors.lastname: Brand
authors.affiliation: U.S. Geological Survey, 431 National Center, Reston, VA, 20192, USA.
authors.forename: Lauren
authors.initials: L
authors.lastname: Brandes
authors.affiliation: Max-Planck-Institute for Biogeochemistry, P.O. Box 10 01 64, 07701, Jena, Germany.
authors.forename: Heike
authors.initials: H
authors.lastname: Geilmann
authors.affiliation: Department of Geological Sciences, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Arndt
authors.initials: A
authors.lastname: Schimmelmann
mesh_terms: D002247:Carbon Isotopes / Q000032:analysis* / Q000737:chemistry; D018698:Glutamic Acid / Q000032:analysis* / Q000737:chemistry*; D013058:Mass Spectrometry / Q000592:standards*; D009587:Nitrogen Isotopes / Q000032:analysis* / Q000737:chemistry; D012015:Reference Standards; D015203:Reproducibility of Results
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D018698:Glutamic Acid