Analytical and Bioanalytical Chemistry

, Volume 405, Issue 1, pp 159–176 | Cite as

δ15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

  • Natacha Gentile
  • Michel J. Rossi
  • Olivier Delémont
  • Rolf T. W. Siegwolf
Original Paper

Abstract

Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ15N by elemental analysis (EA)–isotope ratio mass spectrometry (IRMS). Although the bulk nitrogen isotope analysis of organic material is not to be questioned, literature from different disciplines using IRMS provides hints that the quantitative conversion of nitrate into nitrogen presents difficulties. We observed abnormal series of δ15N values of laboratory standards and nitrates. These unexpected results were shown to be related to the tailing of the nitrogen peak of nitrate-containing compounds. A series of experiments were set up to investigate the cause of this phenomenon, using ammonium nitrate (NH4NO3) and potassium nitrate (KNO3) samples, two organic laboratory standards as well as the international secondary reference materials IAEA-N1, IAEA-N2—two ammonium sulphates [(NH4)2SO4]—and IAEA-NO-3, a potassium nitrate. In experiment 1, we used graphite and vanadium pentoxide (V2O5) as additives to observe if they could enhance the decomposition (combustion) of nitrates. In experiment 2, we tested another elemental analyser configuration including an additional section of reduced copper in order to see whether or not the tailing could originate from an incomplete reduction process. Finally, we modified several parameters of the method and observed their influence on the peak shape, δ15N value and nitrogen content in weight percent of nitrogen of the target substances. We found the best results using mere thermal decomposition in helium, under exclusion of any oxygen. We show that the analytical procedure used for organic samples should not be used for nitrates because of their different chemical nature. We present the best performance given one set of sample introduction parameters for the analysis of nitrates, as well as for the ammonium sulphate IAEA-N1 and IAEA-N2 reference materials. We discuss these results considering the thermochemistry of the substances and the analytical technique itself. The results emphasise the difference in chemical nature of inorganic and organic samples, which necessarily involves distinct thermochemistry when analysed by EA-IRMS. Therefore, they should not be processed using the same analytical procedure. This clearly impacts on the way international secondary reference materials should be used for the calibration of organic laboratory standards.

Figure

Control chart of the δ15N value of IAEA-N1, IAEA-NO-3 and NH4NO3 analysed a) with oxygen injection (analytical cycle 70 s, oxygen for 60 s, sample start and stop at 18 s/20 s), b) with oxygen injection (analytical cycle 70 s, oxygen for 60 s, sample start and stop at 0 s/2 s and 5 s/7 s), c) without oxygen injection (analytical cycle 70 s, sample start and stop at 18 s/20 s).

Keywords

Nitrate Nitrogen Isotope ratio mass spectrometry Ammonium Elemental analysis–isotope ratio mass spectrometry Inorganic 

Supplementary material

216_2012_6471_MOESM1_ESM.pdf (2 mb)
ESM 1(PDF 2061 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  • Natacha Gentile
    • 1
  • Michel J. Rossi
    • 2
  • Olivier Delémont
    • 1
  • Rolf T. W. Siegwolf
    • 2
  1. 1.Institut de Police Scientifique, Ecole des Sciences CriminellesUniversity of LausanneLausanneSwitzerland
  2. 2.Laboratory of Atmospheric ChemistryPaul Scherrer InstitutVilligenSwitzerland

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