δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

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Serval ID
serval:BIB_601F3C0C3DEA
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure
Journal
Analytical and Bioanalytical Chemistry
Author(s)
Gentile Natacha, Rossi Michel J., Delémont Olivier, Siegwolf Rolf T. W.
ISSN
1618-2642
1618-2650
ISSN-L
1618-2642
Publication state
Published
Issued date
01/2013
Peer-reviewed
Oui
Volume
405
Number
1
Pages
159-176
Language
english
Abstract
Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ 15N 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 δ 15N 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, δ 15N 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.
Keywords
δ 15N, nitrogen, isotope ratio mass spectrometry, nitrate, ammonium, EA-IRMS, organic, inorganic, Analytical Chemistry, Biochemistry
Pubmed
Web of science
Open Access
Yes
Create date
08/01/2013 8:49
Last modification date
01/10/2019 7:18
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