Jahrestagung der Arbeitsgemeinschaft
Stabile Isotope e.V.
26.–29. September 2021 | TU Darmstadt
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Block 1.1: Analytik, Methoden, Technik und Qualitätssicherung stabiler Isotope
14:00 - 14:15
Compound Specific Stable Sulfur Isotope Analysis (δ34S and δ33S) of Organic Compounds Using Gas Chromatography Hyphenated with Multiple Collector Inductively Coupled Plasma Mass Spectrometry (GC-MC-ICPMS)
1Helmholtz-Zentrum für Umweltforschung GmbH - UFZ, Leipzig, Germany; 2Geological Survey of Israel, Jerusalem, Israel; 3Westfälische Wilhelms-Universität Münster, Institut für Geologie und Paläontologie, Münster, Germany
Stable sulfur isotope analysis is applicable in various fields in forensics and environmental analytics to investigate the sources and degradation of organic compounds, many of them being priority pollutants in groundwater and the atmosphere. A broader use of sulfur isotopes of organic compounds in environmental studies is still hampered by the availability of precise and easy-to-use techniques. Here we present a method for the determination of stable sulfur isotope ratios using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS) which can be used for both δ34S and δ33S analysis. The method was evaluated using the reference materials IAEA-S-1, IAEA-S-2 and IAEA-S-3 which were converted offline to SF6 prior to analysis. Standardization was carried out by a two-point calibration approach. The δ34S obtained by our method are in good agreement (within analytical uncertainty) with the results obtained by the conventional dual inlet method. Additionally, the impact of the used mass resolution (low and medium), the influence of auto-protonation of sulfur isotopes and the effect of isobaric interferences of O2+ on the obtained isotopic ratios were investigated. Overall analytical uncertainty including normalization and reproducibility for δ34S and δ33S was usually better than ±0.2 mUr (1σ) for analytes containing at least 100 pmol of S. Thus, the presented compound-specific online method should be sufficiently precise to address a wide variety of research questions involving mass independent isotope effects of sulfur-containing organic compounds to discriminate sources or biological and chemical reactions in the environment.
14:15 - 14:30
New Frontiers in Compound-Specific δ2H Analysis
1University of Bristol, UK; 2University of Plymouth, UK; 3Silicon Austria Labs, Austria; 4International Atomic Energy Agency, Austria; 5Elementar UK Ltd, UK
The hydrogen isotopic composition (δ2H) of lipid “biomarker” compounds (molecules synthesized by and traceable to living organisms) have long been of interest to biogeochemists, with applications ranging from the investigation of food authenticity, to the reconstruction of ancient climate and environment. The preferred method of stable isotope analysis of such lipids employs gas chromatography-isotope ratio mass spectrometry (GC-IRMS), which effectively limits applications to those which measure compounds of relatively low molecular weight and polarity (i.e. compounds which elute from a typical capillary GC column at c. 320-350°C). As such, only very few compounds of molecular weight > c. 500 g/mol have been successfully analyzed intact by GC-IRMS to determine δ2H. However, the hydrogen isotopic composition of larger and/or polar compounds can be of significant interest.
Here we present two pioneering new techniques for the analysis of larger and/or more polar organic molecules of biogeochemical interest which are traditionally considered unsuitable for GC-IRMS analysis. This includes a rapid one-step derivatization procedure for the isotope analysis of the non-exchangeable hydrogen in mono and disaccharides and subsequent conversion to H2 by chromium reduction (Abrahim et al, 2020), and the development of a high-temperature GC-IRMS (HTGC-IRMS) methodology employed for the analysis of a suite of compounds of interest (Lengger et al., 2021). In particular we present the successful HTGC-IRMS analysis of triacylglcerides (TAGs), whose derivative fatty acids are often employed in studies of archaeological diet and modern food authenticity analysis, tetraether lipids including glycerol dialkyl glycerol tetrethers (GDGTs) which have been widely employed as proxies for paleoclimate and environmental analysis based on their relative distributions, and longer chain n-alkanes, the lower molecular weight homologues of which have been widely employed for paleohydrological analysis. We will present initial results demonstrating the performance and validation of the techniques, and their potential application to organic biogeochemistry.
Lengger S.K., Weber Y., Taylor K.W. R, Kopf S. H., Berstan R,, Bull I,D., Mayser J., Leavitt W. D., Blewett J. & Pearson A. (2021). Determination of the δ2H values of high molecular weight lipids by high temperature GC coupled to isotope ratio mass spectrometry. Rapid Communications in Mass Spectrometry 35:e8983.https://doi.org/10.1002/rcm.898310
Abrahim, A., Cannavan, A., & Kelly, S. (2020). Stable isotope analysis of non-exchangeable hydrogen in carbohydrates derivatised with N-methyl-bis-trifluoroacetamide by gas chromatography–Chromium silver reduction/High temperature Conversion-isotope ratio mass spectrometry (GC-CrAg/HTC-IRMS). Food Chemistry 318 (126413) https://doi.org/10.1016/j.foodchem.2020.126413
14:30 - 14:45
A quick one-step sample preparation method with 2,2-Dimethoxypropane (DMP) for isotopic fingerprint analysis of vegetable oil – how does DMP influence the accuracy of the C- and H-CSIA by GC-C/Py-IRMS?”
Stoffstrommanagement und Ressourcenwirtschaft, Institut IWAR, TU Darmstadt
Compound-specific stable isotope analysis (CSIA) of fatty acids (FAs) is an important tool for the investigation of authentication of vegetable oil. Stable isotope ratios of carbon (δ13C) and hydrogen (δ2H) of individual FAs are to be determined from the δ13C and δ2H of fatty acid methyl ester (FAMEs), which are produced by reactions of FAs in oil and methyl group (–CH3) of methanol in the presence of catalysts. This transmethylation process generally involves multiple steps thus often becomes a bottleneck process of the investigation with a large number of samples.
Garcés and Mancha (1993) developed a quick sample preparation method to generate FAMEs directly from seeds in one step in a single reactor. They optimized transmethylation efficiency by adding 2,2-Dimethoxypropane (DMP). The applicability of this one-step method was verified to determine the lipid content and the FA profile of different oilseeds. However, the verification of this method in determining the isotope composition of individual FAs has not yet been reported.
In this study, we evaluated the feasibility of the one-step method for C- and H-CSIA of individual FAMEs in rapeseed samples with the following two concerns: (1) the influence of the aggregate states of samples on the reproducibility of C- and H-CSIA (2) the influence of DMP on the accuracy of δ13C and δ2H values of FAMEs, consequently FAs, by Gas Chromatography-Combustion/Pyrolysis-Isotope Ratio Mass Spectrometry (GC-C/Py-IRMS). DMP in the one-step method will contribute to producing extra methanol as a reaction-intermediate, which can be consumed to generate FAMEs. We investigated the isotope composition of FAMEs produced with and without DMP and evaluated the extent of the involvement of the reaction intermediate methanol in the production of FAMEs in the one-step method.
Our results showed that the reproducibility of the one-step method in C- and H-CSIA for the solid rapeseeds and fluid oil was comparable. Further, we confirmed that no significant differences arose in the carbon and hydrogen isotope compositions of the selected main FAMEs produced with and without DMP, except for the H-CSIA value of C18:3. The reproducibility of the one-step method for rapeseed was in the range of ±0.1 mUr to ± 0.3 mUr for C-CSIA and ±1 mUr to ±3 mUr for H-CSIA of the main FAMEs. The performance of the one-step method for rapeseed samples for the determination of δ13C and δ2H values of FAMEs is satisfying.
14:45 - 15:00
Revisiting SPIN-MIRMS: N-isotopic composition of nitrate and ammonium from aquaeous solutions by isotope ratio mass spectrometry
1Universität Göttingen, Kompetenzzentrum Stabile Isotope, Deutschland; 2Thuenen Institut für Agrarrelevante Klimaforschung, Braunschweig, Deutschland
Analyses of N-isotopic composition of nitrate and ammonium from aqueeous solutions are difficult - especially at natural abundance. The SPIN-MIRMS technique presented previously has the key advantage that no sample pretreatment is necessary and analysis is fast.
Here we present recent insights into the performance and substrate specifity of the approach.
15:00 - 15:15
δ13C Analysis of DOC from Liquid Samples - Proof of Concept
Max-Planck-Institut für Biogeochemie, Deutschland
Both, dissolved organic carbon (DOC) and its corresponding δ13C signature are important parameters for the investigation of the carbon cycle. For example, DOC concentration analyses reveal the organic matter flow in aquatic systems, while δ13C values of DOC can give insights on origin and transformation of organic matter. The combined analysis of DOC and δ13C can require either time-consuming and laborious sample preparation or a wet chemical oxidation method which can underestimate DOC concentrations and lead to isotopic fractionation.
Here we investigate the feasibility of using an elemental analyzer – isotope ratio mass spectrometer (EA-IRMS) system. By attaching a liquid autosampler to the EA and installing a Nafion water trap between the reactor and the GC-column we analyze DOC concentrations and δ13C values of liquids analogous to the way solid samples are measured. With this minor adjustment to the instrument configuration, δ13C measurements with a precision of < 0.1 ‰ can be made on DOC samples with a concentration of 10 mg/ml.
Preliminary tests were designed to test different parameters including linearity, memory effect, and stability. We also investigated whether dissolved carbonates yield a similar measurement precision as dissolved organic compounds, and whether a mix of DOC and DIC affect the overall fidelity of the measurements. For the analytical tests and data evaluation several in-house and international standards including urea, caffeine (IAEA-600), glutamic acid (USGS-40 and 41), carbonates (as inorganic standard) and sugars were dissolved in ultra-pure water. In the future, the method will be optimized for natural samples with smaller DOC concentrations by changing the injection amount or implementing a concentrating step. The preliminary results show that our method is easy to apply in laboratories for quick routine δ13C characterization of high concentration samples.
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