Enhanced forensic discrimination of pollutants by position-specific isotope analysis using isotope ratio monitoring by (13)C nuclear magnetic resonance spectrometry

Potential of stable isotope analysis to deduce anaerobic biodegradation of ethyl tert-butyl ether (ETBE) and tert-butyl alcohol (TBA) in groundwater: a review

Understanding anaerobic biodegradation of ether oxygenates beyond MTBE in groundwater is important, given that it is replaced by ETBE as a gasoline additive in several regions. The lack of studies demonstrating anaerobic biodegradation of ETBE, and its product TBA, reflects the relative resistance of ethers and alcohols with a tertiary carbon atom to enzymatic attack under anoxic conditions. Anaerobic ETBE- or TBA-degrading microorganisms have not been characterized. Only one field study suggested anaerobic ETBE biodegradation. Anaerobic (co)metabolism of ETBE or TBA was reported in anoxic microcosms, indicating their biodegradation potential in anoxic groundwater systems. Non-isotopic methods, such as the detection of contaminant loss, metabolites, or ETBE- and TBA-degrading bacteria are not sufficiently sensitive to track anaerobic biodegradation in situ. Compound- and position-specific stable isotope analysis provides a means to study MTBE biodegradation, but isotopic fractionation of ETBE has only been studied with a few aerobic bacteria (εC -0.7 to -1.7‰, εH -11 to -73‰) and at one anoxic field site (δ2H-ETBE +14‰). Similarly, stable carbon isotope enrichment (δ13C-TBA +6.5‰) indicated TBA biodegradation at an anoxic field site. CSIA and PSIA are promising methods to detect anaerobic ETBE and TBA biodegradation but need to be investigated further to assess their full potential at field scale.

NMR-based isotopic and isotopomic analysis

Molecules exist in different isotopic compositions and most of the processes, physical or chemical, in living systems cause selection between heavy and light isotopes. Thus, knowing the isotopic fractionation of the common atoms, such as H, C, N, O or S, at each step during a metabolic pathway allows the construction of a unique isotope profile that reflects its past history. Having access to the isotope abundance gives valuable clues about the (bio)chemical origin of biological or synthetic molecules. Whereas the isotope ratio measured by mass spectrometry provides a global isotope composition, quantitative NMR measures isotope ratios at individual positions within a molecule. We present here the requirements and the corresponding experimental strategies to use quantitative NMR for measuring intramolecular isotope profiles. After an introduction showing the historical evolution of NMR for measuring isotope ratios, the vocabulary and symbols - for describing the isotope content and quantifying its change - are defined. Then, the theoretical framework of very accurate quantitative NMR is presented as the principle of Isotope Ratio Measurement by NMR spectroscopy, including the practical aspects with nuclei other than ²H, that have been developed and employed to date. Lastly, the most relevant applications covering three issues, tackling counterfeiting, authentication, and forensic investigation, are presented, before giving some perspectives combining technical improvements and methodological approaches.

Intramolecular isotope effects during permanganate oxidation and acid hydrolysis of methyl tert-butyl ether

Stable isotopes have been widely used to monitor remediation of environmental contaminants over the last decades. This approach gives a good mechanistic description of natural or assisted degradation of organic pollutants, such as methyl tert-butyl ether (MTBE). Since abiotic degradation seems to be the most promising assisted attenuation method, the isotopic fractionation associated with oxidation and hydrolysis processes need to be further investigated in order to understand better these processes and make their monitoring more efficient. In this study, position-specific isotope effects (PSIEs) associated with permanganate oxidation and acid hydrolysis of MTBE were determined using isotope ratio monitoring by ¹³C Nuclear Magnetic Resonance Spectrometry (irm-¹³C NMR) combined with isotope ratio monitoring by Mass Spectrometry (irm-MS). The use of this Position-Specific Isotopic Analysis (PSIA) method makes it possible to observe a specific normal isotope effect (IE) associated with each of these two abiotic degradation mechanisms. The present work demonstrates that the ¹³C isotope pattern of the main degradation product, tert-butyl alcohol (TBA), depends on the chemical reaction by which it is produced. Furthermore, this study also demonstrates that PSIA at natural abundance can give new insights into reaction mechanisms and that this methodology is very promising for the future of modeling the remediation of organic contaminants.

An untargeted 13C isotopic evaluation approach for the discrimination of fermented food matrices at natural abundance: Application to vinegar

A non-destructive and comprehensive ¹³C isotopic evaluation approach based on ¹H NMR spectroscopy was developed. The carbon isotope distribution (CID) of most of the components (S/N ≥ 1000) in food matrices were evaluated using frequency distribution of peak area ratios (PAR) of decoupling to non-¹³C-decoupling spectra at natural abundance. The approach was applied successfully to vinegar and it was found that the PAR of fermented vinegars is obviously narrower than that of the blended one and the one produced via chemically defined culture medium. Besides, the extra additives can also be evaluated by their characteristic PAR values. It was found that the sugars are the most commonly added components into the blended vinegars. The results obtained from the developed approach show good validity (baseline effect, RSD < 0.1%) and internal reproducibility (RSD < 0.1%). Practicability of the method is confirmed by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) detection method. The results imply that the proposed approach could be used not only for the discrimination but also for the primary authentication of the blended components in the fermented food matrices.

Forensic application of position-specific isotopic analysis of trinitrotoluene (TNT) by NMR to determine 13C and 15N intramolecular isotopic profiles

2,4,6-trinitrotoluene (TNT) is a molecule which is easily identified with current instrumental techniques but it is generally impossible to distinguish between sources of the same substance (TNT). To overcome this difficulty, we present a multi stable isotope approach using isotope ratio monitoring by mass spectrometry (irm-MS) and Nuclear Magnetic Resonance spectrometry (irm-NMR). In the one hand, irm-MS provides bulk isotopic composition at natural abundance in ¹³C and ¹⁵N. The range of variation between samples is rather small particularly for ¹³C. In the other hand, irm-¹³C NMR and irm-¹⁵N NMR enable the determination of positional intramolecular ¹³C/¹²C ratios (δ¹³C_i) and ¹⁵N/¹⁴N ratios (δ¹⁵N_i) with high precision that lead to larger variation between samples. The present work reports an application of the recent methodology using irm-¹⁵N NMR to determine position-specific ¹⁵N isotope content of TNT. The interest of this methodology is compared to irm-¹³C NMR and irm-MS (¹³C and ¹⁵N) in terms of TNT samples discrimination. Thanks to the use of irm-NMR the results show a unique isotopic fingerprint for each TNT which enable origin discrimination between the samples without ambiguity.

Simulating Stable Isotope Ratios in Plumes of Groundwater Pollutants with BIOSCREEN-AT-ISO

BIOSCREEN is a well-known simple tool for evaluating the transport of dissolved contaminants in groundwater, ideal for rapid screening and teaching. This work extends the BIOSCREEN model for the calculation of stable isotope ratios in contaminants. A three-dimensional exact solution of the reactive transport from a patch source, accounting for fractionation by first-order decay and/or sorption, is used. The results match those from a previously published isotope model but are much simpler to obtain. Two different isotopes may be computed, and dual isotope plots can be viewed. The dual isotope assessment is a rapidly emerging new approach for identifying process mechanisms in aquifers. Furthermore, deviations of isotope ratios at specific reactive positions with respect to "bulk" ratios in the whole compound can be simulated. This model is named BIOSCREEN-AT-ISO and will be downloadable from the journal homepage.

The new face of isotopic NMR at natural abundance

The most widely used method for isotope analysis at natural abundance is isotope ratio monitoring by Mass Spectrometry (irm-MS) which provides bulk isotopic composition in ² H, ¹³ C, ¹⁵ N, ¹⁸ O or ³⁴ S. However, in the 1980s, the direct access to Site-specific Natural Isotope Fractionation by Nuclear Magnetic Resonance (SNIF-NMR^TM ) was immediately recognized as a powerful technique to authenticate the origin of natural or synthetic products. The initial - and still most popular - application consisted in detecting the chaptalization of wines by irm-² H NMR. The approach has been extended to a wide range of methodologies over the last decade, paving the way to a wide range of applications, not only in the field of authentication but also to study metabolism. In particular, the emerging irm-¹³ C NMR approach delivers direct access to position-specific ¹³ C isotope content at natural abundance. After highlighting the application scope of irm-NMR (² H and ¹³ C), this article describes the major improvements which made possible to reach the required accuracy of 1‰ (0.1%) in irm-¹³ C NMR. The last part of the manuscript summarizes the different steps to perform isotope analysis as a function of the sample properties (concentration, peak overlap) and the kind of targeted isotopic information (authentication, affiliation). Copyright © 2016 John Wiley & Sons, Ltd.