Absolute 13C/12C Isotope Amount Ratio for Vienna Pee Dee Belemnite from Infrared Absorption Spectroscopy

13CH4/12CH4 sensing using Raman spectroscopy

The paper presents a technique for measuring the concentration of 13CH4 in natural methane using Raman spectroscopy. The peak positions and the relative scattering cross-sections of the Q-branches for the most intense vibrational bands of 13CH4 are determined. Features of the 13CH4/12CH4 ratio measurement methods using Q-branches of the ν1 and ν3 bands were considered. It was shown that the 13CH4/12CH4 ratio can be determined by simulation of the ν3 bands of these molecules without the use of experimental spectra. In our experiments the measurement error of δ13C value was 10 ‰ using the 100-s exposure spectrum at a gas pressure close to 1 atm recorded on the developed Raman spectrometer. In addition, the Raman spectra of alkanes (up to n-hexane) in the range of 2850-3050 cm-1 at a resolution of 0.4 cm-1 are presented, and their integrated intensities in the ranges of the characteristic bands of 13CH4 and 12CH4 are provided. The data obtained make it possible to expand the capabilities of Raman gas analyzers in the mud gas logging industry.

Isotopic measurements of carbon dioxide: the role of measurement science and standards

Isotopic measurements provide valuable information about the origin of greenhouse gases - as carbon dioxide levels increase, there is a corresponding shift towards lighter isotopic composition similar to that of fossil fuels. Detecting such isotopic shifts, however, requires extremely precise measurements, which must also be globally reproducible in order to make reliable policy decisions. This feature article outlines the collective search for the ideal standard for carbon isotope measurements since the 1950s. This tragicomedy of errors, if you wish, has strengthened the reliability of today's measurements and has taken us from fictional oceans, to toilet seat marbles, and complex mathematical conventions that separate data from reliable results.

Rovibrational Energies of 13C16O2 Determined with Kilohertz Accuracy

Accurate spectroscopic data of carbon dioxide are widely used in many important applications, such as carbon monitoring missions. Here, we present comb-locked cavity ring-down saturation spectroscopy of the second most abundant isotopologue of CO2, 13C16O2. We determined the positions of 88 lines in three vibrational bands in the 1.6 μm region, 30011e/30012e/30013e-00001e, with an accuracy of a few kHz. Based on the analysis of combination differences, we obtained for the first time the ground-state rotational energies with kHz accuracy. We also provide a set of hybrid line positions for 150 13C16O2 transitions. The rotational energies (J < 50) in the 30013e vibrational state can be fitted by a set of rotational and centrifugal constants with deviations within a few kHz, indicating that the 30013e state is free of perturbations. These precise isotopic line positions will be utilized to improve the Hamiltonian model and quantitative remote sensing of carbon dioxide. Moreover, they will help to track changes in the carbon source and sink through isotopic analysis.

Comparison of Carbon Isotope Ratio Measurement of the Vanillin Methoxy Group by GC-IRMS and 13C-qNMR

Site-specific carbon isotope ratio measurements by quantitative 13C NMR (13C-qNMR), Orbitrap-MS, and GC-IRMS offer a new dimension to conventional bulk carbon isotope ratio measurements used in food provenance, forensics, and a number of other applications. While the site-specific measurements of carbon isotope ratios in vanillin by 13C-qNMR or Orbitrap-MS are powerful new tools in food analysis, there are a limited number of studies regarding the validity of these measurement results. Here we present carbon site-specific measurements of vanillin by GC-IRMS and 13C-qNMR for methoxy carbon. Carbon isotope delta (δ13C) values obtained by these different measurement approaches demonstrate remarkable agreement; in five vanillin samples whose bulk δ13C values ranged from -31‰ to -26‰, their δ13C values of the methoxy carbon ranged from -62.4‰ to -30.6‰, yet the difference between the results of the two analytical approaches was within ±0.6‰. While the GC-IRMS approach afforded up to 9-fold lower uncertainties and required 100-fold less sample compared to the 13C-qNMR, the 13C-qNMR is able to assign δ13C values to all carbon atoms in the molecule, not just the cleavable methoxy group.

Methane accumulation and its potential precursor compounds in the oxic surface water layer of two contrasting stratified lakes

Methane (CH4) supersaturation in oxygenated waters is a widespread phenomenon despite the traditional perception of strict anoxic methanogenesis. This notion has recently been challenged by successive findings of processes and mechanisms that produce CH4 in oxic environments. While some of the processes contributing to the vertical accumulation of CH4 in the oxygenated upper water layers of freshwater lakes have been identified, temporal variations as well as drivers are still poorly understood. In this study, we investigated the accumulation of CH4 in oxic water layers of two contrasting lakes in Germany: Lake Willersinnweiher (shallow, monomictic, eutrophic) and Lake Stechlin (deep, dimictic, eutrophic) from 2019 to 2020. The dynamics of isotopic values of CH4 and the role of potential precursor compounds of oxic CH4 production were explored. During the study period, persistent strong CH4 supersaturation (relative to air) was observed in the surface waters, mostly concentrated around the thermocline. The magnitude of vertical CH4 accumulation strongly varied over season and was generally more pronounced in shallow Lake Willersinnweiher. In both lakes, increases in CH4 concentrations from the surface to the thermocline mostly coincided with an enrichment in 13C-CH4 and 2H-CH4, indicating a complex interaction of multiple processes such as CH4 oxidation, CH4 transport from littoral sediments and oxic CH4 production, sustaining and controlling this CH4 supersaturation. Furthermore, incubation experiments with 13C- and 2H-labelled methylated P-, N- and C- compounds clearly showed that methylphosphonate, methylamine and methionine acted as potent precursors of accumulating CH4 and at least partly sustained CH4 supersaturation. This highlights the need to better understand the mechanisms underlying CH4 accumulation by focusing on production and transport pathways of CH4 and its precursor compounds, e.g., produced via phytoplankton. Such knowledge forms the foundation to better predict aquatic CH4 dynamics and its subsequent rates of emission to the atmosphere.

Effects of 13C isotope-labeled allelochemicals on the growth of the invasive plant Alternanthera philoxeroides

The secondary metabolites of indigenous plants have significant allelopathic inhibitory effects on the growth and development of invasive alien plants. Methyl palmitate (MP) and methyl linolenate (ML) were used as exogenous allelopathic substances. The research investigated the differences of inhibitory effects of MP and ML on the growth of seedlings of Alternanthera philoxeroides, and calculated their morphological characteristics, biomass, physiological indicators and the response index (RI). The synthetical allelopathic index (SE) of 1 mmol/L MP was the smallest (- 0.26) and the allelopathic inhibition was the strongest; therefore, it was selected as a 13C-labeled allelochemical. The distribution of 1 mmol/L MP in different parts of A. philoxeroides and the correlation between the biomass ratios of roots, stems and leaves and the 13C content were studied by 13C stable isotope tracing experiments. Atom percent excess (APE) between roots, stems and leaves of A. philoxeroides treated with 1 mmol/L MP were significantly different in terms of magnitude, with leaves (0.17%) > roots (0.12%) > stems (0.07%). The root, stem and leaf biomass ratios of invasive weeds had great significant positive correlation with 13C content (p < 0.01, R2 between 0.96 and 0.99). This current research provides a new idea and method for the control of A. philoxeroides, but large-scale popularization remains to be studied.

A Spectroscopic Description of Asymmetric Isotopologues of CO2

A polyad-conserving algebraic model applied to vibrational excitations of asymmetric isotopologues of CO2 is presented. First, the problem of vibrational excitations is studied by taking into account only the minimum subspace of states to characterize the Fermi interaction. This analysis allows an estimation of the force constants as well as the feasibility of describing the system in a local mode scheme, in terms of SU(2) operators associated with Morse ladder operators for the stretches. This description together with the algebraic U(3) for the bends establishes the dynamical group SU1(2) × U(3) × SU2(2) for a series of isotopologues. Six isotopologues are considered, namely, 16O12C18O, 16O12C17O, 16O13C18O, 16O13C17O, 17O13C18O, and 17O12C18O in their electronic ground states. For isotopologues 16O12C18O, 16O12C17O, 17O12C18O, and 16O13C18O, the vibrational description was carried out using a Hamiltonian involving 14 parameters. For this series of isotopologues with a number of energy terms 90, 57, 42, and 40, the deviations obtained were rms = 0.15, 0.10, 0.06, and 0.07 cm-1, respectively. For 16O13C17O, with 28 experimental energies and involving 13 parameters, the deviation was rms = 0.05 cm-1, while for 17O13C18O, a different strategy was proposed since only 12 experimental energy levels. In all cases, the polyad scheme P212 = 2(ν1 + ν3) + ν2 was considered. In addition, a new criterion of locality/normality degree is proposed, embracing the case of molecules with normal mode behavior, in particular, the isotopologues of CO2.

Effect of Non-Markovian Collisions on Measured Integrated Line Shapes of CO

Using cavity ring-down spectroscopy to probe R-branch transitions of CO in N_{2}, we show that the spectral core of the line shapes associated with the first few rotational quantum numbers, J, can be accurately modeled using a sophisticated line profile, provided that a pressure-dependent line area is introduced. This correction vanishes as J increases and is always negligible in CO-He mixtures. The results are supported by molecular dynamics simulations attributing the effect to non-Markovian behavior of collisions at short times. This work has large implications because corrections must be considered for accurate determinations of integrated line intensities, and for spectroscopic databases and radiative transfer codes used for climate predictions and remote sensing.

NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets

Faraday rotation spectroscopy (FRS) exploits the magneto-optical effect to achieve highly selective and sensitive detection of paramagnetic molecules. Usually, a solenoid coil is used to provide a longitudinal magnetic field to produce the magneto-optical effect. However, such a method has the disadvantages of excessive power consumption and susceptibility to electromagnetic interference. In the present work, a novel FRS approach based on a combination of a neodymium iron boron permanent magnet ring array and a Herriott multipass absorption cell is proposed. A longitudinal magnetic field was generated by using 14 identical neodymium iron boron permanent magnet rings combined in a non-equidistant form according to their magnetic field's spatial distribution characteristics. The average magnetic field strength within a length of 380 mm was 346 gauss. A quantum cascade laser was used to target the optimum 4₄₁ ← 4₄₀ Q-branch nitrogen dioxide transition at 1613.25 cm^-1 (6.2 μm) with an optical power of 40 mW. Coupling to a Herriott multipass absorption cell, a minimum detection limit of 0.4 ppb was achieved with an integration time of 70 s. The low-power FRS nitrogen dioxide sensor proposed in this work is expected to be developed into a robust field-deployable environment monitoring system.

Site-specific carbon isotope measurements of vanillin reference materials by nuclear magnetic resonance spectrometry

Vanillin, one of the world's most popular flavor used in food and pharmaceutical industries, is extracted from vanilla beans or obtained (bio)-synthetically. The price of natural vanillin is considerably higher than that of its synthetic alternative which leads increasingly to counterfeit vanillin. Here, we describe the workflow of combining carbon isotope ratio combustion mass spectrometry with quantitative carbon nuclear magnetic resonance spectrometry (¹³C-qNMR) to obtain carbon isotope measurements traceable to the Vienna Peedee Belemnite (VPDB) with 0.7‰ combined standard uncertainty (or expanded uncertainty of 1.4‰ at 95% confidence level). We perform these measurements on qualified Bruker 400 MHz instruments to certify site-specific carbon isotope delta values in two vanillin materials, VANA-1 and VANB-1, believed to be the first intramolecular isotopic certified reference material (CRMs).

Absolute Quantification of Pure Free Radical Reagents by Combination of the Effective Magnetic Moment Method and Quantitative Electron Paramagnetic Resonance Method

An absolute quantitative analysis of free radicals by combining the effective magnetic moment method and the quantitative electron paramagnetic resonance [qEPR] method is proposed. This combined method utilizes the advantages of both the analytical methods and compensates for their disadvantages. In the effective magnetic moment method, the magnetic moment under a constant magnetic field is measured accurately using a superconducting quantum interference device. The qEPR method compares a "primary standard sample" and "secondary standard sample". The effective magnetic moment method was used to determine the purity of the primary standard sample. The qEPR method realizes a simple purity analysis of free-radical reagents with traceability to the International System of Units (SI). The purity of the free radicals by the qEPR method for pure 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl benzoate [4HTB], 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine [TEMPOL], and di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium [DPPH] reagents was obtained with a relative expanded uncertainty of 0.7% for 4HTB to 1.5% for the DPPH. These small uncertainties were almost equal to those of the purity of the primary standard samples and were achieved by adopting in-plane positioning of the measured sample perpendicular to the cylindrical axis of the sample space of the superconducting quantum interference device. Some purity values of the free radicals for these reagents differed from those stated by the manufacturers. This combined method enables short-time quality control of pure radical reagents, instead of quality control by separation analytical methods or titrations.

Ames-2021 CO2 Dipole Moment Surface and IR Line Lists: Toward 0.1% Uncertainty for CO2 IR Intensities

A highly accurate CO₂ ab initio dipole moment surface (DMS), Ames-2021, is reported along with ¹²C¹⁶O₂ infrared (IR) intensity comparisons approaching a 1-4‰ level of agreement and uncertainty. The Ames-2021 DMS was accurately fitted from CCSD(T) finite-field dipoles computed with the aug-cc-pVXZ (X = T, Q, 5) basis for C atom and the d-aug-cc-pVXZ (X = T, Q, 5) basis for O atoms, and extrapolated to the one particle basis set limit. Fitting σ_rms is 3.8 × 10^-7 au for 4443 geometries below 15 000 cm^-1. The corresponding IR intensity, S_Ames-2021, are computed using the Ames-2 potential energy surface (PES), which is the best PES available for CO₂. Compared to high accuracy IR studies for 2001i-00001 and 3001i-00001 bands, S_Ames-2021 matches NIST experiment-based intensities [S_NIST-HIT16 or S_HIT20] to -1.0 ± 1.3‰, or matches DLR experiment-based intensities [S_{DLR-HIT16/UCL/Ames}] to 1.9 ± 3.7‰. This indicates the systematic deviations and uncertainties have been significantly reduced in S_Ames-2021. The S_UCL2015 (or S_HITRAN2016) have larger deviations (vs S_DLR) and uncertainties (vs S_DLR, S_NIST) which are attributed to the less accurate Ames-1 PES adopted in UCL-296 line list calculation. The S_Ames-2021 intensity of ¹²C¹⁶O₂ and ¹³C¹⁶O₂ is utilized to derive new absolute ¹³C/¹²C ratios for Vienna PeeDee Belemnite (VPDB) with uncertainty reduced by ¹/₃ or ²/₃. Further evaluation of S_Ames-2021 intensities are carried out on those CO₂ bands discussed in the HITRAN2020 update paper. Consistent improvements and better accuracies are found in band-by-band analysis, except for those bands strongly affected by Coriolis couplings, or very weak bands measured with relatively larger experimental uncertainties. The Ames-2021 296 K IR line lists are generated for 13 CO₂ isotopologues, with 18 000 cm^-1 and S_296 K > 1 × 10^-31 cm/molecule cutoff and then combined with CDSD line positions (except ¹⁴C¹⁶O₂). The Ames-2021 DMS and 296 K IR line lists represent a major improvement over previous CO₂ theoretical IR intensity studies, including Ames-2016, UCL-296, and recent UCL DMS 2021 update. A real 1 permille level of agreement and uncertainty will definitely require both more accurate PES and more accurate DMS.

Subpromille Measurements and Calculations of CO (3-0) Overtone Line Intensities

Intensities of lines in the near-infrared second overtone band (3-0) of ^{12}C^{16}O are measured and calculated to an unprecedented degree of precision and accuracy. Agreement between theory and experiment to better than 1‰ is demonstrated by results from two laboratories involving two independent absorption- and dispersion-based cavity-enhanced techniques. Similarly, independent Fourier transform spectroscopy measurements of stronger lines in this band yield mutual agreement and consistency with theory at the 1‰ level. This set of highly accurate intensities can provide an intrinsic reference for reducing biases in future measurements of spectroscopic peak areas.

Absolute Carbon Stable Isotope Ratio in the Vienna Peedee Belemnite Isotope Reference Determined by 1H NMR Spectroscopy

The Vienna Peedee Belemnite (VPDB) isotope reference defines the zero point of the carbon stable isotope scale that is used to describe the relative abundance of ¹³C and ¹²C. An accurate and precise characterization of this isotope reference is valuable for interlaboratory comparisons and conducting robust carbon stable isotope analyses in a vast array of fields, such as chemical forensics, (bio)geochemistry, ecology, or (astro)biology. Here, we report an absolute ¹³C/¹²C ratio for VPDB that has been obtained, for the first time, using proton nuclear magnetic resonance spectroscopy (¹H NMR). Four different NMR instruments were used to determine ¹³C/¹²C ratios in a set of glycine reference materials from the US Geological Survey (USGS64, USGS65, and USGS66) and a set of formate samples that were characterized by isotope ratios mass spectrometry (IRMS). Intercalibration of the NMR-derived ¹³C/¹²C ratios with relative abundance (δ¹³C_VPDB) measurements from IRMS yields a value of 0.011100 for the absolute ¹³C/¹²C ratio in VPDB, with an expanded uncertainty of ±0.000026 (2σ, n = 114). This is significantly different from the value of 0.011180 that is commonly used but falls within the range of values recently revised using IRMS and infrared absorption measurements. ¹H NMR was found to be an effective method for measuring absolute ¹³C/¹²C ratios due to its ability to simultaneously detect signals associated with ¹²C and ¹³C. Results provide a new and independent measure of the carbon isotope composition of VPDB, improving our understanding of this important isotope reference.

Discontinuity in the Realization of the Vienna Peedee Belemnite Carbon Isotope Ratio Scale

By convention, carbon isotope ratios are expressed relative to VPDB defined by the calcite standard NBS19 in the 1980s. [See T. Coplen, Pure Appl. Chem. 1994, 66, 273-276.] To improve the realization of the VPDB scale, a second fixed point (lithium carbonate, LSVEC) was introduced in 2006 [T. Coplen et al. Anal. Chem. 2006, 78, 2439-2441], which is now known to be isotopically unstable. [Assonov, S. Rapid Commun. Mass Spectrom., 2018, 32, 827-830.] With the high-quality reference materials made available in 2020, it is now possible to realize the VPDB scale with high confidence. [Assonov, S. et al. Rapid Commun. Mass Spectrom., 2020, 34, e8867; Assonov, S. Rapid Commun. Mass Spectrom. 2021, 35, e9014; Qi, H. et al. Rapid Commun. Mass Spectrosc. 2021, 35, e9006.] Here, we report the analysis of 25 reference materials using isotope ratio combustion mass spectrometry, show the discontinuity between the values measured against the new IAEA reference materials and the values currently assigned to these reference materials on the VPDB2006, and provide a link bringing these materials onto the new VPDB2020.