A new approach to detecting sugar syrup addition to honey: Stable isotope analysis of hexamethylenetetramine synthesised from honey monosaccharides (fructose and glucose)

One of the most common types of adulteration of honey involves the addition of invert sugar syrups. A new method was developed to measure the stable isotope ratios of carbon and carbon-bound non-exchangeable (CBNE) hydrogen from specific molecular positions in fructose and glucose in honey. This was achieved through periodate oxidation of the sugars to produce formaldehyde, followed by reaction with ammonia to form hexamethylenetetramine (HMT). The preparation was simplified, optimized, and validated by isotopic analysis of replicate syntheses of HMT from fructose, glucose, sugar syrup and a representative authentic honey sample. The optimized method had a repeatability standard deviation from 1.5‰ to 3.0‰ and from 0.1‰ to 0.4‰ for δ2H and δ13C, respectively. This methodology has advantages over alternative isotopic methods, for measuring CBNE hydrogen isotope ratios in sugars, in terms of time, sensitivity and operability and offers a complementary method to differentiate authentic honey from invert sugar syrups.

Apparent fractionation of hydrogen isotope from precipitation to leaf wax n-alkanes from natural environments and manipulation experiments

Knowledge of hydrogen isotopic fractionation (ε) of plant leaf waxes is the foundation for applying hydrogen isotope values (δ²H) in environmental reconstructions. In this work, we systematically investigated plant ε values (ε_{alk/precipitation}, ε_{alk/soil water}, ε_{alk/leaf water} and ε_{alk/lake water}, representing the isotopic fractionation between plant n-alkane δ²H and precipitation δ²H, soil water δ²H, leaf water δ²H and lake water δ²H) from the natural environments and manipulation experiments. The results show that the ε_{alk/precipitation} values of terrestrial plants have large variations (from -190 ‰ to -20 ‰) and become more negative with increasing aridity index. This phenomenon is possibly caused by the δ²H changes in source water (from precipitation to soil water and then to leaf water) during plant leaf wax synthesis under various evapotranspiration conditions in different climatic zones. The rainfall manipulation experiments show that leaf water δ²H values are generally higher than soil water δ²H values, and the latter are higher than precipitation δ²H values. This finding further demonstrates that the evapotranspiration effect on source water δ²H affects the quantification of the leaf wax apparent ε values (ε_{alk/leaf water} < ε_{alk/soil water} < ε_{alk/precipitation}). The ε_{alk/lake water} values of submerged plants display a smaller range (-153 ± 5 ‰) than the ε_{alk/precipitation} values of terrestrial plants, which is close to the terrestrial ε_{alk/precipitation} values in humid areas. Therefore, the biosynthetic ε value of terrestrial plant leaf waxes is relatively constant (ca. -153 ± 5 ‰), and the observed variable apparent ε_{alk/precipitation} values are possibly caused by the varied degree of evapotranspiration effect on the water that plants used in different climatic conditions. This effect should be considered when applying δ²H values of leaf waxes to trace environmental changes.

Stable carbon and hydrogen isotope fractionation of volatile organic compounds caused by vapor-liquid equilibrium

Several types of laboratory experiments were conducted to evaluate isotope fractionation caused by phase transfer process for a selection of common environmental contaminants. Carbon and hydrogen isotope fractionation caused by vaporization of non-aqueous phase liquid (NAPL), by volatilization from water and by dissolution into an organic solvent (tetraethylene glycol dimethylether or TGDE) under equilibrium conditions was investigated with closed system experimental setups to isolate the air-liquid partitioning process. A selection of aromatic, aliphatic and chlorinated compounds along with one fuel oxygenate (methyl tert-butyl ether or MTBE) were evaluated to determine isotope enrichment factor related to respective phase transfer process. During NAPL vaporization, the residual mass of aromatic compounds, aliphatic compounds and MTBE became progressively depleted in heavy carbon and hydrogen isotopes. In contrast, during volatilization from water, the residual mass of aromatic compounds and MTBE dissolved in the water became progressively enriched in heavy hydrogen isotopes, whereas no significant change in carbon isotope was observed, except for MTBE showing a significant depletion. For the air-TGDE partitioning process, most of the aromatic compounds tested led to no significant carbon (except ethylbenzene) or hydrogen (except toluene and o-xylene) isotope fractionation. In contrast, significant carbon isotope fractionation was observed for aliphatic and chlorinated compounds and hydrogen isotope fractionation for aliphatic compounds, and are comparable to progressive NAPL vaporization in direction and magnitude. The isotope fractionation factors determined in this study are key for interpreting the change in isotope ratios when assessing the fate of gas-phase VOCs present in the soil air or when gas-phase VOCs are sampled using TGDE as the sink matrix. The results of this study contribute to expand the list of common environmental contaminants that can be assessed by the compound-specific isotope analysis (CSIA) method deployed in the frame of gas-phase studies.

Intra-leaf heterogeneities of hydrogen isotope compositions in leaf water and leaf wax of monocots and dicots

Several recent studies showed that leaf wax n-alkane δ²H values (δ²H_wax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ²H_wax values is general for various species, how δ²H_wax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ²H_wax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ²H_lw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ²H_wax and δ²H_lw, i.e., base-to-tip ²H-enrichment for monocots (except Hemerocallis citrina, and Dactylis glomerata) whereas base-to-tip and center-to-edge increases in δ²H_wax and δ²H_lw for dicots. The consistent occurrence of variations of δ²H_lw and δ²H_wax values within a leaf imply that δ²H_wax values probably inherit point-to-pint from in-situ δ²H_lw values, and thus the intra-leaf δ²H_wax heterogeneity mainly results from the spatial pattern of intra-leaf δ²H_lw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ²H_wax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.

Stable isotope data of daily precipitation during the period of 2013-2017 from K-puszta (regional background monitoring station), Hungary

Precipitation was collected on daily basis at K-puszta regional background monitoring station located near Kecskemét, in the western part of the Great Hungarian Plain, between 1 April 2013 and 31 December 2017 for stable hydrogen and oxygen analyses. The sample collection period covered 24 hours, from 07:00 to 07:00 h LT (Local Time) the next day. Stable hydrogen and oxygen isotope composition of the daily precipitation samples were measured using a Liquid Water Isotope Analyser (LWIA-24d) manufactured by Los Gatos Research Ltd. The dataset includes 472 stable isotopic data representing the continuation of the beforehand monitoring started in 2012 [1]. The dataset provides a unique opportunity to combine daily meteorological data and stable isotope composition of daily precipitation which can help to improve our understanding of the processes and factors at relatively high resolution that govern δD and δ¹⁸O values of the precipitation. In addition, the dataset can be used as an isotope hydrological benchmark in comparison with stable isotope dataset obtained from surface- and groundwater or other sources (e.g. climate proxies, agricultural products). Thus, research related to isotope hydrology, agriculture, paleoclimate can benefit from this dataset. Interpretation of this dataset focusing on the relationship between meteorological factors and stable isotope composition of precipitation is in progress.

Hydrogen isotope assimilation and discrimination in green turtles

Although hydrogen isotopes (δ2H) are commonly used as tracers of animal movement, minimal research has investigated the use of δ2H as a proxy to quantify resource and habitat use. While carbon and nitrogen are ultimately derived from a single source (food), the proportion of hydrogen in consumer tissues originates from two distinct sources: body water and food. Before hydrogen isotopes can be effectively used as a resource and habitat tracer, we need estimates of (net) discrimination factors (Δ2HNet) that account for the physiologically mediated differences in the δ2H values of animal tissues relative to that of the food and water sources they use to synthesize tissues. Here, we estimated Δ2HNet in captive green turtles (Chelonia mydas) by measuring the δ2H values of tissues (epidermis and blood components) and dietary macromolecules collected in two controlled feeding experiments. Tissue δ2H and Δ2HNet values varied systematically among tissues, with epidermis having higher δ2H and Δ2HNet values than blood components, which mirrors patterns between keratinaceous tissues (feathers, hair) and blood in birds and mammals. Serum/plasma of adult female green turtles had significantly lower δ2H values compared with juveniles, likely due to increased lipid mobilization associated with reproduction. This is the first study to quantify Δ2HNet values in a marine ectotherm, and we anticipate that our results will further refine the use of δ2H analysis to better understand animal resource and habitat use in marine ecosystems, especially coastal areas fueled by a combination of marine (e.g. micro/macroalgae and seagrass) and terrestrial (e.g. mangroves) primary production.

Environmental behaviors of PAHs in Ordovician limestone water of Fengfeng coal mining area in China

In this study, we collected a total of 15 Ordovician limestone (OL) water, 4 shallow groundwater, 3 mine water, 2 surface water, and 2 coal bedrock water samples, aiming to analyze the characteristics of distributions and sources of polycyclic aromatic hydrocarbons (PAHs) in OL water in a typical exploited coal mine named as Fengfeng mining area. Firstly, the PAHs behaviors and characteristics in different types of water of the mining area were investigated and summarized. And then, the hydrogen and oxygen isotopes were combined with isomer ratio method to determine the characteristics, sources, and behaviors of PAHs in OL water, respectively. Results showed that the concentration of PAHs ranged from 0.06 to 0.56 ng/L in OL water of Fengfeng Mine. Among them, the dominant 2-4 cyclic PAHs, including Nap, Phe, Flt, and Flu, were detected at a low concentration level with high detection rate. Characteristic compound ratios Ant/(Ant + Phe) and Flt/(Flt + Pyr) showed that the PAHs were derived from the combustion of the coal and biomass. The results of δD/δ¹⁸O and δD/Phe testing showed that the PAHs in most OL water came from rainfall infiltration recharge with coal and biomass combustion products in exposed bedrock area at high altitude. The PAHs of some polluted areas were derived from leakage recharge of shallow groundwater, mine water, and coal bedrock water.

Using δDn-alkane as a proxy for paleo-environmental reconstruction: A good choice to sample at the site dominated by woods

Some studies have demonstrated that leaf wax δD_n-alkane values for a single species varied significantly with seasons. However, it is still not clear that the seasonality patterns of leaf wax δD_n-alkane values in higher plants. Meanwhile, few efforts have been pursued to assess the effect of the light slopes (sunny vs. cloudy) on leaf wax δD_n-alkane values. In this study, we systematically investigated plant wax δD_n-alkane values and soil n-alkane δD values along different light slopes in different seasons (spring vs. autumn), as well as the relationship of n-alkane δD values between plant leaves and soil. We found that plant wax δD_n-alkane values were D-enriched by ca. 20‰ in spring relative to autumn, and ca. 10‰ in the sunny slope than in the cloudy slope. Moreover, surface soil n-alkane δD values varied consistently with plant wax δD_n-alkane values for different seasons and light slopes. More importantly, plant wax δD_n-alkane values showed clear seasonal variations, but varied slightly with light slopes. The variations of plant wax δD_n-alkane values can be recorded in soil n-alkane δD_n-alkane values. In addition, we found that leaf wax δD_n-alkane values in a majority of species differed significantly among woods, non-woods and grasses at a site. Therefore, we suggested a good choice to sample at the site dominated by woods when leaf wax δD_n-alkane values are utilized as a proxy for the reconstruction of the paleoenvironment.

Validation of GC-IRMS techniques for δ13C and δ2H CSIA of organophosphorus compounds and their potential for studying the mode of hydrolysis in the environment

Compound-specific stable isotope analysis (CSIA) is among the most promising tools for studying the fate of organic pollutants in the environment. However, the feasibility of multidimensional CSIA was limited by the availability of a robust method for precise isotope analysis of heteroatom-bearing organic compounds. We developed a method for δ ¹³C and δ ²H analysis of eight organophosphorus compounds (OPs) with different chemical properties. In particular, we aimed to compare high-temperature conversion (HTC) and chromium-based HTC (Cr/HTC) units to explore the limitations of hydrogen isotope analysis of heteroatom-bearing compounds. Analysis of the amount dependency of the isotope values (linearity analysis) of OPs indicated that the formation of HCl was a significant isotope fractionation process leading to inaccurate δ ²H analysis in HTC. In the case of nonchlorinated OPs, by-product formation of HCN, H₂S, or PH₃ in HTC was observed but did not affect the dynamic range of reproducible isotope values above the limit of detection. No hydrogen-containing by-products were found in the Cr/HTC process by use of ion trap mass spectrometry analysis. The accuracy of gas chromatography - isotope ratio mass spectrometry was validated in comparison with elemental analyzer - isotope ratio mass spectrometry. Dual-isotope fractionation yielded Λ values of 0 ± 0 at pH 7, 7 ± 1 at pH 9, and 30 ± 6 at pH 12, indicating the potential of 2D CSIA to characterize the hydrolysis mechanisms of OPs. This is the first report on the combination of δ ²H and δ ¹³C isotope analysis of OPs, and this is the first study providing a systematic evaluation of HTC and Cr/HTC for hydrogen isotope analysis using OPs as target compounds. Graphical Abstract Comparison of δ²H measurement of non-chlorinated and chlorinated OPs via GC-Cr/HTC-IRMS and GC-HTC-IRMS system.

Flow and geochemical modeling of drainage from Tomitaka mine, Miyazaki, Japan

The chemistry and flow of water in the abandoned Tomitaka mine of Miyazaki, western Japan were investigated. This mine is located in a non-ferrous metal deposit and acid mine drainage issues from it. The study was undertaken to estimate the quantities of mine drainage that needs to be treated in order to avoid acidification of local rivers, taking into account seasonal variations in rainfall. Numerical models aimed to reproduce observed water levels and fluxes and chemical variations of groundwater and mine drainage. Rock-water interactions that may explain the observed variations in water chemistry are proposed. The results show that: (1) rain water infiltrates into the deeper bedrock through a highly permeable zone formed largely by stopes that are partially filled with spoil from excavations (ore minerals and host rocks); (2) the water becomes acidic (pH from 3 to 4) as dissolved oxygen oxidizes pyrite; (3) along the flow path through the rocks, the redox potential of the water becomes reducing, such that pyrite becomes stable and pH of the mine drainage becomes neutral; and (4) upon leaving the mine, the drainage becomes acidic again due to oxidation of pyrite in the rocks. The present numerical model with considering of the geochemical characteristics can simulate the main variations in groundwater flow and water levels in and around the Tomitaka mine, and apply to the future treatment of the mine drainage.