Résultat de la recherche
13C- and 15N‑Isotope Analysis of Desphenylchloridazon by Liquid Chromatography−Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography−Isotope-Ratio Mass Spectrometry
The widespread application of herbicides impacts surface water and groundwater. Metabolites (e.g., desphenylchloridazon from chloridazon) may be persistent and even more polar than the parent herbicide, which increases the risk of groundwater contamination. When parent herbicides are still applied, metabolites are constantly formed and may also be degraded. Evaluating their degradation on the basis of concentration measurements is, therefore, difficult. This study presents compound-specific stableisotope analysis (CSIA) of nitrogen- and carbon-isotope ratios at natural abundances as an alternative analytical approach to track the origin, formation, and degradation of desphenylchloridazon (DPC), the major degradation product of the herbicide chloridazon. Methods were developed and validated for carbon- and nitrogen-isotope analysis (δ13C and δ15N) of DPC by liquid chromatography−isotope-ratio mass spectrometry (LC-IRMS) and derivatization gas chromatography−IRMS (GC-IRMS), respectively. Injecting standards directly onto an Atlantis LC-column resulted in reproducible δ13C-isotope analysis (standard deviation <0.5‰) by LC-IRMS with a limit of precise analysis of 996 ng of DPC on-column. Accurate and reproducible δ15N analysis with a standard deviation of <0.4‰ was achieved by GC-IRMS after derivatization of >100 ng of DPC with 160-fold excess of (trimethylsilyl)diazomethane. Application of the method to environmental-seepage water indicated that newly formed DPC could be distinguished from “old” DPC by the different isotopic signatures of the two DPC sources.
Adsorbing vs. Nonadsorbing tracers for assessing pesticide transport in arable soils
The suitability of two different tracers to mimic the behavior of pesticides in agricultural soils and to evidence the potential for preferential flow was evaluated in outdoor lysimeter experiments. The herbicide atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] was used as a model compound. Two tracers were used: a nonadsorbing tracer (bromide) and a weakly adsorbing dye tracer (uranine). Two soils that are expected to show a different extent of macropore preferential flow were used: a well-drained sandy-loamy Cambisol (gravel soil) and a poorly drained loamy Cambisol (moraine soil). Conditions for preferential flow were promoted by applying heavy simulated rainfall shortly after pesticide application. In some of the experiments, preferential flow was also artificially simulated by injecting the solutes through a narrow tube below the root zone. With depth injection, preferential leaching of atrazine occurred shortly after application in both soil types, whereas with surface application, it occurred only in the moraine soil. Thereafter, atrazine transport was mainly through the porous soil matrix, although contributions of preferential flow were also observed. For all the application approaches and soil types, after 900 d, <3% of the applied amount of atrazine was recovered in the drainage water. Only uranine realistically illustrated the early atrazine breakthrough by transport through preferential flow. Uranine broke through during the first intense irrigation at the same time as atrazine. Bromide, however, appeared earlier than atrazine in some cases. The use of dye tracers as pesticide surrogates might assist in making sustainable decisions with respect to pesticide application timing relative to rainfall or soil potential for preferential flow.
Compound-Specific Chlorine Isotope Analysis of the Herbicides Atrazine, Acetochlor, and Metolachlor
A gas chromatography−single quadrupole mass spectrometry method was developed and validated for compound-specific chlorine isotope analysis (Cl-CSIA) of three chlorinated herbicides, atrazine, acetochlor, and metolachlor, which are widespread contaminants in the environment. For each compound, the two most abundant ions containing chlorine (202/200 for atrazine, 225/223 for acetochlor, and 240/238 for metolachlor) and a dwell time of 30 ms were determined as optimized MS parameters. A limit of precise isotope analysis for ethyl acetate solutions of 10 mg/ L atrazine, 10 mg/L acetochlor, and 5 mg/L metolachlor could be reached with an associated uncertainty between 0.5 and 1‰. To this end, samples were measured 10-fold and bracketed with two calibration standards that covered a wide range of δ37Cl values and for which amplitudes matched those of the samples within 20% tolerance. The method was applied to investigate chlorine isotope fractionation during alkaline hydrolysis of metolachlor, which showed a shift in δ37Cl of +46‰ after 98% degradation, demonstrating that chlorine isotope fractionation could be a sensitive indicator of transformation processes even when limited degradation occurs. This method, combined with large-volume solid-phase extraction (SPE), allowed application of Cl-CSIA to environmentally relevant concentrations of widespread herbicides (i.e., 0.5−5 μg/L in water before extraction). Therefore, the combination of largevolume SPE and Cl-CSIA is a promising tool for assessing the transformation processes of these pollutants in the environment.
Carbon and Chlorine Isotope Fractionation Patterns Associated with
To use compound-specific isotope analysis for confidently assessing organic contaminant attenuation in the environment, isotope fractionation patterns associated with different transformation mechanisms must first be explored in laboratory experiments. To deliver this information for the common groundwater contaminant chloroform (CF), this study investigated for the first time both carbon and chlorine isotope fractionation for three different engineered reactions: oxidative C−H bond cleavage using heat-activated persulfate, transformation under alkaline conditions (pH ∼ 12) and reductive C− Cl bond cleavage by cast zerovalent iron, Fe(0). Carbon and chlorine isotope fractionation values were −8 ± 1‰ and −0.44 ± 0.06‰ for oxidation, −57 ± 5‰ and −4.4 ± 0.4‰ for alkaline hydrolysis (pH 11.84 ± 0.03), and −33 ± 11‰ and −3 ± 1‰ for dechlorination, respectively. Carbon and chlorine apparent kinetic isotope effects (AKIEs) were in general agreement with expected mechanisms (C−H bond cleavage in oxidation by persulfate, C−Cl bond cleavage in Fe(0)-mediated reductive dechlorination and E1CB elimination mechanism during alkaline hydrolysis) where a secondary AKIECl (1.00045 ± 0.00004) was observed for oxidation. The different dual carbon-chlorine (Δδ13C vs Δδ37Cl) isotope patterns for oxidation by thermally activated persulfate and alkaline hydrolysis (17 ± 2 and 13.0 ± 0.8, respectively) vs reductive
Dual element (C-Cl) isotope approach to distinguish abiotic reactions of chlorinated methanes by Fe(0) and by Fe(II) on iron minerals at neutral and alkaline pH
A dual element C-Cl isotopic study was performed for assessing chlorinated methanes (CMs) abiotic transformation reactions mediated by iron minerals and Fe(0) to further distinguish them in natural attenuation monitoring or when applying remediation strategies in polluted sites. Isotope fractionation was investigated during carbon tetrachloride (CT) and chloroform (CF) degradation in anoxic batch experiments with Fe(0), with FeCl2(aq), and with Fe-bearing minerals (magnetite, Mag and pyrite, Py) amended with FeCl2(aq), at two different pH values (7 and 12) representative of field and remediation conditions. At pH 7, only CT batches with Fe(0) and Py underwent degradation and CF accumulation evidenced hydrogenolysis. With Py, thiolytic reduction was revealed by CS2 yield and is a likely reason for different Λ value (Δδ13C/Δδ37Cl) comparing with Fe(0) experiments at pH 7 (2.9±0.5 and 6.1±0.5, respectively). At pH 12, all CT experiments showed degradation to CF, again with significant differences in Λ values between Fe(0) (5.8±0.4) and Fe-bearing minerals (Mag, 2±1, and Py, 3.7±0.9), probably evidencing other parallel pathways (hydrolytic and thiolytic reduction). Variation of pH did not significantly affect the Λ values of CT degradation by Fe(0) nor Py. CF degradation by Fe(0) at pH 12 showed a Λ (8±1) similar to that reported at pH 7 (8±2), suggesting CF hydrogenolysis as the main reaction and that CF alkaline hydrolysis (13.0±0.8) was negligible. Our data establish a base for discerning the predominant or combined pathways of CMs natural attenuation or for assessing the effectiveness of remediation strategies using recycled minerals or Fe(0). keywords (6 words): CSIA, carbon tetrachloride, chloroform, pyrite, Fe(0), degradation pathways
Compound-Specific Chlorine Isotope Analysis of Tetrachloromethane and Trichloromethane by Gas Chromatography-Isotope Ratio Mass Spectrometry vs Gas Chromatography-Quadrupole Mass Spectrometry: Method Development and Evaluation of Precision and Trueness
Compound-specific chlorine isotope analysis of tetrachloromethane (CCl4) and trichloromethane (CHCl3) was explored by both, gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and GC-quadrupole MS (GC-qMS), where GC-qMS was validated in an interlaboratory comparison between Munich and Neuchâtel with the same type of commercial GC-qMS instrument. GC-IRMS measurements analyzed CCl isotopologue ions, whereas GC-qMS analyzed the isotopologue ions CCl3, CCl2, CCl (of CCl4) and CHCl3, CHCl2, CHCl (of CHCl3), respectively. Lowest amount dependence (good linearity) was obtained (i) in H-containing fragment ions where interference of 35Cl- to 37Cl-containing ions was avoided; (ii) with tuning parameters favoring one predominant rather than multiple fragment ions in the mass spectra. Optimized GC-qMS parameters (dwell time 70 ms, 2 most abundant ions) resulted in standard deviations of 0.2‰ (CHCl3) and 0.4‰ (CCl4), which are only about twice as large as 0.1‰ and 0.2‰ for GC-IRMS. To compare also the trueness of both methods and laboratories, samples from CCl4 and CHCl3 degradation experiments were analyzed and calibrated against isotopically different reference standards for both CCl4 and CHCl3 (two of each). Excellent agreement confirms that true results can be obtained by both methods provided that a consistent set of isotopically characterized reference materials is used.