Isotope Effects on the Vaporization of Organic Compounds from an Aqueous Solution-Insight from Experiment and Computations.

An isotope fractionation analysis of organic groundwater pollutants can assess the remediation at contaminated sites yet needs to consider physical processes as potentially confounding factors. This study explores the predictability of water-air partitioning isotope effects from experiments and computational predictions for benzene and trimethylamine (both H-bond acceptors) as well as chloroform (H-bond donor). A small, but significant, isotope fractionation of different direction and magnitude was measured with ε = -0.12‰ ± 0.07‰ (benzene), εC = 0.49‰ ± 0.23‰ (triethylamine), and εH = 1.79‰ ± 0.54‰ (chloroform) demonstrating that effects do not correlate with expected hydrogen-bond functionalities. Computations revealed that the overall isotope effect arises from contributions of different nature and extent: a weakening of intramolecular vibrations in the condensed phase plus additional vibrational modes from a complexation with surrounding water molecules. Subtle changes in benzene contrast with a stronger coupling between intra- and intermolecular modes in the chloroform-water system and a very local vibrational response with few atoms involved in a specific mode of triethylamine. An energy decomposition analysis revealed that each system was affected differently by electrostatics and dispersion, where dispersion was dominant for benzene and electrostatics dominated for chloroform and triethylamine. Interestingly, overall stabilization patterns in all studied systems originated from contributions of dispersion rather than other energy terms.

Toward Improved Accuracy in Chlorine Isotope Analysis: Synthesis Routes for In-House Standards and Characterization via Complementary Mass Spectrometry Methods.

Increasing applications of compound-specific chlorine isotope analysis (CSIA) emphasize the need for chlorine isotope standards that bracket a wider range of isotope values in order to ensure accurate results. With one exception (USGS38), however, all international chlorine isotope reference materials (chloride and perchlorate salts) fall within the narrow range of one per mille. Furthermore, compound-specific working standards are required for chlorine CSIA but are not available for most organic substances. We took advantage of isotope effects in chemical dehalogenation reactions to generate (i) silver chloride (CT16) depleted in 37Cl/35Cl and (ii) compound-specific standards of the herbicides acetochlor and S-metolachlor (Aceto2, Metola2) enriched in 37Cl/35Cl. Calibration against the international reference standards USGS38 (-87.90 ‰) and ISL-354 (+0.05 ‰) by complementary methods (gas chromatography-isotope ratio mass spectrometry, GC-IRMS, versus gas chromatography-multicollector inductively coupled plasma mass spectrometry, GC-MC-ICPMS) gave a consensus value of δ37ClCT16 = -26.82 ± 0.18 ‰. Preliminary GC-MC-ICPMS characterization of commercial Aceto1 and Metola1 versus Aceto2 and Metola2 resulted in tentative values of δ37ClAceto1 = 0.29 ± 0.29 ‰, δ37ClAceto2 = 18.54 ± 0.20 ‰, δ37ClMetola1 = -4.28 ± 0.17 ‰ and δ37ClMetola2 = 5.12 ± 0.27 ‰. The possibility to generate chlorine isotope in-house standards with pronounced shifts in isotope values offers a much-needed basis for accurate chlorine CSIA.

Compound-Specific Stable Isotope Fractionation of Pesticides and Pharmaceuticals in a Mesoscale Aquifer Model.

Compound-specific isotope analysis (CSIA) receives increasing interest for its ability to detect natural degradation of pesticides and pharmaceuticals. Despite recent laboratory studies, CSIA investigations of such micropollutants in the environment are still rare. To explore the certainty of information obtainable by CSIA in a near-environmental setting, a pulse of the pesticide bentazone, the pesticide metabolite 2,6-dichlorobenzamide (BAM), and the pharmaceuticals diclofenac and ibuprofen was released into a mesoscale aquifer with quasi-two-dimensional flow. Concentration breakthrough curves (BTC) of BAM and ibuprofen demonstrated neither degradation nor sorption. Bentazone was transformed but did not sorb significantly, whereas diclofenac showed both degradation and sorption. Carbon and nitrogen CSIA could be accomplished in similar concentrations as for "traditional" priority pollutants (low µg/L range), however, at the cost of uncertainties (0.4-0.5‰ (carbon), 1‰ (nitrogen)). Nonetheless, invariant carbon and nitrogen isotope values confirmed that BAM was neither degraded nor sorbed, while significant enrichment of (13)C and in particular (15)N corroborated transformation of diclofenac and bentazone. Retardation of diclofenac was reflected in additional (15)N sorption isotope effects, whereas isotope fractionation of transverse dispersion could not be identified. These results provide a benchmark on the performance of CSIA to monitor the reactivity of micropollutants in aquifers and may guide future efforts to accomplish CSIA at even lower concentrations (ng/L range).

Characteristic isotope fractionation patterns in s-triazine degradation have their origin in multiple protonation options in the s-triazine hydrolase TrzN.

s-Triazine herbicides (atrazine, ametryn) are groundwater contaminants which may undergo microbial hydrolysis. Previously, inverse nitrogen isotope effects in atrazine degradation by Arthrobacter aurescens TC1 (i) delivered highly characteristic (13C/12C, 15N/14N) fractionation trends for pathway identification and (ii) suggested that the s-triazine ring nitrogen was protonated in the enzyme s-triazine hydrolase (TrzN) where (iii) TrzN crystal structure and mutagenesis indicated H+-transfer from the residue E241. This study tested the general validity of these conclusions for atrazine and ametryn with purified TrzN and a TrzN-E241Q site-directed mutant. TrzN-E241Q lacked activity with ametryn; otherwise, degradation consistently showed normal carbon isotope effects (εcarbon=-5.0‰±0.2‰ (atrazine/TrzN), εcarbon=-4.2‰±0.5‰ (atrazine/TrzN-E241Q), εcarbon=-2.4‰±0.3‰ (ametryn/TrzN)) and inverse nitrogen isotope effects (εnitrogen=2.5‰±0.1‰ (atrazine/TrzN), εnitrogen=2.1‰±0.3‰ (atrazine/TrzN-E241Q), εnitrogen=3.6‰±0.4‰ (ametryn/TrzN)). Surprisingly, TrzN-E241Q therefore still activated substrates through protonation implicating another proton donor besides E241. Sulfur isotope effects were larger in enzymatic (εsulfur=-14.7‰±1.0‰, ametryn/TrzN) than in acidic ametryn hydrolysis (εsulfur=-0.2‰±0.0‰, pH 1.75), indicating rate-determining C-S bond cleavage in TrzN. Our results highlight a robust inverse 15N/14N fractionation pattern for identifying microbial s-triazine hydrolysis in the environment caused by multiple protonation options in TrzN.