Normal and Inverse Diffusive Isotope Fractionation of Deuterated Toluene and Benzene in Aqueous Systems

Abstract

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Magnitude of Diffusion- and Transverse Dispersion-Induced Isotope Fractionation of Organic Compounds in Aqueous Systems

“…Therefore, the observed isotope fractionation induced by diffusion of benzene, toluene, ethylbenzene, BAM, and metolachlor at natural isotopic abundance in aqueous phase was weak to negligible. The calculated β-values of the isotopologue pairs for our target compounds are without exception smaller than 0.1 showing a similarly weak mass dependence as observed in other studies 6,7,16−18 for CO 2 , CH 4 , C 2 H 6 , TCE, 1,2-DCA, and 15 In panel (d), the black dashed line represents the regression (β 0 = 0.37) without the anomalously high D H /D L value of Ar isotopes, 46 and the black dash-dotted line represents the regression (β 0 = 0.04) without low D H /D L -values of Ar and He isotopes data. 22,30 Most of the compounds data points are labeled with compound names except for some of the NatOrgs due to the limit of space.…”

“…cis-DCE at natural isotopic abundance (Table 1). This dependence is particularly weak when compared to much larger β-values (β = 0.4 to 0.5) reported for labeled toluene and ethylbenzene 7,15,19 (Figure 3; note that we have omitted stark outliers from labeled studies in this comparison, which will be discussed later on). This raises the question of the underlying reasons for the different behavior of labeled and nonlabeled organic compounds.…”

“…Finally, stark outliers have been reported for labeled molecules, where the inconsistent diffusion-induced isotope fractionation of labeled benzene and toluene in different experimental setups stand out. Rolle et al 15 observed significant normal diffusion-induced isotope fractionation for labeled toluene and ethylbenzene, and reverse isotope fractionation for labeled benzene, whereas Kopinke et al 20 reported negligible isotope fractionation of labeled benzene, toluene, and cyclohexane and hypothesized that the different solvent matrix (aqueous diffusion in agar gel vs water) may induce the contradictory mass dependence. Presently, it is difficult to explain this inconsistency—future studies may explore whether it can be traced back to specific features of the experimental setups, or even to measurement protocols, the linearity and accuracy of which are not as stringently established for GC–MS as for GC–IRMS.…”

“… 19 , 37 − 39 In such simulations, diffusion coefficients of heavy and light isotopologues were usually estimated by the Enskog 27 or the Worch relation, 40 implying an exponent β = 0.5 or 0.53 in eq 2 , which resulted in large dispersion-induced isotope fractionation in these models. 15 , 19 , 37 , 38 , 41 …”

“…19,37−39 In such simulations, diffusion coefficients of heavy and light isotopologues were usually estimated by the Enskog 27 or the Worch relation, 40 implying an exponent β = 0.5 or 0.53 in eq 2, which resulted in large dispersion-induced isotope fractionation in these models. 15,19,37,38,41 This study aims at experimentally re-examining isotope fractionation by diffusion and transverse dispersion for organic compounds at natural isotopic abundance. We applied compound-specific isotope analysis to investigate the diffusion-induced isotope fractionation with increasing molecular mass and decreasing mass ratio of heavy to light isotopologues for benzene, toluene, ethylbenzene, 2,6-dichlorobenzamide (BAM), and metolachlor in the aqueous phase.…”