¹³C- and ¹⁵N-Isotope Analysis of Desphenylchloridazon by Liquid Chromatography–Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography–Isotope-Ratio Mass Spectrometry

Abstract

Widespread application of herbicides impacts surface water and groundwater. Their metabolites (e.g., desphenylchloridzon 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 in addition be degraded. Evaluating their degradation based on concentration measurements is, therefore, difficult. This study presents compound-specific stable isotope analysis (CSIA) of nitrogen and carbon isotope ratios at natural abundances as alternative analytical approach to track 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 (δ 13 C and δ 15 N) 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 δ 13 C isotope analysis (standard deviation < 0.5 ‰) by LC-IRMS with a limit of precise analysis of 996 ng DPC on-column. Accurate and reproducible δ 15 N analysis with a standard deviation < 0.4 ‰ was achieved by GC-IRMS after derivatization of > 100 ng 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 different isotopic signatures of the two DPC sources.

Dual-Element Isotope Analysis of Desphenylchloridazon to Investigate Its Environmental Fate in a Systematic Field Study: A Long-Term Lysimeter Experiment

“…Additionally, this study confirmed that the application of CSIA in combination with solid-phase extraction 27,39 is feasible for the analysis of polar micropollutants in drainage water at environmentally relevant concentrations. Thus, it can be also applied to studies in agricultural soil and groundwater from common unconsolidated sand and/or gravel aquifers with catchment areas within agricultural production.…”

“…On the other hand, for both moraine and gravel soil, δ 15 N values of the DPC formed are, as hypothesized in Figure S1, close to the nitrogen isotope signature of the applied CLZ. Based on the findings of Lingens et al 20 , the pyridazinone-ring of the CLZ molecule is not involved in the first transformation steps (dioxygenation of the phenyl-ring, Figure S1) so that no significant nitrogen isotope fractionation is expected during CLZ transformation to DPC 27,29,48,49 . As the isotope effect during multi-step reactions is reflected by the rate-limiting steps, our results indicate that the amidase-driven cleavage of the moiety (2-hydroxymuconate) at the C-N bond, may be not ratelimiting.…”

“…Derivatization of DPC for Nitrogen Isotope Analysis. Nitrogen isotope analysis was conducted using the derivatization procedure proposed by Melsbach et al 27 . Briefly, DPC was methylated to MDPC by adding an excess of greater than 160 nanalyte/nTMSD (140 μL of a 2 M TMSD solution) into a vial containing a standard or a SPE extract reconstituted in 1 mL methanol.…”

“…The method is briefly summarized in the Supporting Information (II.7.) 27 . Additionally, both DPC and MDPC fractions were used for δ 15 N isotope analysis when possible.…”

“…Correction Procedure for Isotope Analysis. Analogous to the correction procedure described by Melsbach et al 27 , all samples and standards were measured in triplicate and their isotope ratios are reported as the arithmetic means with their respective estimated standard deviations (± σ). In addition to the calibration of the measurement gas, samples are bracketed within the sequences by in-house standards of DPC and MDPC, whose isotopic signature had been determined with EA-IRMS (Table S4).…”