Molecularly Imprinted Polymers for Compound-Specific Isotope Analysis of Polar Organic Micropollutants in Aquatic Environments

Novel extraction methods and compound-specific isotope analysis of methoxychlor in environmental water and aquifer slurry samples

Multi-element compound-specific stable isotope analysis (ME-CSIA) allows monitoring the environmental behavior and transformation of most common and persistent contaminants. Recent advancements in analytical techniques have extended the applicability of ME-CSIA to organic micropollutants, including pesticides. Nevertheless, the application of this methodology remains unexplored concerning harmful insecticides such as methoxychlor, a polar organochlorine pesticide usually detected in soil and groundwater. This study introduces methods for dual carbon and chlorine compound-specific stable isotope analysis (δ13C-CSIA and δ37Cl-CSIA) of both methoxychlor and its metabolite, methoxychlor olefin, with a sensitivity down to 10 and 100 mg/L, and a precision lower than 0.3 and 0.5 ‰ for carbon and chlorine CSIA, respectively. Additionally, three extraction and preconcentration techniques suitable for ME-CSIA of the target pesticides at environmentally relevant concentrations were also developed. Solid-phase extraction (SPE) and liquid-solid extraction (LSE) effectively extracted methoxychlor (107 ± 27 % and 87 ± 13 %, respectively) and its metabolite (91 ± 27 % and 106 ± 14 %, respectively) from water and aquifer slurry samples, respectively, with high accuracy (Δδ13C and Δδ37Cl ≤ ± 1 ‰). Combining CSIA with polar organic chemical integrative samplers (POCISs) for the extraction of methoxychlor and methoxychlor olefin from water samples resulted in insignificant fractionation for POCIS-CSIA (Δδ13C ≤ ± 1 ‰). A relevant sorption of methoxychlor was detected within the polyethersulfones membranes of the POCISs resulting in temporary carbon isotope fractionation depending on the sorbed mass fraction during the first deployment days. This highlights the critical role of the interactions of polar analytes with POCIS sorbents and membranes in the performance of this method. Altogether, this study proposes a proof of concept for ME-CSIA of methoxychlor and its metabolites, opening the door for future investigations of their sources and transformation processes in contaminated sites.

Quartz Crystal Microbalance as a Holistic Detector for Quantifying Complex Organic Matrices during Liquid Chromatography: 2. Compound-Specific Isotope Analysis

In carbon-compound-specific isotope analysis (carbon CSIA) of environmental micropollutants, purification of samples is often required to guarantee accurate measurements of a target compound. A companion paper has brought forward an innovative approach to couple a quartz crystal microbalance (QCM) with high-performance liquid chromatography (HPLC) for the online quantification of matrices during a gradient HPLC purification. This work investigates the benefit for isotope analysis of polar micropollutants typically present in environmental samples. Here, we studied the impact of the natural organic matter (NOM) on the isotopic integrity of model analytes and the suitability of the NOM-to-analyte ratio as a proxy for the sample purity. We further investigated limitations and enhancement of HPLC purification using QCM on C18 and C8 phases for single and multiple targets. Strong isotopic shifts of up to 3.3% toward the isotopic signature of NOM were observed for samples with an NOM-to-analyte ratio ≥10. Thanks to QCM, optimization of matrix removal of up to 99.8% of NOM was possible for late-eluting compounds. The efficiency of HPLC purification deteriorated when aiming for simultaneous purification of two or three compounds, leading to up to 2.5% less NOM removal. Our results suggest that one optimized HPLC purification can be achieved through systematic screening of 3 to 5 different gradients, thereby leading to a shift of the boundaries of accurate carbon CSIA by up to 2 orders of magnitude toward lower micropollutant concentrations.

Assessment of stable carbon isotope 13С/12С ratio in phthalates from surface waters using HPLC-HRMS-TOF approach

A method for estimating the ratio of stable carbon isotopes 13С/12С in the composition of phthalates from surface water at a trace concentration level is proposed. It is based on the concentration of hydrophobic components of water using an analytical reversed phase HPLC column followed by their gradient separation and detection of eluted phthalates using a high-resolution time-of-flight mass spectrometer (ESI-HRMS-TOF) in the form of molecular ions. The ratio of stable carbon isotopes 13С/12C in phthalates is calculated as a ratio of integrals under the monoisotopic [M+1+H]+ and [M+H]+ peaks. The Δ13C value is calculated relatively to the 13C/12C ratio in commercial DnBP and DEHP phthalates used as standards. The minimal concentration of DnBP and DEHP in water required for a reliable determination of Δ13C value is estimated by the level of ca. 0.2 μg L-1. The technique has been verified during the monitoring of priority phthalates in the waters of Lake Baikal.

Avoiding Interferences in Advance: Cyclodextrin Polymers to Enhance Selectivity in Extraction of Organic Micropollutants for Carbon Isotope Analysis

Compound-specific isotope analysis (CSIA) of organic water contaminants can provide important information about their sources and fate in the environment. Analyte enrichment from water remains nonetheless a critical yet inevitable step before measurement. Commercially available solid-phase extraction (SPE) sorbents are inherently nonselective leading to co-extraction of concurrent dissolved organic matter (DOM) and in turn to analytical interferences, especially for low-occurring contaminants. Here, we (i) increased extraction selectivity by synthesizing cyclodextrin polymers (α-, β-, γ-CDP) as SPE sorbents, (ii) assessed their applicability to carbon isotope analysis for a selection of pesticides, and (iii) compared them with commonly used commercial sorbents. Extraction with β-CDP significantly reduced backgrounds in gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and enhanced sensitivity by a factor of 7.5, which was further confirmed by lower carbon-normalized C_DOM/C_analyte ratios in corresponding extracts as derived from dissolved organic carbon (DOC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Gibbs free energies of adsorption demonstrated weak competition between DOM and analyte on the three CDPs. No isotopic fractionation (Δδ¹³C within ± 0.3‰) was observed for the investigated pesticides after using β-CDP as an SPE sorbent covering a range of concentrations (5-500 μg L^-1), flow velocities (5-40 cm min^-1), and sorbent regeneration (up to six times). The present study highlights the benefit of selecting innovative extraction sorbents to avoid interferences in advance. This strategy in combination with existing cleanup approaches offers new prospects for CSIA at field concentrations of tens to hundreds of nanograms per liter.

Microbial hydrolysis of atrazine in contaminated groundwater

Degradation of the widespread herbicide atrazine has been intensively studied in soils, while its degradation in groundwater has received less attention. This work studied atrazine degradation in contaminated groundwater adjacent to its production plant. The degradation potential was first explored in groundwater enrichment cultures. A broad potential for microbial atrazine degradation was observed when atrazine served as the sole nitrogen source, even when incubated with nitrate. Hydroxyatrazine was formed by the cultures, while desethylatrazine and desisopropylatrazine were not detected. Both the atzA and the trzN genes were identified by quantitative PCR analysis, with a clear dominance of atzA. Carbon isotope enrichments throughout the degradation process varied between the different cultures, with ε values ranging from -0.6 to -5.5‰. This implies corresponding uncertainties when using compound-specific isotope analysis to estimate degradation extents. In the field samples, in-situ degradation was reflected by a high percentage of metabolites, with hydroxyatrazine accounting for >95% of the metabolites in most wells. Both atzA and trzN were detected in the groundwater at quantities of ≈10² to 10⁶ copies mL^-1, with a dominance of atzA over trzN. These results provide evidence of the high potential for atrazine hydrolysis in the contaminated groundwater.

Discovering Nature's Fingerprints: Isotope Ratio Analysis on Bioanalytical Mass Spectrometers

For a generation or more, the mass spectrometry that developed at the frontier of molecular biology was worlds apart from isotope ratio mass spectrometry, a label-free approach done on optimized gas-source magnetic sector instruments. Recent studies show that electrospray-ionization Orbitraps and other mass spectrometers widely used in the life sciences can be fine-tuned for high-precision isotope ratio analysis. Since isotope patterns form everywhere in nature based on well-understood principles, intramolecular isotope measurements allow unique insights into a fascinating range of research topics. This Perspective introduces a wider readership to current topics in stable isotope research with the aim of discussing how soft-ionization mass spectrometry coupled with ultrahigh mass resolution can enable long-envisioned progress. We highlight novel prospects of observing isotopes in intact polar compounds and speculate on future directions of this adventure into the overlapping realms of biology, chemistry, and geology.

Simple extraction methods for pesticide compound-specific isotope analysis from environmental samples

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Extractions from soil, plants, and water were tested for pesticide C and N CSIA.

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Pesticide recoveries strongly varied among compounds and matrices properties.

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Tested extraction methods caused no effect on δ¹³C and δ¹⁵N of pesticides.

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C and N pesticide CSIA can be applied in situ to agricultural water samples.

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Pesticide CSIA for soil and sediment samples are limited to source areas.

Compound-specific isotope analysis (CSIA) is a powerful approach to evaluate the transformation of organic pollutants in the environment. However, the application of CSIA to micropollutants, such as pesticides, remains limited because appropriate extraction methods are currently lacking. Such methods should address a wide range of pesticides and environmental matrices, while recovering sufficient mass for reliable CSIA without inducing stable isotope fractionation. Here, we present simple extraction methods for carbon and nitrogen CSIA for different environmental matrices and six commonly used herbicides, i.e., atrazine, terbutryn, acetochlor, alachlor, butachlor, and S-metolachlor, and three fungicides, i.e., dimethomorph, tebuconazole, and metalaxyl. We examined the potential of several extraction methods for four types of soils or sediments, three types of environmental waters and aerial and root plant samples for multielement (ME)-CSIA.•

Pesticide extraction recoveries varied depending on the physical characteristics of the pesticides and matrix properties for environmental water (77 to 87%), soil and sediment (35 to 82%), and plant (40 to 59%) extraction.

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The tested extraction methods did not significantly affect the carbon and nitrogen stable isotope signatures of pesticides (Δ(¹³C) <0.9‰ for Δ(¹⁵N) <1.0‰).

Understanding adsorption and biodegradation in granular activated carbon for drinking water treatment: A critical review

Drinking water treatment plants use granular activated carbon (GAC) to adsorb and remove trace organics, but the GAC has a limited lifetime in terms of adsorptive capacity and needs to be replaced before it is exhausted. Biological degradation of target contaminants can also occur in GAC filters, which might allow the GAC to remain in service longer than expected. However, GAC biofiltration remains poorly understood and unpredictable. To increase the understanding of adsorption and biodegradation in GAC, previous studies have conducted parallel column tests that use one column of GAC (potentially biologically active) to assess overall removal via both adsorption and biodegradation, and one column with either sterilized GAC or biological non-adsorbing media to assess adsorption or biodegradation alone. Mathematical models have also been established to give insight into the adsorption and biodegradation processes in GAC. In this review, the experimental and modeling approaches and results used to distinguish between the role of adsorption and biodegradation were summarized and critically discussed. We identified several limitations: (1) using biological non-adsorbing media in column tests might lead to non-representative extents of biodegradation; (2) sterilization methods may not effectively inhibit biological activity and may affect adsorption; (3) using virgin GAC coated with biofilm could overestimate adsorption; (4) potential biofilm detachment during column experiments could lead to biased results; (5) the parallel column test approach itself is not universally applicable; (6) competitive adsorption was neglected by previous models; (7) model formulations were based on virgin GAC only. To overcome these limitations, we proposed four new approaches: the use of gamma irradiation for sterilization, a novel minicolumn test, compound-specific isotope analysis to decipher the role of adsorption and biodegradation in situ, and a new model to simulate trace organic adsorption and biodegradation in a GAC filter .

Phase Transfer and Biodegradation of Pesticides in Water-Sediment Systems Explored by Compound-Specific Isotope Analysis and Conceptual Modeling

Current approaches are often limited to evaluating the contribution of pesticide dissipation processes in water-sediment systems as both degradation and phase transfer, that is, sorption-desorption, contribute to the apparent decrease of pesticide concentration. Here, the dissipation of widely used herbicides acetochlor and S-metolachlor was examined in laboratory by water-sediment microcosm experiments under oxic and anoxic conditions. Compound-specific isotope analysis (CSIA) emphasized insignificant carbon isotope fractionation in the sediment, indicating prevailing pesticide degradation in the water phase. Conceptual modeling accounting for phase transfer and biodegradation indicated that biodegradation may be underestimated when phase transfer is not included. Phase transfer does not affect carbon isotope fractionation for a wide spectrum of molecules and environmental conditions, underscoring the potential of pesticide CSIA as a robust approach to evaluate degradation in water-sediment systems. CSIA coupled with the identification of transformation products by high-resolution tandem mass spectrometry suggests the degradation of acetochlor and S-metolachlor to occur via nucleophilic substitution and the predominance of oxalinic acids as transformation products under both anoxic and oxic conditions. Altogether, combining the pesticide CSIA, the identification of transformation products, and the use of conceptual phase-transfer models improves the interpretation of pesticide dissipation in water-sediment systems.

Isotope fractionation of micropollutants during large-volume extraction: heads-up from a critical method evaluation for atrazine, desethylatrazine and 2,6-dichlorobenzamide at low ng/L concentrations in groundwater

Micropollutants are frequently detected in groundwater. Thus, the question arises whether they are eliminated by natural attenuation so that pesticide degradation would be observed with increasing residence time in groundwater. Conventional analytical approaches rely on parent compound/metabolite ratios. These are difficult to interpret if metabolites are sorbed or further transformed. Compound-specific stable isotope analysis (CSIA) presents an alternative for identifying degradation based on the analysis of natural isotope abundances in pesticides and their changes during degradation. However, CSIA by gas chromatography-isotope ratio mass spectrometry is challenged by the low concentrations (ng/L) of micropollutants in groundwater. Consequently, large amounts of water need to be sampled requiring enrichment and clean-up steps from interfering matrix effects that must not introduce artefacts in measured isotope values. The aim of this study was to evaluate the accuracy of isotope ratio measurements of the frequently detected micropollutants atrazine, desethylatrazine and 2,6-dichlorobenzamide after enrichment from large water volumes (up to 100 L) by solid-phase extraction with consecutive clean-up by HPLC. Associated artefacts of isotope discrimination were found to depend on numerous factors including organic matter content and extraction volume. This emphasizes the necessity to perform a careful method evaluation of sample preparation and sample pre-treatment prior reliable CSIA.

Recent advances and applications of molecularly imprinted polymers in solid-phase extraction for real sample analysis

Sample pretreatment is essential for the analysis of complicated real samples due to their complex matrices and low analyte concentrations. Among all sample pretreatment methods, solid-phase extraction is arguably the most frequently used one. However, the majority of available solid-phase extraction adsorbents suffer from limited selectivity. Molecularly imprinted polymers are a type of tailor-made artificial antibodies and receptors with specific recognition sites for target molecules. Using molecularly imprinted polymers instead of conventional adsorbents can greatly improve the selectivity of solid-phase extraction, and therefore molecularly imprinted polymer-based solid-phase extraction has been widely applied to separation, clean up and/or preconcentration of target analytes in various kinds of real samples. In this article, after a brief introduction, the recent developments and applications of molecularly imprinted polymer-based solid-phase extraction for determination of different analytes in complicated real samples during the 2015-2020 are reviewed systematically, including the solid-phase extraction modes, molecularly imprinted adsorbent types and their preparations, and the practical applications of solid-phase extraction to various real samples (environmental, food, biological, and pharmaceutical samples). Finally, the challenges and opportunities of using molecularly imprinted polymer-based solid-phase extraction for real sample analysis are discussed.

Isotope analysis method for the herbicide bromoxynil and its application to study photo-degradation processes

Bromoxynil is an increasingly applied nitrile herbicide used for post-emergent control of annual broadleaved weeds. Compound-specific isotope analysis (CSIA) of the compound is of interest for studying its environmental fate, yet is challenging following its polar nature. We present a CSIA method for bromoxynil that includes offline thin-layer chromatography purification followed by an elemental analyzer isotope ratio mass spectrometer (EA-IRMS). This method was shown to be accurate and precise for δ¹³C and δ¹⁵N analysis of the compound (standard deviation of replicate standards <0.5‰). The method was applied to photodegraded samples, either radiated under laboratory condition with a UV lamp, or exposed to sunlight under environmental conditions. Dominating degradation products were similar in both cases. Nevertheless, isotope effects differed, presenting a strong inverse carbon isotope effect (εC = 4.74 ± 0.82‰) and a weak inverse nitrogen isotope effect (εN = 0.76 ± 0.12‰) for the laboratory experiment, and an insignificant carbon isotope effect (εC = 0.34 ± 0.44‰) and a normal nitrogen isotope effect (εN = -3.70 ± 0.30‰) for the natural conditions experiment. The differences in δ¹³C vs. δ¹⁵N enrichment trends suggest different mechanism for the two processes. Finally, the obtained dual isotope trend for natural conditions provide the basis for studying the dominance of photodegradation as a degradation route in the environment.

Molecularly imprinted sol-gel polymers for the analysis of iprodione fungicide in wine: Synthesis in green solvent

Iprodione is a fungicide widely used in viticulture in most agricultural countries. It was banned recently in the European community because of its carcinogenic and endocrine disrupting characters. In this work, a cheap analytical method able to monitor iprodione in a white wine was developed. Molecularly imprinted sol-gel polymers (MIS) specific to iprodione and using green solvents were synthesized. An experimental design having the following factors (solvent volume and crosslinker quantity) was used to prepare an optimal MIS. In terms of selectivity, the optimal MIS showed the best partition coefficient towards iprodione in a white wine containing four other competing fungicides (procymidone, pyrimethanil, azoxystrobin and iprovalicarb). A solid phase extraction method using the optimal MIS was optimized and applied to analyse iprodione in a white wine. Low detection and quantification limits were reached 11.7 and 39.1 µg/L respectively.

Solid-phase extraction method for stable isotope analysis of pesticides from large volume environmental water samples

Compound-specific isotope analysis (CSIA) is a valuable tool for assessing the fate of organic pollutants in the environment. However, the requirement of sufficient analyte mass for precise isotope ratio mass spectrometry combined with prevailing low environmental concentrations currently limits comprehensive applications to many micropollutants. Here, we evaluate the upscaling of solid-phase extraction (SPE) approaches for routine CSIA of herbicides. To cover a wide range of polarity, a SPE method with two sorbents (a hydrophobic hypercrosslinked sorbent and a hydrophilic sorbent) was developed. Extraction conditions, including the nature and volume of the elution solvent, the amount of sorbent and the solution pH, were optimized. Extractions of up to 10 L of agricultural drainage water (corresponding to up to 200 000-fold pre-concentration) were successfully performed for precise and sensitive carbon and nitrogen CSIA of the target herbicides atrazine, acetochlor, metolachlor and chloridazon, and metabolites desethylatrazine, desphenylchloridazon and 2,6-dichlorobenzamide in the sub-μg L^-1-range. ¹³C/¹²C and ¹⁵N/¹⁴N ratios were measured by gas chromatography-isotope ratio mass spectrometry (GC/IRMS), except for desphenylchloridazon, for which liquid chromatography (LC/IRMS) and derivatization-GC/IRMS were used, respectively. The method validated in this study is an important step towards analyzing isotope ratios of pesticide mixtures in aquatic systems and holds great potential for multi-element CSIA applications to trace pesticide degradation in complex environments.

Improved Enantioselectivity for Atenolol Employing Pivot Based Molecular Imprinting

In the last few decades, molecular imprinting technology went through a spectacular evolution becoming a well-established tool for the synthesis of highly selective biomimetic molecular recognition platforms. Nevertheless, there is still room for advancement in the molecular imprinting of highly polar chiral compounds. The aim of the present work was to investigate the favorable kosmotropic effect of a ternary complex involving a polar chiral template (eutomer of atenolol) and a functional monomer, bridged by a central metal ion through well-defined, spatially directional coordinate bonds. The efficiency of the chiral molecular recognition was systematically assessed on polymers obtained both by non-covalent and metal-mediated molecular imprinting. The influence on the chromatographic retention and enantioselectivity of different experimental variables (functional monomers, cross-linkers, chaotropic agents, metal ions, porogenic systems, etc.) were studied on both slurry packed and monolithic HPLC columns. Deliberate changes in the imprinting and rebinding (chromatographic) processes, along with additional thermodynamic studies shed light on the particularities of the molecular recognition mechanism. The best performing polymer in terms of enantioselectivity (α = 1.60) was achieved using 4-vinyl pyridine as functional monomer and secondary ligand for the Co(II)-mediated imprinting of S-atenolol in the presence of EDMA as cross-linker in a porogenic mixture of [BMIM][BF₄]:DMF:DMSO = 10:1:5, v/v/v.