Molecularly-imprinted polymers for compound-specific isotope

which custom-made MIPs enabled the selective cleanup of enriched analytes in .... through the synthesis and application of a custom-made MIP for 1H-BT...
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Molecularly-imprinted polymers for compound-specific isotope analysis of polar organic micropollutants in aquatic environments Rani Bakkour, Jakov Bolotin, Börje Sellergren, and Thomas B. Hofstetter Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00493 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018

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Analytical Chemistry

Molecularly-imprinted polymers for compound-specific isotope analysis of polar organic micropollutants in aquatic environments Rani Bakkour,†,‡, ¶ Jakov Bolotin,† Börje Sellergren,§ and Thomas B. Hofstetter∗,†,‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zürich, Switzerland, , and Department of Biomedical Sciences, Malmö University, Malmö, Sweden E-mail: [email protected]

∗ To

whom correspondence should be addressed

† Eawag ‡ ETH

Zürich address: Technical University of Munich, Chair for Analytical Chemistry and Water Chemistry, München,

¶ present

Germany § Malmö University

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Abstract

1

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Compound-specific isotope analysis (CSIA) of polar organic micropollutants in environ-

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mental waters requires a processing of large sample volumes to obtain the required analyte

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masses for analysis by gas chromatography / isotope-ratio mass spectrometry (GC/IRMS).

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However, the accumulation of organic matter of unknown isotopic composition in standard en-

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richment procedures currently compromises the accurate determination of isotope ratios. We

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explored the use of molecularly-imprinted polymers (MIPs) for selective analyte enrichment

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for 13 C/12 C and 15 N/14 N ratio measurements by GC/IRMS using 1H-benzotriazole, a typical

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corrosion inhibitor in dishwashing detergents, as example of a widely detected polar organic

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micropollutant. We developed procedures for the treatment of > 10 L of water samples, in

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which custom-made MIPs enabled the selective cleanup of enriched analytes in organic sol-

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vents obtained through conventional solid phase extractions. Hydrogen bonding interactions

13

between the traziole moiety of 1H-benzotriazole and the MIP were responsible for selective

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interactions through an assessment of interaction enthalpies and 15 N isotope effects. The pro-

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cedure was applied successfully without causing isotope fractionation to river water samples

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as well as in- and effluents of wastewater treatment plants containing µg/L concentrations of

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1H-benzotriazole and dissolved organic carbon (DOC) loads of up to 28 mg C/L. MIP-based

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treatments offer new perspectives for CSIA of organic micropollutants through the reduction

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of the DOC-to-micropollutant ratios.

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Analytical Chemistry

20

Introduction

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The analysis of stable isotope ratios in organic soil and water contaminants has become a widely used

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approach to identify the sources of pollution and to identify (bio)degradation pathways. 1–7 Whereas

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constant ratios of 13 C/12 C, 2 H/1 H, 15 N/14 N, and of other elements in a compound enable one to infer

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precursor materials, synthesis routes, and formation pathways of pollutants, 8–14 changes of isotope

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ratios lead to stable isotope fractionation patterns that reveal the (bio)chemical reaction by which a

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pollutant is degraded. 15–21 However, due to the poor sensitivity of gas and liquid chromatography

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used in combination with isotope-ratio mass spectrometry, 22–25 the applications of compound-

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specific isotope analysis (CSIA) have largely focused on so-called legacy contaminants such as

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halogenated solvents, nitroaromatic explosives, and fuel constituents 26–33 Those compounds are

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often found in the high µg L−1 to mg L−1 concentration range and can be extracted from the

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environmental matrices in straightforward procedures, for example, through transfer of the analytes

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into the gas phase and enrichment onto solid sorbents. 32–37 Unfortunately, such procedures are

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not necessarily applicable for CSIA of polar organic micropollutants of current interest 38 such as

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pesticides, pharmaceuticals, consumer chemicals, and personal care products. Their sub-µg L−1

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concentrations in natural and treated waters require the processing of large sample volumes greater

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than 5 L by solid-phase extractions (SPE, e.g., 39 ) to obtain the necessary analyte mass for isotope-

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ratio mass spectrometry. Moreover, the inherently poor selectivity of SPE-based procedures will

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very likely lead to a co-enrichment of organic matter of unknown isotopic composition and thus

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compromise accurate isotope ratio measurements.

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One promising option to increase the selectivity of analyte enrichment procedures is the use of

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molecularly-imprinted polymers (MIPs). MIPs are synthetic, typically custom-made materials ca-

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pable of molecular recognition of a target analyte through specific intermolecular interactions. 40–44

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Synthesis is carried out in the presence of the target analyte and involves the polymerization of

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functional monomers that are selected based on their ability to interact with functional groups

45

of the target molecule. Therefore, the final materials exhibit a three-dimensional structure with

46

high specificity. 45 Even though the concept of solid-phase extraction with molecularly-imprinted 3

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47

polymers (MISPE) has been used successfully for years, 40,46–48 it has never been applied in com-

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bination with isotopic analyses. In fact, MISPE based on non-covalent interaction between analyte

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and sorbent seems ideally suited for the treatment of SPE-extracts, in which polar organic micropol-

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lutants are normally enriched from large volumes of water samples. 49 Only µL volumes of organic

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solvents can be injected onto gas chromatographs coupled to isotope-ratio mass spectrometers.

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The substantial analyte enrichment to the mg L−1 level causes concomitant enrichment of organic

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matrix and, therefore, requires further sample treatment. Because those extracts typically consist

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of organic solvents, polar organic micropollutants can be extracted based on H-bonding and ionic

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interactions with the MIP without interferences of water.

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The goal of this work was to explore new sample clean-up strategies based on molecularly-

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imprinted polymers for the stable isotope analysis of polar organic micropollutants in aquatic

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environments. Here, we focused our work on the analysis of

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benzotriazole (1H-BT), a corrosion inhibitor, which is widely used in dishwashing detergents.

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Like many polar organic micropollutants, 1H-BT is too persistent to be degraded in waste water

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treatment plants (WWTP) and therefore found in µg L−1 concentrations in rivers as well as influents

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and effluents of WWTPs containing very different organic matrices. 50–52 The specific objectives of

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our work were to (i) propose sample treatment procedures for MISPE-based stable isotope analysis

64

through the synthesis and application of a custom-made MIP for 1H-BT. (ii) We characterized

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the selectivity of our materials in a comparison with other polar organic micropollutants such as

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triazines and substituted benzotriazoles and (iii) evaluated the potential of method-induced isotope

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fractionation from incomplete analyte recovery. (iv) Finally, we illustrate how MISPE-based

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procedures for CSIA can be applied in different matrices by analysing δ13 C and δ15 N of 1H-BT in

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riverwater, influent and effluent of a WWTPs, as well as in dishwashing detergent taps.

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13 C/12 C

and

15 N/14 N

ratios of 1H-

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Analytical Chemistry

70

Experimental section

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A list of all chemicals including suppliers and purities is provided in the Supporting Information

72

(SI).

73

Safety considerations

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Milling and sieving the synthesized polymers can generate dust. The processing of materials should

75

be carried out under the fumehood. Additionally, gloves, goggles and respiration mask are advised

76

to be worn. The radical generator, azobisisobutyronitrile, decompose at temperature ≥ 45 °C and

77

may explode if large amounts are exposed to high temperatures. Do not exceed temperature of 40

78

°C during its purification.

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Synthesis of molecularly-imprinted polymers

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Molecularly-imprinted polymers were synthesized following an adapted procedure of Chapuis

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et al. 53 for terbutylazine. The polymerization mixture consisted of 119.1 mg (1 mmol) 1H-

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benzotriazole (1H-BT) as template, 344.4 mg (4 mmol) methacrylic acid as a functional monomer,

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3964 mg (20 mmol) ethylene glycol dimethacrylate as crosslinker, 39.4 mg (0.24 mmol) azobi-

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sisobutyronitrile as radical generator, and 5.6 mL dichloromethane as porogene. We dissolved

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the template in the functional monomer, cooled the solution to 0℃ for 15 minutes and added the

86

remaining reagents. The polymerization mixture was purged with N2 in a borosilicate glass tube

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(OD = 18 mm, ID = 15 mm), sealed with a screw-cap, and placed in an ice bath for 30 minutes. The

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same procedure was used for preparation of non-imprinted polymer (NIP) except for the absence

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of the template 1H-BT.

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Polymerization took place over 24 hours at 2 ± 0.2℃ by immersing the glass tubes in a merry-

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go-round photoreactor (DEMA 125, Hans Mangels GmbH, Bornheim-Roisdorf, Germany; Figure

92

S1) filled with a sodium nitrate filter solution (0.15 M, cut-off of wavelength > 320 nm) and

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centred with a medium pressure mercury lamp (150 W) surrounded by a borosilicate glass jacket. 54

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After polymerization, the tubes were removed and crushed to recover the polymer monoliths.

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The polymers were sequentially ground (Mixer Mill MM 301, Retsch GmbH, Germany) and

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sieved through 100 µm sieve (Retsch GmbH, Germany). The particle fraction < 100µm was

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sedimented in methanol to obtain particle size distribution between 20 and 100 µm. We removed

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the template by ten repetitive cycles of Soxhlet extraction using mixtures of methanol and formic

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acid (90/10 vol-%) until ≥ 98% of the 1H-benzotriazole was recovered. Nonimprinted polymers

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were extrated in the identical manner. The identical procedure was applied to synthesize a MIP

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with 5-,6-(CH3 )2 -benzotriazole as template for the quantitative analysis of imprinting efficiency.

102

Possible interferences of residual analytes on the accuracy of stable isotope analysis were evaluated

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exemplarily for 1H-BT through (a) quantification of its concentration before and after the use of

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MIPs with 1H-BT in organic solvent and (b) the comparison of

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1H-BT before and after the use of MIPs with 1H-BT spiked river water samples.

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Sample preparation procedures using molecularly-imprinted polymers

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The multi-step procedure for the enrichment and purification of 1H-BT in (a) in- an effluent of

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waste water treatment plants (WWTP), (b) spiked river water, and (c) dishwashing tabs is shown

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schematically in Figure 1. Generally, sample preparation consisted of two solid phase extraction

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steps (Fig. 1, B and D) with different pre- and post treatments.

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Enrichment of 1H-BT from waste water plant influent and effluent

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Samples from WWTP influent (5 L, sample no. 1 in Figure 1, step A) and effluent (10 L) were

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filtered through 0.7 µm glass fiber filters (Whatman, GF/F 47mm) and acidified with HCl (32 %) to

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pH 2.0. Filtered samples 2 containing 1H-BT were enriched by conventional solid phase extraction

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using OASIS HLB cartridges (Waters, 6 g, 35 mL) in step B using a 12-port vacuum extraction

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manifold (Supelco, Switzerland). Prior to sample loading, cartridges were conditioned with 100

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mL hexane, 100 mL ethyl acetate, 100 mL methanol and 100 mL MiliQ water. Water samples

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2 were percolated through the cartridges using a vacuum pump at a flow rate of ≤ 7.2 mL/min 6

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13 C/12 C

and

15 N/14 N

ratios of

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Analytical Chemistry

WWTP influent/effluent

aqueous sample (1) filtration acidification

B

flow rate 1-CH3 -BT. The observed sequence

307

agrees with the notion that IF are highest for the template used for synthesis. 48 We verified this

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outcome by assessing IF of the same analytes on another custom-made MIP, where 5-,6-(CH3 )2 -

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BT was employed as a template instead of 1H-BT. The IF (CH3 )2 −BT -values are shown in Table 1

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for the 5-,6-(CH3 )2 -BT-imprinted MIP and they are of similar magnitude as IF-values for 1H-

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BT imprinted polymers. IF (CH3 )2 −BT decrease in the sequence 5-,6-(CH3 )2 -BT > 5-CH3 -BT >

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1H-BT > 1-CH3 -BT confirming that the selectivity was highest for the template molecule. With

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both MIPs, we did not observe selective interactions of 1-CH3 -BT. Absence of selectivity for N-

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substituted benzotriazole suggests that the interactions of analytes with the polymer was based on

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hydrogen bonding of the triazole moiety with the carboxylic acid functional group of the monomer

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(methacrylic acid, Figure 3). Substitution at the 1H position on the triazole ring caused complete

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loss of selectivity towards 1-(CH3 )-BT on both polymers.

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Further evidence for the selectivity of the MIPs was obtained from the comparisons of the

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apparent interaction enthalpies, ∆HMIP , 63 with various analytes, and interaction enthalpies of non-

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imprinted polymers, ∆HNIP (Table 1). ∆HMIP were most negative, that is most favourable, for the

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template molecule 1H-BT and became less negative for the substituted benzotriazoles. ∆HMIP -

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values ranged between 7.0 and 16 kJ/mol which is in the lower range of interaction enthalpies for

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hydrogen bonding (8-84 kJ/mol, 64,65 ). Note that interactions of benzotriazoles with non-imprinted

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materials were also exothermic but the difference between MIP and NIP, ∆HMIP − ∆HNIP , revealed

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the larger selectivity of the MIPs. ∆HMIP − ∆HNIP for benzotriazoles followed the same sequence

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reported for IF-values. However, values approached zero for the remaining compounds showing

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the lack of selective interactions of the MIP with benzothiazole, naphthalene, and atrazine.

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Isotope fractionation from incomplete analyte extraction

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We quantified the extent of C and N isotope fractionation from analyte-MIP interactions in a

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separate experiment on MISPE cleanup step E (Figure 1). Table 2 shows mass fractions, mi , 18

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Analytical Chemistry

(a)

(b) HO

HO

H N N

CH3

N

N

N

H O

O

O

N

H

O

O

O

Figure 3. (a) Hydrogen bonding between the template, 1H -BT, and the functional monomer, methacrylic acid, used for imprinting enables selective interactions. (b) Absence of selective binding due to N-methyl substitution in 1-CH3 -BT. 331

and the corresponding δ13 C and δ15 N-values of 1H-BT recovered in three sample fractions (Fi ).

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The total 1H-BT recovery was 99%. F1 corresponds to the fraction of analyte collected after

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breakthrough during the loading of the cartridge with 1H-BT in dichloromethane. This fraction

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amounted to 41% of the total analyte mass. F2 is the fraction of analyte obtained through washing

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of the cartridge with the identical solvent (42%). F3 reflects the remaining 16% of 1H-BT that

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were recovered from the MIP through elution with methanol. The same type of experiment was

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carried out with dichloromethane extracts of river water using five-fold higher MIP (500 mg). As a

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consequence of higher retention capacity, no 1H-BT was detected in F1 , whereas all 1H-BT mass

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was recovered in the two remaining fractions, F2 (74%, Table 2) and F3 (26%).

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The C isotope signatures of 1H-BT in the presence and absence of organic matrix from river

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water samples varied only slightly (±1h) in the three fractions. The extent of C isotope fractionation

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pertinent to the MISPE cleanup step calculated with eq. 4, ∆13 C, was negligible regardless of the

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sample matrix (−0.6 ± 1.4h and 0.1 ± 1.1h, Table 2). By contrast, δ15 N values of 1H-BT varied

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substantially (up to 13h) between the three sample fractions F1 to F3 . Due to the almost complete

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recovery of the analyte after the elution with methanol, MISPE induced N isotope fractionation, 19

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Analytical Chemistry

εN = -5.0 ± 2.2 ‰

-18 -20

15

δ Ni,MIP (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-22 -24 -26 -28 1.0

0.8

0.6

0.4

0.2

0.0

c/c0

Figure 4. N isotope fractionation of 1H -BT through stepwise elution from the MIP. δ15 Ni MIP (±1σ ) stand for the calculated N isotope signatures of 1H -BT remaining on the MIP after elution of different 1H -BT fractions. c/c0 is the fraction of 1H -BT remaining associated with the MIP. The data was generated by using information from Table 2 and eqs. 6 and 7. The solid line was obtained through non-linear regression of eq. 5 with an N of −5.0 ± 2.2h.

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∆15 N is, nevertheless, negligible within uncertainty in both sample matrices (−1.4 ± 1.3h and

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−0.2 ± 1.1h).

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These observations suggests that a non-stoichiometric recovery of the analyte after the MISPE

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cleanup step would not cause systematic C isotope fractionation whereas N isotope signatures could

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be biassed towards more positive δ15 N values. We tentatively quantified an operational N isotope

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enrichment factor, N , from the small data set for the MISPE cleanup step E in absence of organic

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matrix using eq. 5. Figure 4 shows the average N isotope signatures of 1H-BT that remained

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associated with the MIP (δ15 Ni,MIP , eq. 7) after sequential loading step F1 and F2 , respectively

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(blue and green areas). The most enriched fraction of 1H-BT, F3 , with a δ15 N of −18.1 ± 0.9h,

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was eluted from the MIP through the change of solvent (red area in Figure 3). The N isotope

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fractionation trend was quantified with an N of −5.0 ± 2.2h (eq. 5) and illustrates that the 15 N-

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containing 1H-BT is retained preferentially by the MIP. This isotopic preference corresponds to

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an apparent 15 N kinetic isotope effect of 1.0054 ± 0.0022 (eq. 8) under the chosen experimental

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(h) c (h) c

∆13 C ∆15 N

F1

0.0 ± 1.3 0.0 ± 1.2

1.00 ± 0.03 0.41 ± 0.02 −26.1 ± 0.5 −25.4 ± 0.3 −26.9 ± 0.2 −28.3 ± 0.1

reference F3

−0.6 ± 1.4 −1.4 ± 1.3

0.42 ± 0.02 0.16 ± 0.03 −25.8 ± 0.4 −24.9 ± 0.1 −25.6 ± 0.2 −18.1 ± 0.9

no matrix F2 n.d n.d. n.d.

F1

0.1 ± 1.1 −0.2 ± 1.1

0.74 ± 0.02 −26.4 ± 0.2 −30.5 ± 0.3

0.26 ± 0.02 −25.6 ± 0.6 −17.5 ± 0.6

river water extract F2 F3

for F1 and F2 determined from 1H-BT in dichloromethane, F3 in methanol, n.d. = not detected due to larger MIP b uncertainties are ±1σ of triplicate measurements of concentrations or isotope mass used in MISPE cleanup step; c propagated uncertainties (95% confidence intervals). ratios;

a Data

(-) b (h) b (h) b

mi δ13 C δ15 N

Fi

Table 2. Sample fractions of 1H -BT, Fi collected during the MISPE cleanup step E (Figure 1): mass fractions of 1H -BT, mi , C and N isotope signatures, δ13 C and δ15 N, and extent of C and N isotope fractionation, ∆13 C and ∆15 N derived with eq. 4. a

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Page 22 of 31

359

conditions. The observation of N isotope fractionation in the absence of any isotope fractionation

360

for C implies that the N atoms of 1H-BT were involved in the interactions with the MIP and

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confirms the above conclusion that the selectivity of the MIP is indeed based on H-bonding with

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the triazole moiety of the analyte (Figure 3). Finally, this

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recoveries during the MISPE cleanup step of ≥ 81% are sufficient to limit such shifts of δ15 N to

364

≤ 1.1h (eq. 9), that is to the typical precision of N isotope ratio measurements by GC/C/IRMS. 5

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Application in wastewater treatment plants

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We applied the sample preparation procedures shown in Figure 1 to determine the C and N isotope

367

signatures of 1H-BT in the influent and effluent of a WWTP, one of the frequently encountered

368

matrices of the pollutant with DOC contents of several mg C/L. 51,52 The outcome was verified

369

through a comparison the δ13 C and δ15 N values with those from 1H-BT in dishwashing detergents

370

sold in villages that discharge their sewer into the studied WWTP.

15 N

isotope effect implies that 1H-BT

371

Figure 5 shows the δ13 C and δ15 N values of 1H-BT in the influent and effluent of the WWTP

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as well as in dishwashing detergents sold by retailers in the study area. δ13 C and δ15 N values of

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1H-BT in dishwashing detergents span from −27h to −23h and −11h to 1h, respecitvely. δ13 C

374

cover a similar range observed previously by Spahr et al. 55 with a simplified approach whereas our

375

analysis reveals an even wider distribution of δ15 N in 1H-BT. The wastewater samples (2 in Figure 1)

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contained substantial amounts of DOC that amounted to 7.3 and 28 mg C/L for effluent and influent,

377

respectively. The substantial interferences on C isotope ratio measurements were reduced through

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an additional liquid-liquid extraction during step F (Figure 1). δ15 N values of 1H-BT in influent

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and effluent samples were identical within uncertainty (−4.5 ± 0.9h vs. −5.6 ± 1.3h) whereas

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the difference of their δ13 C was only minimal (−26.8 ± 0.2h vs. −26.1 ± 0.4h). Even though a

381

quantitative analysis of the C and N isotope signatures in the WWTP and in dishwashing detergents

382

is beyond the scope of this work, data in Figure 5 reveals some consistent trends which confirm the

383

applicability of MISPE-based procedures for CSIA. Similar C and N isotope signatures of 1H-BT

384

in influent and effluent agree with the frequently made observation that this micropollutant is not 22

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Migros Denner Spar average

Coop Dorfladen Volg

2 0 -2

15

δ N (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

-4 -6 -8 -10 Influent Effluent

-12 -28

-26

-24

13

δ C (‰)

Figure 5. C and N isotope signatures of 1H -BT in the influent and effluent of a WWTP (coloured triangles) as well as in various solid, liquid, and gel-like dishwashing detergents sold by retailers in the study area (grey symbols). The dashed lines correspond to the arithmetic mean of δ13 C and δ15 N of 1H -BT in detergent samples. Error bars represent ±1σ . 1H -BT influent and effluent concentrations were 4.3 and 3.8 µg/L, respectively.

385

degraded during the waste water treatment process. 51,52 Moreover, those values are also within the

386

range of δ13 C and δ15 N values found in dishwashing detergents that are sold in the study area and

387

they are within less than 1h of their average values.

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388

Conclusion

389

Our work illustrates that sample treatment procedures which rely on the selective cleanup of

390

enriched organic micropollutants by molecularly-imprinted polymer offer new perspectives for

391

CSIA. Extractions of organic micropollutants in large volume samples by conventional SPE are

392

an essential step to enrich adequate amounts of analytes for isotope-ratio mass spectrometry but

393

they will inevitably lead to concomitant enrichment of organic matter. This matrix can be removed

394

selectively in organic extracts with MIPs. The application of molecularly-imprinted polymers

395

for CSIA of 1H-BT especially in influent and effluent samples from WWTPs revealed, however,

396

that despite good selectivity, organic matrix interferences may become too large thus making the

397

application of MISPE impracticable. Our analysis suggest that the C-normalized concentration

398

ratio of dissolved organic carbon (DOC) to micropollutant can be a useful parameter to assess the

399

feasibility of MISPE-based sample cleanup for CSIA, particularly for measurements of C isotope

400

ratios. In fact, the C-normalized DOC:1H-BT ratio decreased systematically as a consequence of

401

the series of treatment of WWTP samples. Whereas the DOC:1H-BT ratio in the aqueous WWTP

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influent was 11000 (1 in Figure 1), it decreased to 860 in sample 3 after SPE and 340 in sample

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5 after MISPE. In WWTP effluent, the DOC:1H-BT ratio decreased from 3200 in the aqueous

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effluent (1) to 190 after SPE (3) and 76 (5) after MISPE. Whereas we implemented liquid-liquid

405

extraction in order to further reduce DOC:1H-BT ratios in WWTP samples, alternative approaches

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such as preparative HPLC on sample 5 could also be applied. 39 A systematic and quantitative

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assessment of the acceptable ratio of matrix:micropollutant is warranted to assess the feasibility

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of MISPE sample preparation for CSIA more generally. Such efforts will enable the application

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of stable isotope analyses for tracking sources and transformation processes of many contaminants

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of emerging concerns including, for example, pharmaceuticals, pesticides, and water disinfection

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byproducts. 11,12,16,17,39

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Acknowledgement

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This work was supported by the Swiss National Science Foundation (grant no. CRSII2-141805).

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We thank Stefanie Jucker, Hans-Jürgen Schindler, Samuel Derrer, Marius Wiesmann, Alessandra

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Sodero, Tony Merle, and Aileen Melsbach for experimental support.

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Supporting Information Available

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Description of field site and sampling campaign, additional preparation steps for dishwashing

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detergents, derivation of interaction enthalpies, molecular structure of analytes. This material is

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available free of charge via the Internet at http://pubs.acs.org/.

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