Elsevier

Analytica Chimica Acta

Volume 852, 10 December 2014, Pages 88-96
Analytica Chimica Acta

A novel magnetic ionic liquid modified carbon nanotube for the simultaneous determination of aryloxyphenoxy-propionate herbicides and their metabolites in water

https://doi.org/10.1016/j.aca.2014.09.024Get rights and content

Highlights

  • A new kind of ionic liquid modified carbon nanotube has been synthesized and applied for simultaneous analysis of AOPPs and their metabolites.

  • The potential pollutants, such as metabolites of AOPPs, have been analyzed.

  • The mechanism of absorption has been discussed.

  • Varieties of experiment factors were optimized and selected.

  • This method has been successfully applied in the analysis of real water samples.

Abstract

A reliable, sensitive, rapid and environmentally friendly analysis procedure for the simultaneous determination of the analytes with a wide range of polarity in the environmental water was developed by coupling dispersive magnetic solid-phase extraction (d-MSPE) with high-performance liquid chromatography (HPLC)–diode array detector (DAD) and ultra-high pressure liquid chromatography (UHPLC)-triple quadrupole mass spectrometer (MS/MS), in this work. Magnetic ionic liquid modified multi-walled carbon nanotubes (m-IL-MWCNTs) were prepared by spontaneous assembly of magnetic nanoparticles and imidazolium-modified carbon nanotubes, and used as the sorbent of d-MSPE to simultaneously extract aryloxyphenoxy-propionate herbicides (AOPPs) and their polar acid metabolites due to the excellent π–π electron donor–acceptor interactions and anion exchange ability. The factors, including the amount of sorbent, pH of the sample solution, extraction time and the volume of elution solvent were investigated. Under the optimized conditions, the proposed d-MSPE coupling to HPLC–DAD system had a satisfactory performance, the limits of detection (LODs, defined as the signal to noise ratio of 3) and the limits of quantification (LOQs, defined as the signal to noise ratio of 10) for analytes in Milli-Q water were in the range of 2.8–14.3 and 9.8–43.2 μg L−1 respectively. Calibration curves were linear (r2 > 0.998) over the concentration range from 0.02 to 1 mg L−1. The recoveries of the eight analytes ranged from 66.1 to 89.6% with the RSDs less than 8.6%. In order to extend the method in extremely low concentration analysis, d-MSPE-UHPLC–MS/MS was investigated, which showed better performance in terms of limit of detection and analysis time.

Introduction

The aryloxyphenoxy-propionate herbicide (AOPP) is a kind of selective post-emergence herbicide [1], which was registered for use in controlling annual and perennial grassy weeds for many crops. They interfere with production of fatty acids needed for plant growth by inhibition of acetyl co-enzyme A carboxylase [2], and in most cases AOPPs can be rapidly deesterificated to their acidic form by plants or soil, which increases their polarity and solubility but does not influence their bioactivity [3], [4], [5]. AOPPs are toxic to aquatic organisms [6], especially fish, and could be inducers of liver toxicity and injury [7]. The widespread use contributes to their presence in the environment and positive detectable rate in surface and ground water and other environmental matrices [8], [9], and the accumulation of these herbicides may potentially destroy fish populations in natural water systems and jeopardize human health. Therefore, it is necessary to develop robust methods to simultaneously analyze AOPP and their metabolites residues in water.

Conventional methods for isolation and/or enrichment of AOPPs related chemicals from water involve liquid–liquid extraction (LLE), solid phase extraction (SPE) [9], [10], [11]. LLE is widely used traditional sample pre-treatment method for water samples. However, it is not preferred when water either contains emulsifying agents or analytes present in trace quantities. Moreover it is time consuming, requires large volume of toxic organic solvents and lacks automation. SPE is a simple technique, however it suffers from some drawbacks such as high cost and decline in performance with time. Furthermore, it is hard to find a robust cartridge sorbent for both ester and acid. In recent years, a lot of microextraction techniques have been used in extraction of AOPPs, such as solvent microextraction (SME) [8], [12], [13], microextraction in packed syringe (MEPS) [14] and dispersive liquid–liquid microextraction (DLLME) [15]. Although SME and DLLME are less solvent-consuming than conventional methods, they still require adjusting the pH of the sample before simultaneously extracting ester and acid from water.

Dispersive magnetic solid-phase extraction (d-MSPE) is an excellent extraction method, which is a new procedure of SPE based on the use of magnetic or magnetizable sorbents. Compared with traditional SPE procedure, d-MSPE is indicated as a time and labor effective separation approach, which combines the advantages of magnetic separation technology (MST) and SPE [16], [17], [18], [19], [20]. The sorbents do not need to be packed into the SPE cartridges and the phase separation could be conveniently made by applying an external magnetic field. In d-MSPE, the sorbent plays a key role in obtaining higher enrichment efficiency of analytes. Unfortunately the common magnetic nanoparticles (MNPs), such as Fe3O4 or γ-Fe2O3, almost do not have significant adsorption efficiency. There are two ways to overcome this limitation. First, modify the surface of magnetic nanoparticles. In general, the surface of MNPs are covered with some kind of absorbent material, including poly (ethylene glycol), humic acid, etc. [17], [21], [22], [23], [24]. In this case, most sorbents on the surface of magnetic particles are hydrophobic. These hydrophobic constituents lack ability to interact with polar or ionic substances. Therefore, it makes the extraction of such analytes from environmental or biological sample difficult, which limits its application in some extent. Second, the adsorption composites were composed by MNPs and another kind of nano-sized sorbent. In recent years, the use of carbon nanotubes (CNTs) in SPE is among the most important applications of these materials in analytical science [25]. Their use is based on the properties of CNTs, for example, they have ability to establish π–π interactions as well as excellent Van der Waals interactions with other molecules, in particular with hydrophobic ones. They also possess a large surface area, especially on the outside and interstitial spaces within nanotubes bundles. Their chemical, mechanical and thermal stability should also be considered [26].

But the surface of pristine CNTs lacks active sites to interact with polar or ionic substance which limits their application in environmental pollution analysis [25]. Some research groups have used the chemical modification of CNTs, such as carboxyl multi-walled carbon nanotubes (MWCNTs-COOH) [27], [28], hydroxy multi-walled carbon nanotubes (MWCNTs-OH) [29], amino multi-walled carbon nanotubes (MWCNTs-NH2) [30], polyethylene glycol modified multi-walled carbon nanotubes (MWCNTs-PEG) and octadecylamine modified multi-walled carbon nanotubes (MWCNTs-ODA) [31], polyelectrolyte functionalized multi-walled carbon nanotubes (MWCNTs–PDDA) [32], for analysis of chemical warfare agents, polyhalogenated pollutants and pesticides. However, some of those modifications lack ability to interact with anions that makes them have no significant improvement in simultaneous analysis of ester and acid. Recently, ionic liquid modified multi-walled carbon nanotubes have been synthesized [33], [34]. They have excellent solubility and dispersibility in water and the ionic liquid on the surface of carbon nanotubes have anion exchange groups. Both of the characteristics make IL-MWCNT could be a potential sorbent for simultaneous extraction of the analytes with different polarity.

The aim of the current work is applying the proposed d-MSPE method for the simultaneous preconcentration and determination of the analytes with different polarity (four AOPP herbicides and their bioactive acid forms) in water sample. To achieve this goal, a new kind of magnetic ionic liquids modified multi-walled carbon nanotubes has been synthesized and employed as a d-MSPE sorbent. The effect of certain variables, including the amount of sorbent, pH of the sample solution, extraction time and the volume of elution solvent, on the extraction recovery (ER) of each analyte was evaluated. The proposed method was successfully applied to determine the four esters and four acids in real water samples (ground water and reservoir water). Also a comparison study with the MNPs, ionic liquids modified multi-walled carbon nanotubes and pristine multi-walled carbon nanotubes (MWCNTs) as sorbents for target analytes were conducted.

Section snippets

Standards and reagents

Diclofop-methyl (DM, 99.0%), diclofop acid (DA, 98.5%), cyhalofop-butyl (DB, 99.5%), cyhalofop acid (CA, 99.5%), quizalofop-p-ethyl (QE, 99.0%), quizalofop (QU, 99.0%), haloxyfop-methyl (HM, 99.9%) and haloxyfop acid (HA, 99.5%) were provided by the Institute for Control of Agrochemicals Ministry of Agriculture, and the chemical structures are shown in Fig. 1. The organic solvents used for HPLC determination (via acetonitrile, methanol, formic acid; all HPLC grade) were supplied by Fisher (New

Characterization of IL-MWCNT and m-IL-MWCNT

In order to confirm the successful synthesized IL-MWCNT, FT-IR spectrometry, TME and EDX were performed. In the IR spectrum of IL-MWCNT, the Cdouble bondO band of carboxylic acid group at 1625 cm−1 in o-MWCNT shifted to 1557 cm−1, indicating the formation of the amide bond, while the Cdouble bondN mode of the imidazole ring was also observed at 1467 cm−1 (see Fig. S2 in the Supporting information). TEM gave important morphological information on the hybrid materials and energy dispersive X-ray spectroscopy (EDX)

Conclusions

The d-MSPE method was used for the determination of four AOPPs and their metabolites in environmental samples based on imidazolium salt-based ionic liquids (ILs)-modified carbon nanotubes as sorbent. This was a first analytical report in which the simultaneous enrichment and determination of four AOPPs and their metabolites with different polarity were presented. The method was environmentally friendly because only near 2 mL of methanol was used. Good linearity, reproducibility and lower

Acknowledgements

This work was supported by A Foundation for the Author of National Excellent Doctoral Dissertation of PR China, Program for New Century Excellent Talents in University(NCET09-0738), the National Natural Science Foundation of China (21277171, 21337005, J1210064), the New-Star of Science and Technology supported by Beijing Metropolis, Program for New Century Excellent Talents in University and Program for Changjiang Scholars and Innovative Research Team in University.

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