Extraction of benzoylurea pesticides from tea and fruit juices using deep eutectic solvents
Introduction
The use of pesticides has increased significantly to guarantee the quantity and quality of agricultural production [1], [2], [3]. Benzoylureas (Bus) are widely used insecticides that are used primarily to control insects in cotton, corn, sugar beets, potatoes, grapes, citrus and ornamental plants [4], [5]. These insecticides enter the aquatic environment through runoff after application and continue to accumulate in environments [6]. These insecticide residues can cause soil, water and food contamination and are toxic to aquatic organisms and detrimental to human health [2], [7]. Therefore, many countries and institutions have defined maximum residue limits (MRLs) for BUs; for example, the MRLs of triflumuron, chlorfluazuron, flufenoxuron and hexaflumuron in tea set by the Japanese government are 0.02, 10, 15, and 15 mg kg−1 [8], [9], respectively. The monitoring of these insecticides is essential to ensure environmental and consumer safety.
Sample preparation is a very important step in pesticide residue detection due to the complexity of the sample matrices. The most widely used pretreatment procedures for of samples containing BUs include solid-phase extraction (SPE) and liquid-liquid extraction (LLE) [10], [11]. However, these techniques have some disadvantages, such as requiring large amounts of organic solvents and being time consuming, which have limited their application. As we all know, modern analytical chemistry is based on instrumental analysis. Sample preparation is essential to meet the needs of different instruments before performing the instrumental analysis. It is developing in the direction of miniaturization, economy and environmental friendliness. Therefore, the challenge of modern analytical chemistry for the sample preparation is to simplify these operations and design methods to minimize or even exclude the use of hazardous solvents [12], [13].
In recent years, several economical and environmentally friendly methods have been developed to determine BUs, such as SPE [14], solid-phase microextraction (SPME) [15], dispersive solid-phase extraction (DSPE) [16], liquid-liquid microextraction (LLME) [17] and in situ dispersive liquid-liquid microextraction (in situ DLLME) [18]. However, SPE techniques are limited by the restricted lifetimes of materials and the complex preparation process. DLLME can effectively overcome these shortcomings. DLLME was exploited by Rezaee et al. in 2006 [19]. DLLME methods typically consist of a three-component system of the dispersant, extractant and sample solution. During the extraction, the dispersant and extractant are quickly injected into the sample solution by a syringe to form a turbid solution, and an extraction equilibrium is established due to the increase in the contact area.
A major drawback of traditional DLLME processes is the use of organic chlorinated solvents, such as chloroform and chlorobenzene [9]. These solvents pose potential threats to both the environment and the operators on account of their toxicity and volatility. Thus, the development of green extraction agents as a substitute for organic solvents is of great importance. Deep eutectic solvents (DESs) as green solvents have attracted the interest of researchers in recent years. In 2003, Abbott et al. prepared a DES by mixing choline chloride and urea in a solid molar ratio [20]. The melting point of the DES is lower than the melting point of either single component. Metal oxides such as zinc oxide, copper oxide, and cobalt oxide, are soluble in the DES, and research on DESs has received widespread attention. They are typically formed from a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD) in stoichiometric ratios [21]. Generally, the melting points of DESs are lower than those of each individual component due to self-association or the formation of intramolecular hydrogen bonds [22], [23], [24]. DESs are inexpensive and easily synthesized by simple methods from low toxic, low cost, readily available and biodegradable compounds, which make them attractive alternative solvents for green analytical chemistry [25], [26], [27].
In this study, DESs were synthesized by mixing quaternary phosphonium salts and hydrophobic alcohols. Fourier transform infrared (FT-IR) spectroscopy was applied to elucidate the hydrogen bonding in the compounds and to investigate the functional groups in the DESs. In the experiment, the key factors affecting the extraction efficiency were screened by Plackett-Burman (P-B) design and optimized with the central composite design (CCD). Finally, the optimized conditions were used to determine the BUs in beverage samples.
Section snippets
Reagents and materials
The BUs standards (diflubenzuron, triflumuron, hexaflumuron, flufenoxuron, and chlorfluazuron) were acquired from the Agricultural Environmental Protection Institution (Tianjin, China). Trihexyl (tetradecyl) phosphonium tetrafluoroborate ([P14,6,6,6]Cl) was purchased from J & K Chemical Technology Co., Ltd. (Beijing, China). Decyl alcohol, dodecyl alcohol, and tetradecyl alcohol (all ≥98% mass fraction purity) were acquired from Aladdin Reagent Corporation (Shanghai, China). HPLC-grade
Characterization of the DESs
Literature studies have shown that the components in DESs interact primarily through hydrogen bonding. That is, a hydrogen bond is formed between a quaternary ammonium salt, a halogen in a quaternary phosphonium-based HBA, and a HBD such as a polyol and an acid. In this study, the FT-IR spectra of [P14,6,6,6]Cl, tetradecyl alcohol and DES were investigated to characterize the synthesized DESs, and the results are shown in Fig. 2A. There was a strong and broad band at approximately 3309 cm−1
Conclusions
A DLLME method based on the low-density DES combined with HPLC-UVD was developed. The study results show that the method has good extraction recovery and repeatability and low LODs, which prove that the method has the potential to be applied for the detection of BUs in beverages. The main advantage of this method was the use of a DES as a green and inexpensive extractant instead of halogenated, aromatic, and toxic solvents, which is in line with the current trends toward environmental
CRediT authorship contribution statement
Xinya Liu: Conceptualization, Methodology, Validation, Investigation, Writing - original draft, Writing - review & editing. Mengyuan Chen: Validation, Investigation. Zilin Meng: Investigation. Heng Qian: Investigation. Sanbing Zhang: Project administration. Runhua Lu: Resources, Funding acquisition. Haixiang Gao: Resources, Writing - review & editing, Supervision, Funding acquisition. Wenfeng Zhou: Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors greatly appreciate the financial support from National Key R&D Program of China (Project nos. 2018YFC1604402) and National Natural Science Foundation of China (Project nos. 21677174 and 21507159).
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