Extraction of benzoylurea pesticides from tea and fruit juices using deep eutectic solvents

Highlights

• A rapid UA-DES-DLLME method for benzoylurea insecticides analysis was presented.

• Several deep eutectic solvents (DESs) were synthesized.

• The key analytical arguments were screened by experimental design methodology.

• The proposed method has the advantages of simplicity, miniature and environmental friendliness.

Abstract

In this study, low-density deep eutectic solvent combined with dispersive liquid-liquid microextraction was applied to the extraction of five benzoylurea insecticides (BUs, including diflubenzuron, triflumuron, hexaflumuron, flufenoxuron, and chlorfluazuron) from beverages. Then the extracted and concentrated samples were analyzed and detected using the high-performance liquid chromatography combined with an ultraviolet detector. The DESs were synthesized by [P14,6,6,6]Cl as hydrogen bond acceptor and tetradecyl alcohol as hydrogen bond donor, and then characterized by Fourier transform infrared spectroscopy. In the experiment, the key factors affecting the extraction efficiency were screened by Plackett-Burman design and optimized with the central composite design. The extraction recovery rates were 85.91–95.12%. The limits of detection and correlation coefficients of the method were 0.30–0.60 μg L⁻¹ and 0.9992–0.9997. Finally, the method was applied to determine the BUs in four beverage samples, and satisfactory recoveries, within the range of 76.87–101.19% were achieved. The present method has the potential to be applied to the detection of BUs in aqueous samples.

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.