Development of magnetic dispersive solid phase extraction using toner powder as an efficient and economic sorbent in combination with dispersive liquid–liquid microextraction for extraction of some widely used pesticides in fruit juices

Highlights

• A new extraction method based on MDSPE–DLLME–GC–FID was developed.

• For the first time, toner powder was used as a magnetic sorbent.

• The sorbent did not need synthesis, pretreatment or conditioning.

• The method was applied for the analysis of some pesticides in fruit juices.

• EFs and ERs between 495–754 and 49–75% were obtained, respectively.

Abstract

In this study, for the first time, a magnetic dispersive solid phase extraction method using an easy–accessible, cheap, and efficient magnetic sorbent (toner powder) combined with dispersive liquid–liquid microextraction has been developed for the extraction and preconcentration of some widely used pesticides (diazinon, ametryn, chlorpyrifos, penconazole, oxadiazon, diniconazole, and fenazaquin) from fruit juices prior to their determination by gas chromatography–flame ionization detection. In this method, the magnetic sorbent is mixed with an appropriate dispersive solvent (methanol–water, 80:20, v/v) and then injected into an aqueous sample containing the analytes. By this action the analytes are rapidly adsorbed on the sorbent by binding to its carbon. The sorbent particles are isolated from the aqueous solution in the presence of an external magnetic field. Then an appropriate organic solvent (acetone) is used to desorb the analytes from the sorbent. Finally, the obtained supernatant is mixed with an extraction solvent and injected into deionized water in order to achieve high enrichment factors and sensitivity. Several significant factors affecting the performance of the introduced method were investigated and optimized. Under the optimum experimental conditions, the extraction recoveries of the proposed method for the selected analytes ranged from 49–75%. The relative standard deviations were ≤7% for intra- (n = 6) and inter-day (n = 4) precisions at a concentration of 10 μg L^–1 of each analyte. The limits of detection were in the range of 0.15–0.36 μg L^–1. Finally, the applicability of the proposed method was evaluated by analysis of the selected analytes in some fruit juices.

Introduction

Pesticides are chemical compounds that their consumption in agriculture is increased day by day in order to protect the agricultural products in counter to molds, fungi, insects, and any other agents that can affect crops quality and yield [1]. Although the use of pesticides has many merits, but overuse of them can constitute a great menace to the environment and human's health. Hence, it is indispensable to develop simple and sensitive analytical methods which are capable of detecting their trace residue concentrations in food and environmental samples. To date, the methods used for the analysis of pesticides are mostly based on the chromatographic techniques such as high–performance liquid chromatography [2], [3] and gas chromatography (GC) [4], [5]. In order to achieve the reliable results performing a sample preparation step before using the chromatographic techniques is essential. A perfect sample preparation method should be able to extract the analytes from sample matrix and transfer them into a suitable phase for injection into instrumental system [6]. Up to now, various sample preparation methods such as liquid–liquid extraction [7], solid phase extraction (SPE) [8], homogeneous liquid–liquid extraction [9], solid phase microextraction [10], single drop microextraction [11], dispersive liquid–liquid microextraction (DLLME) [12] and etc. have been used for isolation and preconcentration of pesticides from different matrices, and bioactive compounds [13] using also deep eutectic solvents and ionic liquids [14]. Among the above–mentioned methods, SPE is one of the mostly used for matrix simplification and enrichment because of its matured technology, facility of the operation, and the good adaptability towards the multiple sample matrices [15], [16]. Nevertheless, SPE has also suffers some problems like obstruction of cartridges, and necessity of pretreatment of sorbents before extraction procedure [17], [18], [19]. In order to overcome these problems, several other methods based on the traditional SPE methodology, such as dispersive solid phase extraction (DSPE) [20], [21] and magnetic dispersive solid phase extraction (MDSPE) [22], [23] have been developed. In these cases, the sorbent is directly dispersed into the sample solution instead of its packing into a cartridge. Dispersing the sorbent into the sample solution can increase the contact interface between the sorbent and the analytes significantly, and it can improve the mass transfer of the analytes and extraction efficiency [24], [25].

DSPE was introduced by Anastassiades et al. in 2003 [26]. In DSPE, an SPE sorbent is mixed with an appropriate organic solvent (disperser solvent) and dispersed into a sample solution containing the target analytes. After extraction, the sorbent containing the retained analytes is settled by centrifugation. Finally, the analytes are desorbed with an appropriate solvent. The sorbents such as primary secondary amine, C₁₈ (octadecylsilane), and graphitized carbon black are mostly used in this method [27], [28], [29]. In spite of the simplicity and low cost of DSPE, this method needs centrifugation that is time–consuming and can increase the extraction time. MDSPE is a new version of DSPE in which a magnetic sorbent is used instead of the conventional non–magnetic sorbents. The main advantage of this sorbent is that, it can be collected and separated from the aqueous phase using an external magnetic field, which eliminates the centrifugation step and makes the sample pretreatment procedure more convenient, time–saving, and economic. In MDSPE, the type of magnetic sorbent plays a critical role for the effective extraction of the analytes. The commonly used materials for MDSPE include silica (Fe₃O₄@SiO₂) [30], surfactants [31], molecular imprinted polymers (Fe₃O₄@MIPs) [32], and carbon nanotubes (Fe₃O₄@CNTs) [33]. The main disadvantage of these magnetic sorbents is that, their synthesis and pretreatment processes are very time–consuming, tedious, and may involve toxic reagents. Therefore, searching a new magnetic sorbent that is simple, eco–friendly, cost effective, and has a high selectivity towards the selected analytes is highly desirable.

In this study, for the first time, a magnetic sorbent (toner powder) which is easy–accessible, and does not need any synthesis or pretreatment, was used as an efficient sorbent in the MDSPE combined with DLLME for the extraction and preconcentration of the trace concentrations of some pesticides from fruit juices prior to their determination by gas chromatography–flame ionization detection (GC–FID). MDSPE–DLLME does not only allow the analytes to be preconcentrated, but also the other compounds present in the sample matrix to be removed. However, MDSPE–DLLME is a bit more expensive and time–consuming than alone DLLME. In the first step of this method, small amount of the sorbent is mixed with the disperser and dispersed into the aqueous solution containing the analytes. Then, the sorbent is collected with the help of an external magnetic field. In the second step, for more enrichment of the analytes an appropriate organic solvent is used to elute the analytes from the sorbent for the following DLLME procedure. Simplicity in the operation, low cost, high enrichment factors (EFs), and rapidity due to use the sorbent which does not need any synthesis, pretreatment or conditioning step, are the main advantages of the proposed method.