In-syringe low-density ionic liquid dispersive liquid–liquid microextraction for the fast determination of pyrethroid insecticides in environmental water samples by HPLC-DAD

Abstract

A new microextraction technique named in-syringe low-density ionic liquid dispersive liquid–liquid microextraction (LDIL-DLLME) followed by separation using high performance liquid chromatography has been developed to determine the levels of four pyrethroid insecticides (i.e., deltamethrin, fenvalerate, permethrin, and bifenthrin) in environmental water samples. In the developed method, an ionic liquid (IL) was used for the first time instead of an organic solvent, which is most often used in low-density solvent-based microextraction methods. The IL was placed in a long syringe needle using a microsyringe. It was then dispersed by drawing the sample solution into the syringe. The extraction was finished in the syringe, taking full advantage of the low-density property, and making this method easier and quicker. Several parameters affecting the experimental efficiency of LDIL-DLLME, such as the needle's inner diameter, salt addition, the volume of IL and sample, rotation speed and duration of centrifugation and ultrasound were thoroughly studied. Under optimized conditions, in the range of 1 to 500 μg L⁻¹, good linearity was obtained, with coefficients of determination greater than 0.9994. Three spiked water samples were studied, and recovery ranged from 88.0 to 102.8%, with relative standard deviations (RSDs) ranging from 0.3 to 6.7%. The limits of detection (LODs) for the four pyrethroid insecticides were in the range of 0.88–1.71 μg L⁻¹ and enrichment factors (EFs) were in the range of 242 to 257. The proposed method provides an inexpensive, rapid, simple and eco-friendly process for evaluating pyrethroid insecticides in environmental samples, making it a potential method for the pretreatment of experimental samples.

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1. Introduction

Pyrethroids were introduced in Japan in 1987 by Mitsui Chemicals, Inc.¹ They are widely used to control pests in agriculture, public health, forestry and veterinary medicine by inhibiting the normal sodium channel function in the nerve axons of insects. Pyrethroids also have relatively low toxicity to mammals and can rapidly be degraded in the environment.^2,3 However, wide application has resulted in the widespread distribution of pyrethroid residues in the environment, which are considered hazardous to human health. These residues may remain on agricultural commodities, such as fruits and vegetables, and are classified as carcinogens and or toxins.⁴ Because of their high solubility in water, they are typically distributed in aqueous environments through leaching and run off from soil into ground and surface water. To protect consumers, the maximum residue limits (MRLs) for pyrethroid residues has been established in various foods by several organizations. For example, the European Union established MRLs of 0.01–0.2 mg kg⁻¹ in vegetables.⁵ Therefore, a rapid and effective pretreatment method for determining trace-level amounts of pyrethroids in water samples is needed.

There is an increasing demand to develop sensitive and selective methods to preconcentrate pyrethroid residues, which are usually present in trace amounts. Many analytical methods for evaluating pyrethroids have been reported, such as magnetic solid phase extraction (MSPE) followed by high-performance liquid chromatography with ultra violet detection (HPLC-UV),⁴ the QuEChERS method combined with gas chromatography with electron capture detection (GC-ECD),⁶ dispersive liquid–liquid microextraction (DLLME) combined with high-performance liquid chromatography with diode array detection (HPLC-DAD),⁷ single-drop microextraction (SDME) followed by gas chromatography coupled to mass spectrometry (GC-MS),⁸ hollow fibre-based liquid-phase microextraction (HF-LPME) combined with gas chromatography-mass spectrometry (GC-MS), and more. Although these methods have some advantages, they also have some disadvantages; for example, the MSPE method requires a tedious preparation procedure before extraction, QuEChERS uses a high level of organic solvent as the extraction solvent, DLLME consumes high level of dispersive solvent, and SDME and HF-LPME require long extraction times.

Among these methods, DLLME is the most commonly used method to evaluate pesticide residue concentrations because it is simple, efficient, and inexpensive. Additionally, it has higher preconcentration factors and uses less toxic extraction agents. It was introduced by Rezaee et al. in 2006.⁹ This method has been used in many analytical applications and as a successful aqueous sample pretreatment method. Based on these advantages, several DLLME methods have been successfully introduced, such as DLLME based on the solidification of floating organic drops (DLLME-SFO),¹⁰ ultrasound-assisted dispersive liquid–liquid microextraction (UA-DLLME),¹¹ elevated-temperature dispersive liquid–liquid microextraction (ET-DLLME),¹² in situ ionic liquid dispersive liquid–liquid microextraction (in situ IL-DLLME),¹³ low-density solvent dispersive liquid–liquid microextraction (LDS-DLLME),¹⁴ and supramolecular-based DLLME (SM-DLLME).¹⁵

In traditional DLLME, a high-density extraction solvent, such as tetrachloroethane, carbon tetrachloride, chloroform, dichloromethane, or chlorobenzene, is generally required, and these solvents are environmentally unfriendly, highly toxic and may limit the applicability of the method. Another disadvantage is that the GC peaks of halogenated hydrocarbons partially overlap with those of some analytes. In addition, because they are volatile, they must evaporate to dryness. Additionally, analytes must be reconstituted in a suitable solvent prior to LC, which is a laborious approach.^16–18 Therefore, a low-density solvent dispersive liquid–liquid microextraction (LDS-DLLME) technique was developed. The range of organic solvents that have a lower density than water is broader than that of high-density solvents.¹⁹ Several types of extraction solvents that are less dense than water can be selected, including alkanes, alcohols, ethers, ketones and acetates.²⁰ However, the most commonly used extraction solvents in LDS-DLLME are toluene, n-hexane and 1-octanol which are considered hazardous to the health of analysts or are dangerous liquids because they are inflammable or explosive.

Other alternative extractants for DLLME are ionic liquids, which have negligible vapor pressures, variable viscosities and high thermal stabilities. IL has been successfully used in numerous DLLME studies, such as in situ IL-DLLME,¹³ magnetic stirrer IL-DLLME,²¹ ultrasound-assisted IL-DLLME,²² temperature-assisted IL-DLLME²³ and gas-assisted IL-DLLME,²⁴ for the detection of pesticides, pharmaceuticals, organic pollutants and metal ions in water, food, urine and serum.^25–33 However, most ILs are imidazolium salts, and their densities are higher than that of water. In this work, the IL [P_14,6,6,6]Cl, with a density of 0.89 g cm⁻¹,^3,34,35 was used for the first time as an extraction solvent in DLLME. ILs are consist of organic cations and organic or inorganic anions. As both the anion and cation can be altered, these solvents can be designed for a particular set of properties, leading to ionic liquids known as “Designer Solvents”.³⁶ Some quaternary phosphonium salt and quaternary ammonium salt ionic liquids have a lower density than water, and they can be used in the low-density ionic liquid DLLME.^37–40 The proposed method broadens the range of extractants that can be used in DLLME, but it also increases the number of applications of the low-density methods.

To overcome the collection problem after phase separation, significant efforts have been made to improve extraction devices or collection strategies. Several interesting types of special extraction devices have been designed and applied in the LDS-DLLME procedure.^14,41–46 Most of them are designed to have a narrow neck, which is followed by squeezing the device or adding some water to allow for the movement of the light organic extract into the narrow stem, making them collectable. When the liquid moves, it is easy for some of the liquid to remain on the wall of the vessel, especially for high viscosity liquids. Therefore, we propose a special method for use with high viscosity extraction collection solvents, such as ILs, that takes advantage of their high viscosity. The extraction is finished in a syringe and, after centrifugation, when the IL is stuck to the syringe wall, the aqueous phase is discarded from the top of the syringe. The extractant is then collected by pulling the plunger to the top of syringe. This method is simple and inexpensive.

In conventional DLLME, a dispersion solvent is needed to dissipate the extraction solvents in the aqueous solution. Most of these solvents are volatile organic compounds that can damage the health of analysts and pollute the environment. In addition, they can easily dissolve the analytes, resulting in decreased recovery. To avoid these problems, a new dispersion method, named impulsion dispersion, was adopted. The IL was added in a long syringe needle using a microsyringe; then, it was dispersed by drawing the sample solution into the syringe, which is easy and quick.

In this study, a novel and efficient method, named in-syringe LDIL-DLLME combined with HPLC-DAD, was developed for the first time to determine the levels of four pyrethroid insecticides in environmental water samples. The IL was used as an extraction solvent in the low-density solvent based method. The IL was dispersed by drawing the sample solution into the syringe, and the extraction was finished in the syringe with a special procedure. The effects of some experimental parameters, such as the needle's inner diameter, salt addition, the volume of IL and sample, the rotation speed and time of centrifugation and ultrasound were thoroughly studied. Finally, the optimized conditions were used for determination of pyrethroid insecticides in real environmental water samples.

2. Materials and methods

2.1. Reagents

Four pyrethroid insecticides (deltamethrin, fenvalerate, permethrin and bifenthrin) were supplied by the Agricultural Environmental Protection Institution (Tianjin, China) and the purities are in the range of 97% to 98%. Trihexyl(tetradecyl)phosphonium tetrafluoroborate ([P_14,6,6,6]Cl) was obtained from J & K Chemical Technology Co., Ltd (Beijing, China). Milli-QSP Reagent Water System (Millipore, Bedford, MA, USA) was used to purify deionized water. HPLC-grade acetonitrile was supplied by Dikma Limited (Beijing, China). Sodium chloride (analytical grade) was supplied by Beijing Chemical Reagent Company.

Mixed standard solution of 100 mg L⁻¹ of four pyrethroid insecticides were prepared in acetonitrile and the solutions were stored at 4 °C. The working standard aqueous solutions were prepared daily by diluting an appropriate amount of the mixed standard solution in different concentrations using acetonitrile.

Three river water samples from Qujiang (Quzhou, Zhejiang Province), Tongli (Sushou, Jiangsu Province) and Changyuan He (Jinzhong, Shanxi Province) and one reservoir water sample from Yinjiaju (Chuxiong, Yunnan Province) were used for method validation. The environmental water samples were filtered through a 0.22 μm mixed cellulose membrane and stored in the dark at 4 °C prior to use.

2.2. Instruments

An Agilent 1200 HPLC system (California, USA), which was equipped with an automated sample injector and a diode array detector (DAD) system, was used to perform the chromatographic analysis of the four pyrethroid pesticides. An Agilent Eclipse Plus C18 analytical column (5 μm, 4.6 mm × 250 mm) with Spursil C18 guard cartridges (5 μm, 2.1 mm × 10 mm, Dikma Limited) was used for the separations. The Agilent Chem-Station software was used to operate the HPLC-UV system and perform data analysis. Both a high-speed centrifuge was purchased from USTC Zonkia Scientific Instruments Co., Ltd. (Anhui, China) and an ultrasonic cleaner (KQ3200DE, Kunshan, China) were used for sample treatment. The mobile phase was a methanol/water mixture (82 : 18, v/v), delivered at a flow rate of 1 mL min⁻¹, and the column temperature was maintained at 25 °C. The detection wavelength was 230 nm and the sample injection volume was 10 μL. A Mettler-Toledo AL104 electronic balance (Shanghai, China) was used to weight the materials. A 10 mL syringe was purchased from Cheng Dou Xin Jin Shi Feng Medical Apparatus & instruments Co., Ltd. A long syringe needle was obtained from Wen Zhou Needle Factory (Zhejiang, China). Sodium chloride (analytical grade) was supplied by Beijing Chemical Reagent Company.

2.3. LDIL-DLLME procedure

An aliquot of [P_14,6,6,6]Cl (40 μL) was injected into a 10 cm needle using a microsyringe, and the needle was connected to a 10 mL syringe. Then, 10 mL of deionized water that was spiked with four pyrethroid insecticides was drawn into the syringe, forming a cloudy solution. A gel cap was used to seal the top of the syringe. The syringe was then placed in an ultrasound bath for 90 s to make the extraction more complete, and 0.1 g sodium chloride was added to demulsify the emulsion to promote separation. Then, the plunger was pulled out and the syringe was centrifuged at 6000 rpm for 10 min; the IL remained on the wall of the syringe. The gel cap was removed, and the aqueous phase completely flowed out of the syringe. Finally, the IL was collected at the top of the syringe plunger by withdrawing the plunger to the top of the syringe. A 30 μL aliquot of acetonitrile was added through the injector nipple to dilute the IL phase; 10 μL of the mixture was directly injected into the HPLC system for analysis. The procedure is schematically shown in Fig. 1.

Fig. 1 Schematic procedure for in-syringe LDIL-DLLME.

2.4. Calculation of EF and R%

Enrichment factors (EFs) and extraction recoveries (ER%) were determined to evaluate the extraction efficiency of different experimental conditions by a step-by-step optimization procedure, and they were calculated using the following equations:

where C_IL, C_water, V_IL and V_water are the concentration of the analytes in the sediment phase, initial concentration of the analytes in the water sample, volume of the sediment phase and volume of the water sample, respectively.

3. Results and discussion

3.1. Optimization of the in-syringe LDIL-DLLME method

3.1.1. Optimization of the needle's inner diameter. The needle's inner diameter has a significant effect on the strength of the produced impulsion and has a direct influence on the dispersion result. Four different inner diameters (0.7, 0.9, 1.1 and 1.3 mm) were evaluated. As shown in Fig. 2, a needle with an inner diameter of 0.7 mm obtained the highest recovery, while the other needle inner diameters yielded similar results but with a marginally lower recovery. Due to the small inner diameter, a high impulsion can be obtained when withdrawing the samples. When the inner diameter is large, the impulsion is small. Additionally, in this situation, some IL remained on the wall of the needle and could not be washed into the syringe, decreasing the extraction efficiency. Furthermore, lower impulsion cannot effectively disperse IL. Therefore, a needle with an inner diameter of 0.7 mm was selected for subsequent experiments.

Fig. 2 Effect of the needle's inner diameter on the recoveries of the pyrethroid insecticides (extraction conditions: salt addition, 1%; volume of IL, 40 μL; volume of sample, 10 mL; rotation speed, 7000 rpm; centrifugation time, 10 min; and ultrasound time, 60 s).

3.1.2. Optimization of salt addition. In this paper, the extraction solvent, [P_14,6,6,6]Cl, has a hydrophobic long-chain alkane and hydrophilic chloride ion making it an emulsifying agent. Then, salt is needed for the demulsification process; otherwise, the IL cannot be completely separated. Therefore, the effect of salt addition on recoveries was studied using NaCl concentrations in the range of 0% to 5% (w/v). The results are shown in Fig. 3. When no salt was added to the sample, an extremely low recovery was obtained because less IL could be separated from the aqueous samples, but when the salt concentration increased to 1%, the recovery of all analytes increased noticeably, indicating that a small amount of salt has a positive effect on demulsification. However, the extraction efficiency decreased as the salt concentration increased to more than 1%. This phenomenon can be explained by the fact that a high salt concentration can cause an increase in the viscosity, which conceivably inhibited the extraction by slowing the mass transfer of the analytes and thus decreases the extraction efficiency. For these reasons, 1% NaCl in the aqueous phase was selected.

Fig. 3 Effect of salt addition on the recoveries of the pyrethroid insecticides (extraction conditions: needle's inner diameter, 0.7 mm; volume of IL, 40 μL; volume of sample, 10 mL; rotation speed, 7000 rpm; centrifugation time, 10 min; and ultrasound time, 60 s).

3.1.3. Optimization of the volume of IL. In this study, the volume of IL had a direct influence on the extraction efficiency; a sufficient amount of extraction solvent can completely extract the analytes and ensure a good recovery. The maximum level of IL the needle (0.7 mm inner diameter) can hold is 40 μL; therefore, the influence of the IL volume on the extraction performance was determined by evaluating volumes of [P_14,6,6,6]Cl between 30 and 40 μL. As shown in Fig. 4, the recovery increased when the volume increased from 30 to 40 μL and reached the maximum at 40 μL. To more thoroughly determine the influence the volume of IL, volumes of 45 and 50 μL were used in the needle with an inner diameter of 0.9 mm. There was a slight increase when larger volumes of IL were used, but higher RSDs were obtained due to the larger inner diameter. Furthermore, a larger extraction solvent volume may decrease the enrichment factor, which can reduce the sensitivity of this method. As a result, 40 μL of [P_14,6,6,6]Cl was adopted in the following LDIL-DLLME experiments.

Fig. 4 Effect of the volume of IL on the recoveries of the pyrethroid insecticides (extraction conditions: needle's inner diameter, 0.7 mm; salt addition, 1%; volume of sample, 10 mL; rotation speed, 7000 rpm; centrifugation time, 10 min; and ultrasound time, 60 s).

3.1.4. Optimization of the volume of sample. The volume of sample has three different effects on the extraction efficiency. First, higher volume water samples can dissolve more extraction solvent, leading to a decrease in the recovery. Second, increasing the volume of the water samples can wash the extraction solvent from the needle more completely, leading to an increase in the extraction efficiency. Finally, a higher water sample volume increases the pyrethroid insecticides levels that need to be extracted. This can decrease the extraction efficiency if the extractant is insufficient to extract all of the analytes. Water samples of 5–15 mL were used to evaluate the extraction efficiency. Other factors were at previously determined optimum levels. Fig. 5 shows the results for the four pyrethroid insecticides. Similar recovery was obtained for the 5 and 8 mL samples, but the recovery slightly increased when the sample volume increased up to 10 mL. The results can be explained by the fact that IL has a low solubility in water, and the volume of IL that was used is sufficient to extract all analytes from 10 mL samples. Furthermore, because IL was drawn into a needle, its high viscosity makes it stick to the needle wall. A high water volume (larger than 10 mL) can wash IL completely out of the syringe when drawing the water sample into the syringe, increasing the recovery. Considering all of these observations, a 10 mL water sample volume was selected for the following experiments.

Fig. 5 Effect of the volume of sample on the recoveries of the pyrethroid insecticides (extraction conditions: needle's inner diameter, 0.7 mm; salt addition, 1%; volume of IL, 40 μL; rotation speed, 7000 rpm; centrifugation time, 10 min; and ultrasound time, 60 s).

3.1.5. Optimization of the rotation speed and centrifugation time. In the proposed method, centrifugation was needed to separate the IL and aqueous phases from the cloudy solution. This procedure has a direct effect on the amount of collected IL, and can significantly influence the recovery. The effects of rotation speed, in the range of 2000–10 000 rpm, and centrifugation duration, in the range of 6–14 min, on the extraction efficiency were studied to obtain the optimal conditions. Fig. 6 shows the effect of the centrifugation rotation speed on the recoveries of the four compounds. As the rotation speed increased from 2000 to 6000 rpm, the recoveries for all analytes increased accordingly. When the rotation speed increased to more than 6000 rpm, the percentage of recovery decreased noticeably. When the rotation speed is too high, the syringe cannot withstand the force of rotation and can be distorted or even broken. Therefore, 6000 rpm was adopted as the centrifugation rotation speed. Fig. 7 illustrates the recovery obtained from different centrifugation durations. Recovery for all analytes increased with increasing duration from 6 to 10 min and then an equilibrium state was reached. As a result, centrifugation for 10 min at 6000 rpm was used for further investigation.

Fig. 6 Optimization of the rotation speed on the recoveries of the pyrethroid insecticides (extraction conditions: needle inner diameter, 0.7 mm; salt addition, 1%; volume of IL, 40 μL; volume of sample, 10 mL; centrifugation time, 10 min; and ultrasound time, 60 s).

Fig. 7 Effect of centrifugation time on the recoveries of the pyrethroid insecticides (extraction conditions: needle inner diameter, 0.7 mm; salt addition, 1%; volume of IL, 40 μL; volume of sample, 10 mL; rotation speed, 6000 rpm; and ultrasound time, 60 s).

3.1.6. Optimization of ultrasound time. Ultrasound treatment, which making the extraction more complete, is a key factor in this method. To study the effect of ultrasound on the extraction efficiency, a series of experiments was performed with ultrasound times in the range of 0–90 s. As shown in Fig. 8, a sufficient ultrasound time accelerates the formation of a fine dispersive mixture and results in higher recovery levels. Nearly one hundred percent recovery was obtained when the 90 s ultrasound duration was adopted. Therefore, the ultrasound duration was set at 90 s.

Fig. 8 Effect of ultrasound time on the recoveries of the pyrethroid insecticides (extraction conditions: needle inner diameter, 0.7 mm; salt addition, 1%; volume of IL, 40 μL; volume of sample, 10 mL; rotation speed, 6000 rpm; and centrifugation time, 10 min).

3.2. Method validation

The optimal conditions selected for LDIL-DLLME were as follows: the needle's inner diameter was 0.7 mm, containing 40 μL [P_14,6,6,6]Cl. The sample solution volume was 10 mL, and 1% salt solution was added. An ultrasonic treatment time of 90 s was used, and the samples were centrifuged for 10 min at 6000 rpm. Using the above mentioned optimum conditions, parameters such as the linearity, repeatability, limits of detection (LODs) and EFs were determined. Three replicate extractions were performed at each concentration level. Because fenvalerate has a chiral structure⁴⁷ and the two peaks were connected, they were considered as a single peak in qualitative and quantitative analyse. Permethrin has two structure types, cis- and trans-,⁴⁸ and both of them were used in quantitative analyse. The two peaks are independent, and the lower peak was used in qualitative analyse. Water samples with deltamethrin in the range of 1–500 μg L⁻¹ and three other analytes in the range of 2–500 μg L⁻¹ were studied. The characteristic calibration data are summarized in Table 1. Good linearity and repeatability were obtained for all four pyrethroids, with R² values ranging from 0.9994 to 0.9999 and relative standard deviations (RSDs) ranging from 0.7 to 2.9%. The limits of detection for the four pyrethroid insecticides, calculated at S/N = 3, were in the range of 0.83–1.71 μg L⁻¹ and the limit of quantification were in the range of 2.77–5.71 μg L⁻¹. High EF and recovery of the four analytes were obtained, ranging from 242 to 257 and 96.6 to 102.6%, respectively. In conclusion, the proposed LDIL-DLLME method proved to be an efficient and facile method for the preconcentration and detection of pyrethroids in spiked water samples.

Table 1 The performance characteristics of the LDIL-DLLME method combined with HPLC-UV analysis

Analytes	Linearity equation	Linearity (μg L^−1)	R^2	RSD (%)	Enrichment factor	LOD (μg L^−1)	LOQ (μg L^−1)	Recovery (%)
Deltamethrin	Y = 6.08X − 6.98	1–500	0.9994	0.7	257	0.83	2.77	102.6
Fenvalerate	Y = 5.48X + 1.10	2–500	0.9999	2.9	256	1.40	4.65	102.3
Permethrin	Y = 6.61X − 12.43	2–500	0.9994	1.9	242	1.71	5.71	96.7
Bifenthrin	Y = 6.10X − 9.84	2–500	0.9994	1.5	244	1.20	4.00	96.6

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3.3. Analysis of spiked real water samples

Three river water samples from Qujiang (Quzhou, Zhejiang Province), Tongli (Sushou, Jiangsu Province), and Changyuan He (Jinzhong, Shanxi Province) and one reservoir water sample from Yijiaju (Chuxiong, Yunnan Province) were used for pyrethroid determinations to evaluate the applicability of the in-syringe LDIL-DLLME method. No significant matrix effects were present in this method. No target analytes were detected in the blank samples or in the residues were below the detectable level (as shown in Fig. 9a). The environmental samples were spiked at three concentrations (10, 50 and 100 μg L⁻¹) and were used to evaluate the matrix effects. The resulting typical chromatograms are shown in Fig. 9. The analytical results are summarized in Table 2. As can be seen, the average recoveries for the four pyrethroids were in the range of 88.0% to 102.8%, and the RSDs ranged from 0.3 to 6.7% (n = 3). These results indicated that the LDIL-DLLME method was reliable and appears to be a promising method for evaluating of pyrethroids in water samples.

Fig. 9 The HPLC chromatograms of pyrethroids in the spiked and blank environmental water samples: (1) deltamethrin; (2) fenvalerate; (3) permethrin; (4) bifenthrin. In chromatograms a, b, c and d, the spiked levels were 0, 10, 50 and 100 μg L⁻¹, respectively.

Table 2 Spiked recoveries (%) of the lake water and river water^a

Analytes	Spiked level (μg L^−1)	Qujiang		Tongli		Changyuan He		Yijiaju
		ER	RSD	ER	RSD	ER	RSD	ER	RSD
a ER: extraction recovery (%); RSD: relative standard deviation (%).
Deltamethrin	10	88.1	0.7	94.7	3.8	99.3	6.7	95.4	1.8
	50	102.7	2.7	91.5	3.5	100.0	2.0	92.7	4.7
	100	98.7	2.7	96.6	2.9	98.2	5.4	98.0	2.5
Fenvalerate	10	102.8	3.2	100.2	1.5	94.3	4.4	94.5	5.4
	50	100.3	4.0	91.5	4.6	97.2	5.3	94.3	2.3
	100	98.2	2.7	91.1	3.8	96.2	4.8	94.0	5.6
Permethrin	10	90.5	2.4	96.7	4.9	93.5	5.6	91.0	0.8
	50	102.5	1.3	97.2	5.3	94.0	5.3	97.3	4.0
	100	97.8	2.4	94.7	3.7	100.2	4.7	99.9	2.2
Bifenthrin	10	96.4	6.4	89.5	6.3	90.5	4.8	88.0	5.0
	50	102.6	0.3	95.8	4.6	101.8	3.4	95.2	5.9
	100	96.6	3.1	96.1	4.9	97.5	5.9	99.4	2.9

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3.4. Comparison of in-syringe LDIL-DLLME with other analytical methodologies

The proposed in-syringe LDIL-DLLME method was compared with other analytical methodologies for preconcentrating and evaluating pyrethroid insecticides. As shown in Table 3, several analytical methods, including DLME/D-m-SPE,⁵ DLLME,⁴⁹ UA-DLLME-SFO¹¹ and SSIL-DLLME,⁵⁰ were compared in terms of the extraction solvent, extraction time, solvent usage, recovery and enrichment factors. The organic solvent level required for this technique is lower than that required for DLME/D-m-SPE and traditional DLLME, making it more environmentally friendly and better for health of analysts. Furthermore, when compared with UA-DLLME-SFO and SSIL-DLLME, it does not need a solidification procedure, making the method simpler and quicker. The recovery and EFs in the proposed method are superior to those of most other methods. In conclusion, less organic solvent and a lower extraction time were required, and high recovery and EFs were obtained with the proposed method, demonstrating that it is a simple, fast, effective and environmentally friendly technique.

Table 3 Comparison of the proposed LDIL-DLLME method with other methods for the determination of pyrethroid insecticides^a

Method	Extraction solvent	Sample	Solvent usage (μL)	Extraction time (min)	Linearity (μg L^−1)	EFs	Recovery (%)	Ref.
a DLME, dispersive liquid microextraction; D-μ-SPE, dispersive μ-solid phase extraction; SSIL, solidification of sedimentary ionic liquids.
DLME/D-m-SPE-HPLC	1-Octanol	Water	180	4	5–400	51–108	91.7–104.5	5
DLLME-HPLC	Chloroform	Fruit juices	1650	1	2.00–1000	62–84	85.8–94.0	49
UA-DLLME-SFO-GC-FID	1-Dodecanol	Water	300	4	0.5–200	143–813	65.7–73.2	11
SSIL-DLLME-HPLC	[P12,4,4,4][PF6]	Water	30	1	1–500	145–157	90.1–97.4	50
LDIL-DLLME-HPLC	[P14,6,6,6]Cl	Water	30	2	1–500	242–257	97.0–102.6	This work

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3.5. Comparison of IL with the solvent as extraction solvent in LDS-DLLME

In this method, [P_14,6,6,6]Cl was used as the low density extraction solvent, for the first time, in LDS-DLLME. Using ILs as extraction solvent is superior to the use of other solvents in the following ways. (i) ILs are not flammable and have negligible vapor pressures, making them healthier and safer for the analyst than some volatile, flammable and combustible solvents. (ii) ILs can be designed to have special properties. This means that more potential ILs can be found with the property of low density, which can broaden the application of the LDS-DLLME method. (iii) Due to the high viscosity of IL, it can stick to the wall of the extraction vessel and quickly separate from the aqueous phase. The proposed device for LDIL-DLLME is simpler than in other methods. However, the higher viscosity property also limits the devices that can be adopted for the LDIL-DLLME method. Because most of the devices proposed for LDS-DLLME cannot be used in LDIL-DLLME, the development of other devices is necessary. In conclusion, the adoption of IL in LDS-DLLME greatly broadens the application of the low density method and makes these methods sampler and more efficient.

4. Conclusions

In this study, a novel liquid-phase microextraction technique, in-syringe LDIL-DLLME combined with HPLC-DAD, was successfully applied to determine the levels of four pyrethroid insecticides in environmental water samples. An IL was used as the extraction solvent in LDS-DLLME, for the first time, which widely broadened the application of the low density method. With the high viscosity of IL, the proposed method was finished in a syringe, enabling rapid collection of the extraction phase. To avoid the use of a dispersion solvent in traditional DLLME, the process of drawing the sample solution into a long needle was used to disperse the IL, making this method more environmentally friendly. Several significant experimental parameters were studied. Under the optimized conditions, satisfactory results were obtained and no significant matrix effects were observed for evaluating of four pyrethroid insecticides in real samples. This method provides good repeatability, great enrichment factors and high recovery levels. In addition, this approach required less time and organic solvent, resulting in an environmentally friendly, efficient and simple method for determining trace levels of pyrethroid insecticides in water samples.

Acknowledgements

This work was supported by the fund National Natural Science Foundation of China (Project no. 21507159, 21277172 and 21377163) and Chinese Universities Scientific Fund (Project no. 2016QC082).

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