Simultaneous synthesis of a deep eutectic solvent and its application in liquid–liquid microextraction of polycyclic aromatic hydrocarbons from aqueous samples

Abstract

In the present study, a new generation of solvents, named deep eutectic solvents, were simultaneously synthesized and used as an extraction solvent in a liquid–liquid microextraction method for the extraction and preconcentration of some polycyclic aromatic hydrocarbons from aqueous samples prior to their determination by high performance liquid chromatography-diode array detection. In this method, choline chloride and p-chlorophenol (at a 1 : 2 molar ratio) are added to an aqueous phase containing the target analytes and the mixture is shaken manually to obtain a homogeneous solution. After heating, choline chloride and p-chlorophenol form a deep eutectic solvent in whole parts of the solution and a cloudy state is obtained. The solution is centrifuged and the sedimented phase is injected into the separation system. Some important parameters such as deep eutectic solvent composition, ionic strength, pH and temperature of aqueous phase, and centrifuge rate and time were studied. Under the optimum conditions, enrichment factors and extraction recoveries were obtained in the 586–632 and 88–95% ranges, respectively. The linear ranges of calibration curves were wide and the limits of detection and quantification were between 0.19 and 0.92 and 0.61–3.0 ng mL⁻¹, respectively. This method is very simple, rapid and efficient.

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1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are compounds in which two or more aromatic rings are found. These materials are composed from only carbon and hydrogen and they are classified as the most hazardous substances.¹ PAHs are produced during the incomplete burning of fuels, garbage or other organic substances and can also be found in bitumen, crude oil, coal tar, creosote and roofing tar.^2–4 They are on the European Union (EU) main concern pollutants list.⁵ Therefore, it is of great importance for the identification and determination of PAHs. Due to low concentration and/or complex matrix of the samples, a sample preparation step is often inevitable to improve the selectivity and sensitivity of PAHs determination. In general, liquid–liquid extraction (LLE)^6,7 and solid phase extraction (SPE)^8–10 have been applied for extraction and preconcentration of PAHs as traditional methods. It is well known that these traditional methods have disadvantages such as being time-consuming, require intensive labor, result in unsatisfactory enrichment factors (EFs), and they use large quantities of toxic organic solvents, which force analysts to limit their applications. Therefore, numerous other sample preparation techniques have been developed. Among the developed sample pretreatment methods, the miniaturized extraction techniques have attracted much attention. Solid phase microextraction (SPME) is the first microextraction technique that was introduced by Arthur and Pawliszyn in 1990.¹¹ In SPME, the analytes are partitioned between the sample matrix and a fiber coating. Despite many advantages provided by SPME, most commercial fibers are fragile, comparatively expensive and have a limited lifetime. Sample carry-over is a possible problem.^12–14 Due to these shortcomings, liquid phase microextraction (LPME) was developed.^15–17 After publication of the first paper on LPME in 1996,¹⁸ different modes of LPME, including single drop microextraction (SDME),^19,20 hollow fiber-liquid phase microextraction (HF-LPME),^21,22 dispersive liquid–liquid microextraction (DLLME),^23,24 solidification of floating organic drop microextraction (SFODME),²⁵ air-assisted liquid–liquid microextraction (AALLME),^26,27 and liquid–liquid microextraction (LLME),²⁸ have been reported. Liquid microextraction is a preconcentration method in which the analytes are extracted from an aqueous solution into μL-level of an organic solvent. Along with developing sample preparation methods, efforts have been directed toward applying green solvents in microextraction methods. A class of green solvents is ionic liquids (ILs), which have attracted significant attention due to their special physical and chemical properties.²⁹ A new generation of extraction solvents obtained from renewable resources is deep eutectic solvents (DESs).^30,31 A DES is formed by mixing two components, including a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), which can be related with each other according to hydrogen bond interaction. The obtained DES has a melting point lower than those of the individual components. Physical and chemical properties of DESs are similar to ILs but the simple synthesis of them and availability and biodegradability of the components make them good substitutes to ILs. While the individual components of DESs are well toxicologically characterized, there is very little information about the toxicological properties of DESs themselves, and this needs to be further investigated by the scientific community.³² These features make DESs as potential alternatives to replace conventional organic solvents as well as ILs.

The main goal of this study was to develop a new sample preparation method by simultaneous formation of a water-immiscible DES and performing an LLME method. The extraction capability of the produced DES was investigated for the extraction and preconcentration of PAHs from aqueous samples. In this process, initially two individual components of DES (HBA and HBD) were dissolved in an aqueous sample solution to achieve a homogeneous solution. Then, the mixture was heated to form DES and after cooling to room temperature, fine droplets of the formed DES were dispersed in whole parts of the solution. To the best of our knowledge, this is the first report on the formation of DES in an aqueous solution that is used simultaneously in a microextraction process. To achieve high extraction efficiency, different parameters were studied and optimized.

2. Experimental

2.1. Materials and standards

The studied PAHs, including acenaphthene, phenanthrene, anthracene, and pyrene, were purchased from Merck (Darmstadt, Germany). p-Chlorophenol, choline chloride and analytical-grade sodium chloride, hydrochloric acid, and sodium hydroxide were from Merck. HPLC-grade water and acetonitrile were purchased from Chemlab (Zedelgem, Belgium). A stock solution of PAHs (50 mg L⁻¹ of each analyte) was prepared in methanol. Working standard solutions of the target analytes were prepared daily by appropriate dilutions of the stock solution with HPLC-grade water.

2.2. Instrumentation

Quantification of the PAHs was performed on a Hewlett-Packard 1090-II liquid chromatograph (Palo Alto, CA, USA) equipped with a diode array detector (DAD). The separation was carried out on an Altech Altima analytical C18 column (150 × 4.6 mm id, 5 μm particle size) (Fisher Scientific, Massachusetts, USA). The mobile phase was a mixture of acetonitrile–water (45 : 55, v/v) at a 1 mL min⁻¹ flow rate using an isocratic elution. Monitoring of the analytes was done at 254 nm except for acenaphthene, which was monitored at 230 nm. Data acquisition and processing were done using the ChemStation software. All injections were performed manually using a 10 μL sample loop. A Metrohm 654 pH meter (Metrohm, Herisau, Switzerland) equipped with a glass electrode was used in pH adjustments. A Hettich centrifuge model D-7200 (Germany) was used for accelerating phase separation.

2.3. Samples

Two wastewaters including petrochemical wastewater (output of the refinery unit) and refinery wastewater (output of distillation unit) were obtained from Tabriz Petrochemical and Tabriz Refinery, respectively (Tabriz, Iran). Other aqueous samples including municipality wastewater (Tabriz, Iran), tap water, and Zarineh river water (Miyandoab, West Azarbaijan, Iran) were also studied. All the samples were analyzed without dilution.

2.4. DES synthesis and microextraction procedure

In this method, 10 mL HPLC-grade water spiked with PAHs at a 25 ng mL⁻¹ concentration (for each analyte) or sample solution was transferred into a 15 mL glass test tube with a conical bottom. Then, 0.12 g choline chloride and 0.20 g p-chlorophenol (at a 1 : 2 molar ratio) were added to the abovementioned solution and the mixture was shaken manually to obtain a homogeneous solution. The tube was placed in a water bath at 80 °C for 10 min. During this step, the chlorine atom of choline chloride forms a hydrogen bond with the hydrogen atom of p-chlorophenol in the aqueous phase and DES is formed gradually in whole parts of the solution resulting in a cloudy state. In this period, the target analytes are extracted into the tiny droplets of DES and transferred into it. The solution was cooled to room temperature with tap water. Finally, the cloudy solution was centrifuged at 1178 × g for 5 min and the extractant was settled down at the bottom of the tube (15 ± 1 μL). The sedimented phase was completely withdrawn and injected into the HPLC system. The chemistry of DES formation is shown in Scheme 1.

Scheme 1 Interaction of the DES components.

2.5. Calculation of EF and extraction recovery (ER)

EF is defined as the ratio of the analyte concentration in the sedimented phase (C_sed) to the initial concentration of the analyte (C₀) in the sample as follows:

EF = C_sed/C₀ (1)

C_sed is obtained by comparing the obtained peak areas in two cases: direct injection of the PAHs standard solution prepared in methanol and injection of the sedimented phase into an HPLC.

ER is defined as the percentage of the total analyte amount (n₀) that is extracted into the sedimented phase (n_sed) as follows:

where V_sed and V_aq are volumes of the sedimented phase and sample solution, respectively.

3. Results and discussion

In this study, a new sample preparation method based on in situ synthesis and the LLME method was developed. To achieve the highest extraction efficiency and optimum conditions, the effect of some important parameters, such as molar ratio of DES components, ionic strength, temperature, and pH of sample, were evaluated and optimized.

3.1. Optimization of related parameters in synthesis of the DES in an aqueous solution

Formation and eutectic point of a DES are directly related to the molar ratio of its components. In a worse molar ratio, DES is not prepared suitably. Therefore, optimization of the DES component ratio is a key parameter. In this study, p-chlorophenol and choline chloride were selected as HBD and HBA, respectively, and the DES was formed in an aqueous phase for the first time. Therefore, p-chlorophenol and choline chloride were dissolved in the aqueous phase at different molar ratios. For this purpose, 0.13 g choline chloride and different amounts of p-chlorophenol, including 0.06, 0.12, and 0.24 g, were added to the aqueous phase to obtain 1 : 0.5, 1 : 1, and 1 : 2 molar ratios relative to choline chloride : p-chlorophenol, respectively. It is remarkable that these amounts were selected according to the solubility of p-chlorophenol in the aqueous phase. Choline chloride is freely soluble in water, whereas the solubility of p-chlorophenol in water is 27 g L⁻¹ or 0.27 g/10 mL at 25 °C.³³ Therefore, in all the abovementioned ratios, a homogenous solution was obtained. The obtained results showed that only with a 1 : 2 (choline chloride : p-chlorophenol) molar ratio, a water-immiscible DES formed. This confirms the results obtained by the previous report in which chloride ion in choline chloride forms two hydrogen bonding with the HBD agent.³⁴ Therefore, it was selected as the optimum ratio in the following studies.

According to the results obtained from the previous step, choline chloride and p-chlorophenol form a DES at a 1 : 2 (chloride : p-chlorophenol) molar ratio. By keeping this ratio constant and changing the amounts of choline chloride and p-chlorophenol, different volumes of DES could be obtained. Therefore, optimization of choline chloride and p-chlorophenol amounts is an important parameter from the view point of the extractant (DES) volume. To evaluate the amounts of choline chloride and p-chlorophenol, different solutions containing 0.06 : 0.10, 0.08 : 0.15, 0.12 : 0.20, and 0.15 : 0.25 (g : g) of choline chloride : p-chlorophenol were prepared for extraction and preconcentration of the selected PAHs from 10 mL aqueous solution spiked with 25 ng mL⁻¹ of each analyte. Comparison of ER% obtained with different ratios of choline chloride : p-chlorophenol is shown in Fig. 1. Based on these results, ERs% increased until 0.12 : 0.20 of choline chloride : p-chlorophenol and then remained constant. This can be attributed to the volume of the formed DES in different amounts of choline chloride and p-chlorophenol. It was noted that formed DES volume at the bottom of the tube was 5, 9, 15, and 21 μL for 0.06 : 0.10, 0.09 : 0.15, 0.12 : 0.20, and 0.15 : 0.25 (g : g) of choline chloride : p-chlorophenol, respectively. By increasing the DES volume, the volume ratio of organic phase to aqueous phase was increased which led to an increase ER%. Subsequently, 0.12 : 0.20 (g : g) of choline chloride : p-chlorophenol was selected as the optimal amount of DES constitutes (Fig. 2).

Fig. 1 Effect of choline chloride : p-chlorophenol amounts on the efficiency of the developed method. Extraction conditions: aqueous solution volume, 10 mL; analyte concentrations, 25 ng mL⁻¹ of each PAH; aqueous phase temperature, 70 °C; heating time, 5 min; centrifuge rate, 1118 × g; and centrifuge time, 5 min. The error bars indicate the minimum and maximum of three independent determinations.

Fig. 2 (A) Optimization of centrifuging time. Extraction conditions: aqueous phase temperature 80 °C. The other conditions are the same as used in Fig. 1(B) optimization of centrifugation speed. Extraction conditions are the same as was used in (A) except 5 min was selected as the centrifugation time. The error bars indicate the minimum and maximum of three independent determinations.

DESs were produced by just mixing the components and heating them under mild conditions. The nature of the resulting DES depends on the individual components purity, heating temperature, and time. Moreover, in this study, heating could be a driving force for better extraction solvent dispersion (the produced DES) into the aqueous solution so that the contact area between the extractant and sample increased and hence the mass transfer rates of the analytes were improved. Therefore, the effect of temperature was studied within the 65–95 °C range at a constant 5 min heating time. The obtained results showed that by increasing the temperature, ER% increased until 80 °C and then remained almost constant. The migration rates and solubility variations of the analytes in the organic and aqueous phases by temperature can affect the analytes distribution coefficients. Therefore, different ERs were obtained at different temperatures. Therefore, 80 °C was selected for the next experiments. It is remarkable that at a 60 °C temperature or less, the DES was not formed and the method failed to work.

On the other hand, heating time was another important parameter in amount of the produced DES. Therefore, the mixture was heated to 80 °C for 0–7 min. The obtained results showed that in 3 min or less, the DES was not formed and in 3–5 min, the DES volume and analytical signals increased and then were constant for up to 7 min. Therefore, the mixture was heated for 5 min in the following experiments.

3.2. Optimization of microextraction parameters

Salt addition is commonly used in most LPME procedures. Addition of a salt often decreases the target analytes solubility in the aqueous phase and increased their portioning into the extraction solvent. On the other hand, salt addition decreased the extraction solvent solubility into the aqueous phase. In order to evaluate the ionic strength effect, sodium chloride (0–15%, w/v, at 2.5% intervals) was added to the aqueous phase spiked with the target analytes at a 25 ng mL⁻¹ concentration (each analyte), whereas the other experimental conditions were kept constant. Salt addition can have two opposite effects. The positive effect is the salting out effect phenomena by which the ER% should be increased. However, high salt concentration leads to a viscosity increase of the aqueous phase, which in turn exerts a negative effect on ER%. The experimental results show that NaCl addition did not have a significant effect on ER% until 10%, w/v, and then it decreased gradually at higher NaCl concentrations. It seems that there was a balance between positive and negative effects at 10%, w/v, NaCl. The decrease at high NaCl concentration can be attributed to an increase in the aqueous phase viscosity, which leads to a decrease in the analyte diffusion coefficients. In other words, the negative effects of viscosity incrementally overcome the positive effect of salting out at concentrations higher than 10%, w/v, NaCl. Therefore, other experiments were performed without salt addition.

In order to evaluate the effect of aqueous phase pH on DES production and extraction efficiency of the developed method, several solutions at different pH, including 2, 4, 6, 7, 8, 10, and 12, were prepared and the method was performed on them. It was found that in the 4–8 pH range, pH had no effect on the produced DES and ER with the developed method. However, at other pH values (2, 10, and 12), DES was not formed and the method failed to work. The pH of all samples used in this study was between 5 and 8; therefore, there was no need for pH adjustment.

In this method, centrifugation is an important process to achieve a rapid separation of the extractant droplets from the aqueous phase. To obtain the optimum values for centrifugation time and speed, several experiments were performed in the 3–8 min range (3, 4, 5, 6, 7, and 8 min) and 179–2191 × g, respectively. Except for 3 min of centrifugation time, separation of the two phases was performed completely and the analytical signals were nearly constant. Moreover, the centrifuge speed had a little influence on the extraction efficiency at high speeds. Therefore, 1118 × g and 5 min were selected as the optimal centrifuge rate and time, respectively, in the following studies.

3.3. Method validation

In order to validate the proposed method, linearity, precision, limit of detection (LOD), limit of quantification (LOQ), ER, and EF were evaluated and the obtained results are listed in Table 1. The linearity of the method was evaluated using a series of solutions with seven different concentrations being extracted with the developed method. Good linearity values ranging from 0.9961 to 0.9992 were obtained for all the analytes. The LODs, which are typically determined to be in the range where the signal measured as peak height to noise ratio is equal to 3 (S/N = 3), ranged from 0.19 to 0.92 ng mL⁻¹. Moreover, the LOQs (S/N = 10) were in the 0.61–3.0 ng mL⁻¹ range. The EFs and ERs for the target analytes ranged from 586 to 632 and from 88% to 95%, respectively. Relative standard deviations (RSDs) at a 5 ng mL⁻¹ concentration of each analyte were in the 4–7% and 6–9% ranges for intra-day (n = 6) and inter-day (n = 4) precision, respectively, which indicated that the method was satisfactorily repeatable.

Table 1 Quantitative features of the developed method for the selected PAHs

Analytes	LOD^a	LOQ^b	LR^c	r^d	RSD^e%		EF ± SD^f	ER ± SD^g
					Intra-day	Inter-day
a Limit of detection (S/N = 3) (ng mL^−1).b Limit of quantification (S/N = 10) (ng mL^−1).c Linear range (ng mL^−1) (n = 7).d Correlation coefficient.e Relative standard deviation (n = 5, C = 5 ng mL^−1) for intra-day and (n = 4, C = 5 ng mL^−1) for inter-day precision.f Enrichment factor ± standard deviation (n = 3).g Extraction recovery ± standard deviation (n = 3).
Acenaphthene	0.52	1.6	1.6–1000	0.9987	4	6	586 ± 26	88 ± 4
Phenanthrene	0.28	0.91	0.91–1000	0.9992	4	6	632 ± 33	95 ± 5
Anthracene	0.19	0.61	0.61–1000	0.9989	5	6	612 ± 33	92 ± 5
Pyrene	0.92	3.0	3.0–1000	0.9961	7	9	592 ± 39	89 ± 6

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3.4. Real samples analysis

The effectiveness of the method in measuring trace levels of the target analytes was evaluated by analyzing different aqueous samples, including the refinery unit output (petrochemical wastewater), output of the distillation unit (refinery wastewater), municipality wastewater, tap water, and river water samples. None of the analytes were detected in tap and river waters. Anthracene and pyrene were found in the distillation unit output of the Tabriz Refining and municipality wastewater and anthracene and acenaphthene were found in output of the refinery unit of the Tabriz Petrochemical Company. The concentrations of the detected analytes are listed in Table 2. Fig. 3 depicts typical HPLC-DAD chromatograms of output of the refinery unit, output of the distillation unit, and the municipality wastewater along with a standard solution. To evaluate the method's accuracy and matrices effect in different samples, the added-found method was used. The recoveries obtained for the samples compared with those obtained for HPLC-grade water spiked at the same three concentration levels (5, 10, and 30 ng mL⁻¹ of each analyte) are listed in Table 3. Relative recoveries were obtained in the 90–99% range for the samples, indicating that the matrices had no significant effect on the method performance.

Table 2 The concentrations of the studied analytes in different samples. Data are given as analyte concentration (ng mL⁻¹) ± standard deviation (n = 3)

Analyte	River water	Tap water	Municipality wastewater	Output of distillation unit (Tabriz Refining)	Output of the refinery unit (Tabriz Petrochemical Co.)
a Not detected.
Acenaphthene	ND^a	ND	ND	ND	15 ± 2
Phenanthrene	ND	ND	ND	ND	ND
Anthracene	ND	ND	29 ± 2	35 ± 4	19 ± 2
Pyrene	ND	ND	17 ± 1	26 ± 3	ND

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Fig. 3 Typical HPLC-DAD chromatograms of (I) direct injection of standard solution of PAHs prepared in methanol (50 mg L⁻¹ of each analyte), (II) municipality wastewater, (III) output of distillation unit, and (IV) output of the refinery unit. The detection wavelength was 254 nm. For chromatographic conditions, see the Experimental section. Peak identification: (1) acenaphthene, (2) phenanthrene, (3) anthracene, and (4) pyrene.

Table 3 Study of matrix effects in the samples spiked with different concentrations. Analyte sample content was subtracted

Analyte	Mean recovery ± standard deviation (n = 3)
	Acenaphthene	Phenanthrene	Anthracene	Pyrene
All samples were spiked with each analyte at a concentration of 5 ng mL^−1
Tap water	96 ± 4	98 ± 3	97 ± 3	92 ± 4
River water	97 ± 2	99 ± 2	99 ± 3	91 ± 3
Municipality wastewater	92 ± 1	97 ± 3	97 ± 3	97 ± 3
Output of distillation unit	99 ± 3	93 ± 2	97 ± 2	98 ± 4
Output of the refinery unit	98 ± 3	99 ± 4	98 ± 1	96 ± 4
All samples were spiked with each analyte at a concentration of 10 ng mL^−1
Tap water	99 ± 3	97 ± 3	96 ± 2	95 ± 3
River water	97 ± 3	98 ± 4	98 ± 4	97 ± 3
Municipality wastewater	97 ± 4	98 ± 3	90 ± 2	96 ± 3
Output of distillation unit	99 ± 2	97 ± 4	98 ± 2	95 ± 4
Output of the refinery unit	94 ± 4	98 ± 3	97 ± 3	95 ± 3
All samples were spiked with each analyte at a concentration of 30 ng mL^−1
Tap water	97 ± 3	95 ± 3	92 ± 2	92 ± 3
River water	98 ± 3	97 ± 3	98 ± 4	93 ± 3
Municipality wastewater	98 ± 3	98 ± 4	97 ± 4	95 ± 2
Output of distillation unit	99 ± 3	97 ± 3	94 ± 5	98 ± 4
Output of the refinery unit	98 ± 2	98 ± 4	96 ± 2	98 ± 4

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3.5. Comparison of the developed method with other approaches

The analytical characteristics of the proposed method in combination with HPLC-DAD for determining the target analytes were compared with other previously published papers. Some analytical parameters of the reported methods and proposed method are summarized in Table 4. According to the results, the RSD values for the presented method are better than the other methods except for monolith microextraction-HPLC-DAD. On the other hand, the developed method provides low LODs. Moreover, the LR for the developed method was wider than those of other presented methods except for cooling/heating assisted-solid phase microextraction/GC-FID. All these results reveal that the presented method is a sensitive, rapid and reproducible technique that can be used for PAH preconcentration in water samples.

Table 4 Comparison of the proposed method with other methods in determining the selected PAHs

Method	Sample	LOD^a	LR^b	RSD^c (%)	Reference
a Limit of detection.b Linear range (ng mL^−1).c Relative standard deviation.d In tube-solid phase microextraction-high performance liquid chromatography-diode array detector.e Monolith microextraction-high performance liquid chromatography-diode array detector.f Ultrasound-assisted emulsification microextraction-gas chromatography-mass spectrometry.g Cooling/heating assisted-solid phase microextraction-gas chromatography-flame ionization detection.h Liquid–liquid microextraction-high performance liquid chromatography-diode array detector.
In-SPME-HPLC-DAD^d	Aqueous solution	15–20 (ng mL^−1)	50–100	5.6–20.1	35
MLME-HPLC-DAD^e	Aqueous solution	1.7–2.0 (ng mL^−1)	2.0–10 000	3.9–5.7	36
USAEME-GC-MS^f	Spirit water	1.8–6.3 (ng mL^−1)	—	6–13	37
CHA-SPME-GC-FID^g	Soil	0.53–0.89 (ng g^−1)	2.9–10 000	8.46–10.34	38
LLME-HPLC-DAD^h	Tap and river waters; output of refinery and distillation units; and municipality wastewater	0.19–0.92 (ng mL^−1)	0.61–1000	4–7	This method

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4. Conclusion

In the present study, for the first time, a microextraction procedure was proposed using an in situ synthesized DES for extracting some PAHs in aqueous samples followed by their determination by HPLC-DAD. DESs as a new generation of solvents have the advantages of non-volatility, non-flammability, low vapor pressure, good thermal stability, low toxicity, wide liquid range, good biodegradability, and ability to be reutilized. In this method, a new water-immiscible DES was prepared in an aqueous solution and it was simultaneously used in PAHs extraction. By developing the presented method, it is foreseeable that DESs are very promising in the microextraction field for different analytes.

Abbreviations

AALLME	Air-assisted liquid–liquid microextraction
DLLME	Dispersive liquid–liquid microextraction
HF-LPME	Hollow fiber-liquid phase microextraction
LLME	liquid–liquid microextraction
LLE	Liquid–liquid extraction
LPME	Liquid phase microextraction
PAH	Polycyclic aromatic hydrocarbon
SDME	Single drop microextraction
SPE	Solid phase extraction
SPME	Solid phase microextraction
SFODME	Solidification of floating organic drop microextraction

Acknowledgements

The authors thank the Research Council of University of Tabriz for financial support.

References

1. B. Fouillet, P. Chambon, R. Chambon, M. Castegnaro and N. Weill, Bull. Environ. Contam. Toxicol., 1991, 47, 1–7.
2. J. Li, X. Shang, Z. Zhao, R. L. Tanguay, Q. Dong and C. Huang, J. Hazard. Mater., 2010, 173, 75–81.
3. A. I. Barrado-Olmedo, R. M. Perez-Pastor and S. Garcia-Alonso, Talanta, 2012, 101, 428–434.
4. A. Ishizaki, K. Saito, N. Hanioka, S. Narimatsu and H. Kataoka, J. Chromatogr. A, 2010, 1217, 5555–5563.
5. D. Lerda, European Union reference laboratory for polycyclic aromatic hydrocarbons, Belgium, 4th edn, 2011, p. 27.
6. P. K. Wong and J. Wang, Environ. Pollut., 2001, 112, 407–415.
7. L. Tavakoli, Y. Yamini, H. Ebrahimzadeh and S. Shariati, J. Chromatogr. A, 2008, 1196, 133–138.
8. Z. G. Shi and H. K. Lee, Anal. Chem., 2010, 82, 1540–1545.
9. H. Wu, X. C. Wang, B. Liu, J. Lu, B. X. Du, L. X. Zhang, J. J. Ji, Q. Y. Yue and B. P. Han, J. Chromatogr. A, 2010, 1217, 2911–2917.
10. L. C. Romanoff, Z. Li, K. J. Young, N. C. Blakely, D. G. Patterson and C. D. Sandau, J. Chromatogr. B: Anal. Technol. Biomed. Life Sci., 2006, 835, 47–54.
11. C. L. Arthur and J. Pawliszyn, Anal. Chem., 1990, 62, 2145–2148.
12. P. Helena and I. K. Locita, Trends Anal. Chem., 1999, 18, 272–282.
13. A. Mehdinia and M. O. Aziz-Zanjani, Trends Anal. Chem., 2013, 51, 13–22.
14. A. Spietelun, L. Marcinkowski, M. de la Guardia and J. Namiésnik, J. Chromatogr. A, 2013, 1321, 1–13.
15. M. A. Jeannot and F. F. Cantwell, Anal. Chem., 1996, 68, 2236–2240.
16. A. Sarafraz-Yazdi and A. Amiri, Trends Anal. Chem., 2010, 29, 1–14.
17. Q. Xiao, B. Hu, C. H. Yu, L. B. Xia and Z. C. Jiang, Talanta, 2006, 69, 848–855.
18. H. Liu and P. K. Dasgupta, Anal. Chem., 1996, 68, 1817–1821.
19. J. M. Kokosa, Trends Anal. Chem., 2015, 71, 194–204.
20. S. Jahan, H. Xie, R. Zhong, J. Yan, H. Xiao, L. Fan and C. Cao, Analyst, 2015, 140, 3193–3200.
21. J. Cai, G. Chen, J. Qiu, R. Jiang, F. Zeng, F. Zhu and G. Ouyang, Talanta, 2016, 146, 375–380.
22. V. Simão, J. Meri, A. N. Dias and E. Carasek, Food Chem., 2016, 196, 292–300.
23. M. Rezaee, Y. Assadi, M. R. M. Hosseini, E. Aghaee, F. Ahmadi and S. Berijani, J. Chromatogr. A, 2006, 1116, 1–9.
24. R. R. Kozani, Y. Assadi, F. Shemirani, M. R. M. Hosseini and M. R. Jamali, Talanta, 2007, 72, 387–393.
25. J. Martín, J. L. Santos, I. Aparicio and E. Alonso, Talanta, 2015, 143, 335–343.
26. M. A. Farajzadeh and M. R. Afshar Mogaddam, Anal. Chim. Acta, 2012, 728, 31–38.
27. B. Barfi, A. Asghari, M. Rajabi and S. Sabzalian, J. Chromatogr. B: Anal. Technol. Biomed. Life Sci., 2015, 998–999, 15–25.
28. M. Shamsipur and J. Hassan, J. Chromatogr. A, 2010, 1217, 4877–4882.
29. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Chem. Commun., 2003, 7, 70–72.
30. P. Dominguez de Maria and Z. Maugeri, Curr. Opin. Chem. Biol., 2011, 15, 220–225.
31. Q. Zhang, K. De Oliveira Vigier, S. Royer and F. Jerome, Chem. Soc. Rev., 2012, 41, 7108–7146.
32. Y. Dai, J. van Spronsen, G. J. Witkamp, R. Verpoorte and Y. H. Choi, Anal. Chim. Acta, 2013, 766, 61–68.
33. http://pubchem.ncbi.nlm.nih.gov/compound/4-chlorophenol, 11/22/2015.
34. W. Guo, Y. Hou, S. Ren, S. Tian and W. Wu, J. Chem. Eng. Data, 2013, 58, 866–872.
35. M. Sun, J. Feng, Y. Bu and C. Luo, J. Chromatogr. A, 2015, 1408, 41–48.
36. W. Liu, J. Qi, L. Yan, Q. Jia and C. Yu, J. Chromatogr. B: Anal. Technol. Biomed. Life Sci., 2011, 879, 3012–3016.
37. J. I. Cacho, N. Campillo, P. Vinas and M. Hernandez-Cordoba, Food Chem., 2016, 190, 324–330.
38. A. R. Ghiasvand and M. Pirdadeh-Beiranvand, Anal. Chim. Acta, 2015, 900, 56–66.

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