ISSN 1061-9348, Journal of Analytical Chemistry, 2009, Vol. 64, No. 10, pp. 1017–1022. © Pleiades Publishing, Ltd., 2009.

1017

1

Conventional liquid–liquid extraction (

LLE

) isprobably the most widely used method for preconcen-tration of organic pollutants from water samples [1]. Ingeneral, LLE requires the use of large amounts of toxicorganic solvents and is also a time-consuming proce-dure. In recent years, liquid phase microextraction(

LPME

) techniques have attracted great attentions, asthey are similar to LLE but at a miniaturized scale and,therefore, require much less solvent. For example, sin-gle drop liquid phase microextraction (

SDLPME

) onlyrequires one drop of organic solvent and is commonlyconducted in direct-immersion or headspace mode [2,3]. Liquid–liquid–liquid microextraction (

LLLME

) orsolvent microextraction with backextraction supportedby a liquid membrane [1, 4] and dispersive liquid–liq-uid microextraction (

DLLME

) are based on a ternarycomponent solvent system. DLLME consists of twosteps: (1) injection of an appropriate mixture of extrac-tion and disperser solvent into an aqueous sample con-taining analytes, and (2) centrifugation of the cloudysolution followed by instrumental analysis of the ana-lytes in sediment [5–7]. Although SDLPME andLLLME have proven to be powerful alternatives to tra-ditional extraction techniques, like LLE, they are alsotime-consuming processes. DLLME is a simple andrapid microextraction method, but it also involves theuse of volatile organic solvents.

Ionic liquids are gaining worldwide academic andindustrial attention as replacements for volatile organicsolvents in extraction [3, 8, 9], chemical reactions [10,

1

The article is published in the original.

11] and electrochemistry [12, 13]. They have many ofthe properties of conventional organic solvents, such asexcellent solvation qualities and a wide temperaturerange over which they are liquids. One property they donot share with conventional organic solvents is unde-tectable vapor pressure and high thermal stability [14,15]. Therefore, room temperature ionic liquids areregarded to be novel environmentally benign solvents.

The objective of this study is to investigate thepotentiality of IL in sample pretreatment with theDLLME technique, using 4-nitrophenol

(

4-

NP

),2

,4-dimethylphenol (

2,4-

DMP

), bisphenol A (

BPA

)and 2-naphthol (

2-

NOL

) as model compounds. Unlikethe DLLME technique described in literature, theIL-DLLME method demonstrated in the present workis based on a binary component solvent system (waterand IL, no disperser solvent is used), not a ternary com-ponent solvent system. Parameters affecting the extrac-tion efficiency (pH values of the aqueous phase, con-centration of sodium chloride in aqueous solution andextraction time) have been investigated. In addition,this new technique has been evaluated for quantitativeanalysis, and the application to real water samples anal-ysis has been illustrated. The entire pretreatment pro-cess is totally free of volatile organic solvents and isthus environmentally friendly.

EXPERIMENTAL

Reagents, standards and samples.

HPLC-gradeacetonitrile purchased from Tedia (USA), 4-nitrophe-nol (4-NP, Shanghai Chem. Co., A.R.), 2,4-dimeth-

ARTICLES

A Ionic Liquid for Dispersive Liquid–Liquid Microextraction of Phenols

1

Y. C. Fan

a,b

, M. L. Chen

c

, C. Shen-Tu

c

, and Y. Zhu

a

a

Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou, 310028 China

b

Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, 454003 China

c

Biology and Environment Engineering College of Zhejiang Shuren University Hangzhou, 310015 China

Received April, 16, 2008; in final form March, 16, 2009

Abstract

—In the present study, a new solvent-free mode of liquid phase microextraction termed ionic liquiddispersive liquid–liquid microextraction (

IL-DLLME

) was developed. Four phenols were used as model com-pounds in the development and evaluation of the procedure. In this method, 50

µ

L of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF

6

]) and 1.5 mL of sample aqueous solution were placedin a 2.2-mL glass test tube and mixed by aspirating and rapidly injecting by a syringe. This procedure produceda cloudy solution. In this process, phenols in the water sample were extracted into the IL phase. After centri-fuging, the fine droplets of IL sedimented to the bottom of the glass test tube. The settled phase was injectedinto the high performance liquid chromatograph (HPLC) for separation and detection of phenols. Some param-eters that might affect the extraction efficiency were optimized. The main advantages of the proposed methodare high speed, high recovery, good repeatability and environmental friendliness.

DOI:

10.1134/S1061934809100074

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JOURNAL OF ANALYTICAL CHEMISTRY

Vol. 64

No. 10

2009

FAN et al.

ylphenol (2,4-DMP, Datang Chem. Co. Ltd., Nanjing,China, 99%), bisphenol A (BPA, Sinopharm GroupChemical Reagent Co. Ltd., Shanghai, China, CP),2-naphthol (2-NOL, Sinopharm Group ChemicalReagent Co. Ltd., Shanghai, China, AR), 1-chlorobu-tane (Sanyou Chem. Co., Jiangsu, China, 99.5%) and1-methylimidazole (Kaile Chem. Co., Zhejiang, China,99%) were used without further purification. All otherchemicals were analytical grade reagents and Milli-Q(Millipore, Molsheim, France) water was used through-out.

Ionic liquid 1-butyl-3-methylimidazolium hexafluo-rophosphate ([Bmim

][

PF

6

]) was prepared according tothe procedure described in literature [15, 16].

Standard stock solutions (1000 mg/L) of these phe-nols were prepared in acetonitrile and stored in refrig-erator at

4°

C. Working solutions were prepared daily byappropriate dilution of the stock solutions with water.

In this work, three real water samples including tapwater, river water and waste water were used for evalu-ation. Tap water sample was collected from water tap inour laboratory, river water sample was taken from theQiantang River (Hangzhou, China) and the wastewatersample was collected from a paper mill (Gongshu Dis-trict, Hangzhou, China). Before use, all the environ-mental water samples were filtered through

0.45

µ

mmicropore membranes and stored in refrigerator at

4°

C.

Instrumentation and conditions.

The analyses ofphenols were performed by an Agilent 1200 systemincluding a Quat Pump and a VWD detector set at285 nm. Data acquisition and processing were accom-plished with an Agilent ChemStation program (Agilent,USA), and a Dionex Acclaim

TM

120

C

18

column(150 mm

×

4.6

mm, particle size

5 µ

m) was used for theseparation of phenols extracted into the IL.

The mobile phase was a mixture of acetonitrile andwater delivered at a flow rate of 1.0 mL/min. The gradi-ent program was as follows: keeping constant 30% ace-

tonitrile during 0–15 min, then increasing to 45% ace-tonitrile in 15–20 min and keeping constant until24 min, thereafter restored to 30% in 1 min followed bya 3 min re-equilibration time.

Ionic

liquid

dispersive

liquid

-

liquid

microextraction

(

IL

-

DLLME

)

procedure.

Briefly, a 1.5 mL portion ofsample solution and

50

µ

L of [Bmim

][

PF

6

] were placedin a 2.2-mL glass test tube with conical bottom. 1 mLof the above mixture was aspirated into a 1-mL syringe(Agilent), and then the syringe plunger was depressedrapidly to inject the contents into the remaining solu-tion. The above procedure was repeated two times inorder to entirely disperse IL into the aqueous phase,thus inducing the formation of a cloudy solution. Themixture was subsequently centrifuged for 2 min at3000 rpm, which caused the dispersed fine droplets ofIL to sediment to the bottom of test tube (about

20

µ

L).The

5.0

µ

L of sedimented extraction phase was col-lected and injected into the HPLC system.

RESULTS AND DISCUSSION

Salt addition.

In traditional liquid–liquid extrac-tion, the addition of salt often increases the extractionefficiency due to the salting-out effect [17, 18]. In thiswork, the optimization of salt concentration was con-ducted by constantly using

50

µ

L of IL, while varyingthe concentration of sodium chloride (NaCl, 0–20%,w/v) in 1.5 mL of phosphate buffer solution (pH 6)spiked with

400

µ

g

/

L of phenols. The results, shown inFig. 1, indicate that increasing concentration of NaClslightly decrease the extraction efficiency of 2-NOLand BPA, but has little effect on the other analytes. Thereason for this phenomenon is not clear. One possibleinterpretation is that the IL-DLLE does not attain ther-mal equilibrium. The mechanism needs further investi-gation.

0

150

225

75

300

375

5 10 15 20

Peak

are

a

NaCl, % w/v

Fig. 1.

Effect of NaCl concentration on extraction efficiency. Extraction conditions: 1.5 mL sample solution spiked with

400

µ

g

/

Lfor each analyte at pH 6;

50

µ

L IL.

(

�

) 4

-NP,

(

�

) 2

,4-DMP,

(

�

) 2

-NOL, (

�

) BPA.

JOURNAL OF ANALYTICAL CHEMISTRY

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No. 10

2009

A IONIC LIQUID FOR DISPERSIVE LIQUID–LIQUID MICROEXTRACTION 1019

Based on these results, no NaCl was added in thesubsequent experiments.

Effect of pH value of aqueous phase.

Since the tar-get compounds are weak acids with low ionization con-stants (the p

K

a

of 4-NP, 2,4-DMP, 2-NOL and BPA inwater are 7.23, 10.58, 9.57 [19] and 9.59–11.3 [20],respectively), the extraction efficiency depends on thesample pH. Therefore, the effect of pH variation onextraction efficiency was investigated. This was rangedfrom pH 3 to 8 with 20 mM phosphate buffer solution.As can be seen from Fig. 2, better extraction efficiencywas obtained for all tested phenols at pH 3–6. It wasthus deduced that pH 6 is suitable for extraction.

Effect of volume of IL.

In order to evaluate theeffect of extraction solvent volume on extraction effi-ciency, additional experiments were performed usingdifferent volumes of IL (

40–80

µ

L). Figure 3 demon-strates that the largest analytical response was obtainedat

50

µ

L of IL for all the phenols studied. Furtherincrease in IL volume resulted in a decrease in peakarea. This phenomenon can be interpreted by two suc-cessive processes which occurred during extraction. Atthe beginning, more IL was dissolved into the samplesolution, leaving less residual IL to be injected into theHPLC system for analysis. When the IL volume is fur-ther increased, the extraction phase ratio (defined as theratio of aqueous to IL phase volume) decreased. As the

3 4

Peak

are

a

5 6 7 8

50

100

150

200

250

300

350

pH

0

Fig. 2.

Effect of pH on the peak area of phenols. Extractionconditions: 1.5 mL of sample solution spiked with

400

µ

g

/

Lfor each analyte;

50

µ

L IL.

(

�

) 4

-NP,

(

�

) 2

,4-DMP,

(

�

) 2

-NOL, (

�

) BPA.

Peak

are

a

50

100

150

200

250

300

350

40 50 60 70 80

Volume of IL,

µ

L

Fig. 3.

Effect of IL volume on the peak area of phenols.Extraction conditions: 1.5 mL of sample solution spikedwith

400

µ

g

/

L for each analyte, pH of aqueous solution,

6.(

�

) 4

-NP,

(

�

) 2

,4-DMP,

(

�

) 2

-NOL, (

�

) BPA.

Peak

are

a

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30Extraction time, min

Fig. 4.

Effect of extraction time on the peak area of phenols. Extraction conditions: 1.5 mL of sample solution containing spikedwith

400

µ

g/L for each analyte, 50 µL IL, pH of aqueous solution, 6. (�) 4-NP, (�) 2,4-DMP, (�) 2-NOL, (�) BPA.

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JOURNAL OF ANALYTICAL CHEMISTRY Vol. 64 No. 10 2009

FAN et al.

preconcentration factor is proportional to the phaseratio, 50 µL was chosen as the optimal IL volume.

Effect of extraction and centrifuging time.Extraction time is one of the most important factors inmost of the extraction procedures, because mass trans-fer is a time dependent process. In IL-DLLME, extrac-tion time is defined as the interval between the forma-tion of a cloudy solution and before centrifugation.Extraction time was varied within 0–30 min. Asdepicted in Fig. 4, the extraction efficiency did notchange with variations in extraction time. This impliesthat the area of contact of the solvent (IL) and aqueousphase is extremely large, which permits the rapid trans-fer of analytes from aqueous phase to the extractionphase. This demonstrates one of the most importantadvantages of the IL-DLLE technique.

The effect of centrifuging time was also investi-gated. Experiments indicated that centrifuging time hadno significant influence on extraction. It was found thatcentrifugation for 2 min at 3000 rpm is sufficient for thesedimentation of IL. A comparison of the extractiontime of phenols from aqueous samples using IL-DLLME, solid phase microextraction (SPME) andionic liquid based headspace liquid phase microextrac-tion (IL-HSLPME) is summarized in Table 1.

Analytical performance. Under the above selectedconditions, some parameters such as linearity, precision(RSD, n = 5) and limits of detection (LOD) for the fourphenols were obtained. Each analyte exhibited goodlinearity with a correlation coefficient of R2 ≥ 0.9987.The reliability was studied by five repetitions usingaqueous standard solution with 100 µg/L of each ana-lyte. In each case, the relative standard deviations(RSD) were not greater than 4.8%. The limits of detec-tion (LOD), estimated based on signal-to-noise ratio of3 (S/N = 3), were in the range of 0.68–10 µg/L. Theseparameters are summarized in Table 2.

Real water samples. To test the applicability andaccuracy of the proposed method in real samples anal-ysis, different water samples including tap water, riverwater and waste water were analyzed and the recoverieswere determined at a spiked concentration of 100 µg/Lfor each analyte. The results indicate that the contentsof the four phenols in the three samples were belowtheir detection limits. All recoveries were in the rangeof 94.9–108.2% (shown in Table 2). The typical chro-matograms are shown in Fig. 5.

In comparison to other methods, ionic liquid disper-sive liquid–liquid microextraction (IL-DLLME) pro-vides high recovery of compounds within a short periodof time. The method was applied to the extraction of

Table 1. Comparison of proposed IL-DLLME withIL-HSLPME and SPME for extraction of phenols fromwater samples

Methods Extraction time, min

Sample volume, mL Reference

IL-HSLPME 40 20 [21]

SPME 40 20 [22]

IL-DLLME 2* 1.5 Proposed method

* Centrifugation time.

Table 2. Performance of IL-DLLME

Analytes Linear range, µg/L

Correlation coefficient (R2) LOD, µg/L RSD,

% (n = 5)

Recovery, %

tap water river water waste water

4-NP 20–400 0.9988 1.25 3.4 108.2 (7.3)* 104.5 (6.8) 104.1 (9.7)

2,4-DMP 20–400 0.9987 10 4.8 101.1 (5.2) 101.8 (2.3) 97.8 (7.4)

2-NOL 4–400 0.9989 0.68 3.7 100.4 (5.6) 98.3 (6.4) 94.9 (4.2)

BPA 4–400 0.9990 0.85 1.9 101.5 (4.7) 97.8 (5.7) 95.2 (5.0)

* RSD values, % (n = 3).

JOURNAL OF ANALYTICAL CHEMISTRY Vol. 64 No. 10 2009

A IONIC LIQUID FOR DISPERSIVE LIQUID–LIQUID MICROEXTRACTION 1021

phenols from river, tap and waste waters and showed agood recovery. Comparison of this new method withother extraction methods such as LPME and SPMEshows that DLLME is simple and rapid.

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3 6 9 12 15 18 21 240–2

0

2

4

6

Abs

orba

nce,

mA

U

Time, min

3 6 9 12 15 18 21 240–2

0

2

4

6

Abs

orba

nce,

mA

U

Time, min

(‡)

(b)

1

2

3

4

Fig. 5. Typical chromatograms of the four phenols. (a) river water sample spiked with phenols standard (100 µg/L), (b) river waterblank. Peaks identified as: (1) 4-NP, (2) 2,4-DMP, (3) 2-NOL, (4) BPA.

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