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Sensitive determination of phenols from water samples by temperature-controlled ionic liquid dispersive liquid-phase microextraction

Qingxiang Zhou,*ab Yuanyuan Gao,a Junping Xiaoc and Guohong Xied

Received 15th October 2010, Accepted 6th January 2011

DOI: 10.1039/c0ay00619j

This paper established a new determination method for phenols using temperature-controlled ionic

liquid dispersive liquid-phase microextraction prior to high-performance liquid chromatography. In

this experiment, 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6]) was employed as

the extraction solvent for the enrichment of 2-chlorophenol, 2-naphthol, 2,4-dinitrophenol, and

2,4-dichlorophenol. Parameters that may affect the extraction efficiency including the volume of

[C8MIM][PF6], dissoluble temperature, extraction time, sample pH, amount of ethanol, centrifugation

time and salting-out effect have been investigated in detail. Under the optimal conditions, they have

good linear relationships over the concentration range of 1.0-100 ng mL�1 for 2-chlorophenol,

2-naphthol, 2,4-dinitrophenol, and 1.5-150 ng mL�1 for 2,4-dichlorophenol, and excellent detection

sensitivity with limits of detection (LOD, S/N¼ 3) in the range of 0.27–0.68 mg L�1. Intra day and inter

day precisions of the proposed method (RSDs, n ¼ 6) were 2.1–3.7% and 5.1–7.2%, respectively. The

proposed method has been successfully applied to analyze real water samples spiked with two different

concentrations and good spiked recoveries over the range of 85.8–117.0% were obtained. These results

indicated that the proposed method would be competitive in the analysis of phenols in the future.

1. Introduction

Phenolic compounds are important precursors for the manu-

facture of many dyes, drugs, perfumes, insecticides, and surfac-

tants.1 Many investigations had confirmed the presence of

chlorophenols in many ecosystems: surface and ground waters,

bottom sediments, atmospheric air and soils. Possible routes of

human exposure to chlorophenols are inhalation, ingestion and

eye and dermal contact.2 Owing to their high toxicity, persistence

in the environment and potential carcinogenicity, the US Envi-

ronmental Protect Agency (US EPA) and European Community

(EC) have included some phenols, mainly nitrophenols and

chlorophenols in their lists of priority pollutants.3

Analytical procedures have been developed for the separation

and preconcentration of the contaminants due to their low

concentration or complicated matrices in environmental and

biological samples, such as liquid–liquid extraction (LLE),4,5

aSchool of Chemistry and Environmental Sciences, Henan NormalUniversity, Henan Key Laboratory for Environmental pollution Control,Key Laboratory for Yellow River and Huaihe River Water Environmentand Pollution Control, Ministry of Education, Xinxiang, 453007, PRChina. E-mail: zhouqx@cup.edu.cn; zhouqx@henannu.edu.cnbState Laboratory of Petroeum Resource and Prospecting, College ofGeosciences, China University of Petroleum, Beijing Capmus(CUP),Beijing, 102249, ChinacDepartment of Chemistry, University of Science and Technology Beijing,Beijing, 100083, ChinadCollege of Resources and Environment, Henan Institute of Science andTechnology, Xinxiang, 453003, China

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liquid-phase miroextraction (LPME),6,7 headspace liquid-phase

miroextraction (HS-LPME),8 solid-phase extraction (SPE),9–11

solid-phase microextraction (SPME),12–14 headspace solid-phase

microextraction (HS-SPME)15–19 and single-drop micro-

extraction (SDME).20–24

Recently, Assadi and co-workers reported a novel micro-

extraction technique, termed dispersive liquid–liquid micro-

extraction (DLLME).25 It has the advantages of simplicity,

rapidity, low sample volume, low cost, high recovery, and a high

enrichment factor,26 and has been widely used for the pretreat-

ment of organic pollutants and heavy metal pollution.27 In

a DLLME method, it is considerably important to select an

extraction solvent with higher density than water, high extraction

capability of compounds of interest and good chromatographic

behavior.28 Toxic solvents such as chlorobenzene, carbon tetra-

chloride, tetrachloroethylene and carbon disulfide have been

often used as extraction solvents. In order to reduce the effect of

toxic solvent on the environment, environmental friendly

solvents are expected for use.29 Room temperature ionic liquids

(RTILs), known as a new and novel generation of solvents, have

been widely applied in separation and many other fields. They

have many properties include low volatility, chemical and

thermal stability, and good solubility for both organic and

inorganic molecules.30 Moreover, by fine-tuning the structure,

these properties can be designed to match the specific application

requirements. The main reason that made them useful in

analytical chemistry is the negligible vapor pressure of most

RTILs. Ionic liquids have been used in the development of

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DLLME for the enrichment and determination of heavy

metals,31, 32 and our research group has developed a new liquid

phase microextraction technique named as temperature-

controlled ionic liquid dispersive liquid-phase microextraction

based on the similar principle of DLLME using ionic liquids as

the extraction solvents.33

The goal of the present study is to develop a new method for

powerful preconcentration and sensitive detection of four phenols

in water samples using temperature-controlled ionic liquid

dispersive liquid-phase microextraction method. The effects of

various experimental parameters such as the volume of room

temperature ionic liquids, temperature, extraction time, sample

pH, centrifugation time, and salting-out effect were optimized.

2. Experimental

2.1 Instrumentation

A high performance liquid chromatography system, which consisted

of two LC-10 ATvp pumps and an SPD-10 Avp, ultraviolet detector

(Shimadzu, Kyoto, Japan) was used for the analysis and separation.

A reversed-phase SunFire C18 column (150 mm� 4.6 mm, particle

size 5 mm) was used for separation at ambient temperature and

Chromato Solution Light Chemstation for LC system was employed

to acquire and process chromatographic data. The mobile phase was

a mixture of methanol and ultrapure water (containing 1% acetic

acid) (55 : 45, v/v). The mobile phase flow-rate was set at 0.8 mL

min�1, the injection volume and the detection wavelength were set at

20 mL and 275 nm, respectively. An Anke TDL80-2B (Shanghai,

China) centrifuge was used for phase separation.

2.2 Reagents

2,4-Dichlorophenol and 2-chlorophenol (purity, 99%) were

achieved from Acros organics (New Jersey, USA). 2-Naphthol

(Analytical grade) and 2,4-dinitrophenol (Guarantee Reagent)

were obtained from Shanghai Chemical Cooperation (Shanghai,

China). Stock solutions at 500 mgL�1 were prepared by dis-

solving suitable amounts of them each in methanol and stored at

4 �C in the refrigerator. The stock solutions were further diluted

to yield the appropriate working solutions with methanol.

1-Octyl-3-methylimidazolium tetrafluoroborate ([C8MIM][PF6])

was synthesized in our laboratory. HPLC grade methanol and

acetonitrile were obtained from Huaiyin Guoda Chemical Reagent

Co., Ltd. (Huaian, China). Ultrapure water was obtained from

a Milli-Q water purification system (Millipore, Bedford, MA,

USA). The aqueous solutions were prepared daily by diluting the

standard mixture with ultra-pure water. All the other solvents were

analytical grade unless stated. 3 mol L�1 of sodium hydroxide were

used for adjusting the pH value of the water samples. All glassware

used in the experiments was cleaned with ultrapure water, then

soaked in 6 mol L�1 nitric acid for 24 h and rinsed five times with

ultrapure water before use.

2.3 Temperature-controlled ionic liquid dispersive liquid-phase

microextraction

In the temperature-controlled ionic liquid dispersive liquid-phase

microextraction procedure, 10 mL ultra-pure water or sample

was added. This solution was spiked with a concentration of

654 | Anal. Methods, 2011, 3, 653–658

20 mg L�1 for 2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol,

and 40 mg L�1 for 2,4-dichlorophenol. 50 mL 1-octyl-3-methyl-

imidazolium hexafluorophosphate [C8MIM][PF6],

60 mL 1 mol L�1 hydrochloric acid and 700 mL ethanol were

added into a 10 mL conical tube. Then the conical tubes were

heated in the water bath with the temperature controlled at

60 �C. [C8MIM][PF6] was then completely dissolved in the

aqueous solution and mixed with the solution entirely. The

analytes would be transferred into the IL phase based on the

higher solubility of analytes in IL. The tube was thereafter cooled

with icewater and a cloudy solution was formed. The tube was

kept for 20 min to enhance the migration of phenols from the

sample solution into the tiny droplets of [C8MIM][PF6]. Then the

water–ethanol–[C8MIM][PF6] mixture was centrifuged for

20 min at 4000 rpm. The upper aqueous phase was removed with

a syringe, and the residue was dissolved in 200 mL methanol and

20 mL was injected into the HPLC system for analysis.

2.4 Water samples

Four real water samples were collected for validation of the

proposed method. Melted water was obtained from Henan

Normal University in Xinxiang City, Henan province. Lake

waters were collected from Donghu Lake, Xinxiang City, Henan

province, China and Shouxihu Lake, Yangzhou City, Jiangsu

province, China. Wastewater sample was taken from the exit of

a factory, Xinxiang city, Henan province, China. Before use, all

the water samples were filtered through 0.45 mm micro-pore

membranes and stored in brown glass containers at the temper-

ature of 4 �C.

3. Results and discussion

The extraction efficiency of temperature-controlled ionic liquid

dispersive liquid-phase microextraction procedure depends on

some important experimental parameters, such as the amount of

IL, temperature, extraction time, sample pH, the addition of

organic solvent, centrifugation time, and ionic strength, and they

were investigated in detail. In order to calculate the enrichment

factors and recoveries, eqn (1) and (2) were used.

EF ¼ Csed/C0 (1)

EF, Csed and C0 are the enrichment factor, the concentration

of analytes in the sedimented ionic liquid phase and the initial

concentration of analytes in the aqueous samples, respectively.

R% ¼ (CsedVsed)/(C0Vaq) � 100 ¼ EF � Vsed/Vaq � 100 (2)

where R%, Vsed, Vaq, are the extraction recovery, the volume of

the sedimented phase and the volume of the aqueous sample,

respectively.

3.1 Effect of the volume of ionic liquid

The volume of extraction solvent was a crucial parameter which

seriously had an important impact on the extraction perfor-

mance in liquid phase microextraction. Theoretically, a larger

volume of exaction solvent resulted in a higher extraction effi-

ciency. Solutions containing different volumes (40, 45, 50, 55 and

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60 mL) of [C8MIM][PF6] were subjected to the same temperature-

controlled ionic liquid dispersive liquid-phase microextraction

procedure, and the results are shown in Fig. 1. The results indi-

cated that the peak areas of phenols increased along with the

added volume of [C8MIM][PF6] from 40 to 50 mL and reached

the maximum at 50 mL. Maybe the amount of ionic liquid

exceeded the target dispersed amount about 50 mL, and excess IL

was adsorbed on to the wall of the tube, and led to a few loss of

analytes. Meanwhile, the more IL was used, the more sedimented

IL and the concentrations of analytes would be diluted. There-

fore, 50 mL ionic liquid was employed for further use.

Fig. 2 Effect of temperature. Conditions: volume of [C8MIM][PF6],

50 mL; sample volume, 10 mL; spiked concentration, 20 mg L�1 for

2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol, 30 mg L�1 for 2,4-

dichlorophenol; sample pH, 6; extraction time, 30 min; centrifugation

time, 20 min; (-) 2-chlorophenol; (C) 2-naphthol; (:) 2,4-dinitro-

phenol; (;) 2,4-dichlorophenol.

3.2 Effect of dissolving temperature

In this experiment, temperature is the driving force to make

[C8MIM][PF6] dispersed into the sample solution completely.

Further the analytes have the best chance and largest contact

area and migrate into the IL phase. A series of experiments were

designed for the optimization of the effect of temperature

ranging from 40 to 80 �C. From Fig. 2, we can see that 60 �C was

the optimal temperaure for obtaining the best extraction

performance. The reason was that the mass transfer coefficients

were enhanced along with the temperature, but at a higher

temperature, the extraction performance would decrease due to

the volatilization. So 60 �C was used in the further experiments.

3.3 Effect of extraction time

As is known, extraction time is the most vital factor in most

extraction procedures. This procedure is a key step to increase

the transfer of the target compounds from aqueous phase to ionic

liquid phase and then reach equilibrium in the end. In this work,

extraction time means the time from the moment that the solu-

tion was put into an icewater bath to centrifugation. The

experimental data demonstrated that the extraction equilibrium

was obtained at 20 min, and a longer extraction time would

reduce the enrichment efficiency of the analytes due to the

adsorption of ionic liquid onto the tube wall which would not be

Fig. 1 Effect of ionic liquid volume. Conditions: ionic liquid,

[C8MIM][PF6]; sample volume, 10 mL; spiked concentration, 20 mg L�1

for 2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol, 30 mg L�1 for 2,4-

dichlorophenol; sample pH,6; temperature, 60 �C; extraction time,

30 min; centrifugation time, 20 min; (-) 2-chlorophenol; (C) 2-naph-

thol; (:) 2,4-dinitrophenol; (;) 2,4-dichlorophenol.

This journal is ª The Royal Society of Chemistry 2011

sedimented to the bottom of the tube. Hence, in the rest of the

experiments, the extraction time was set at 20 min.

3.4 Effect of sample pH

The sample pH is also an important factor in the enrichment

process, and which can affect the extraction efficiencies of ana-

lytes. The sample pH was investigated in the range of pH 1–9.

The results are exhibited in Fig. 3. From Fig. 3, it was found that

the peak areas of all the phenols increased gradually from pH 1–3

and deceased from pH 4–6. Only 2-naphthol was detected when

the sample pH was at pH 8 and pH 10. The reason is that phenols

are weak acidic compounds and exist as a molecular form at

acidic conditions, and an ionic form at strong alkali conditions.

Fig. 3 Effect of sample pH. Conditions: volume of [C8MIM][PF6],

50 mL; sample volume, 10 mL; spiked concentration, 20 mg L�1 for

2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol, 30 mg L�1 for

2,4-dichlorophenol; extraction time, 20 min; centrifugation time, 20 min;

(-) 2-chlorophenol; (C) 2-naphthol; (:) 2,4-dinitrophenol; (;) 2,4-

dichlorophenol.

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However, phenols will also exist as addition of a [H+] when the

acidity of sample solution was too low, which was helpless to

the enrichment process. Due to these facts, pH 3 was used in the

following experiments.

Fig. 5 Effect of the ethanol addition. Conditions: volume of

[C8MIM][PF6], 50 mL; sample volume, 10 mL; spiked concentration,

20 mg L�1 for 2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol,

30 mg L�1 for 2,4-dichlorophenol; methanol; temperature, 60 �C;

extraction time, 20 min; centrifugation time, 20 min; sample pH, 3; (-) 2-

chlorophenol; (C) 2-naphthol; (:) 2,4-dinitrophenol; (;) 2,4-dichlor-

ophenol.

3.5 Effect of organic solvents addition

Ref. 34 and 35 indicated that organic solvents could help enhance

the extraction efficiency of analytes, and which was mainly due to

the reduction of the adsorption onto the tube walls of analytes. A

series of experiments were performed to investigate the effect of

addition of organic solvents such as methanol, acetonitrile,

ethanol and acetone. The results are demonstrated in Fig. 4.

From Fig. 4, we can see that the largest peak areas were obtained

with adding ethanol. This is identical with the principle of like

dissolving like. The amount of ethanol was investigated over the

range of 0 � 11% (v/v). The results are shown in Fig. 5. It was

found that the largest peak areas of phenols were obtained at the

concentration of 7%. Too large an amount of ethanol would

reduce the IL phase because it could dissolve IL and be soluble in

water in any proportion. Therefore, 7% ethanol (v/v) was

adopted.

3.6 Effect of centrifugation time

The centrifugation process plays an important role in the

DLLME method, because it accelerates phase separation.27 The

centrifugation time was optimized in the range of 5 � 25 min at

4000 rpm. The peak areas of the analytes increased along with

the centrifugation time over the range of 5 � 20 min, when the

time was over 20 min, the peak areas decreased. With the increase

of time, more [C8MIM][PF6] could be completely sedimented

and which lead to the increase of the peak areas of the analytes.

However, a much longer centrifuging time would lead to heat

generation, which would dissolve of parts of [C8MIM][PF6] and

volatilization of some of the analytes. Therefore, 20 min was

selected as the centrifuging time.

Fig. 4 Effect of adding of organic solvents. Conditions: volume of

[C8MIM][PF6], 50 mL; sample volume, 10 mL; spiked concentration,

20 mg L�1 for 2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol,

30 mg L�1 for 2,4-dichlorophenol; extraction time, 20 min; centrifugation

time, 20 min; concentration of acetone/methanol–ethanol–acetonitrile,

7%; sample pH, 3. (B) 2-chlorophenol; (C) 2-naphthol; (D) 2, 4-dinitro-

phenol; (E) 2, 4-dichlorophenol.

656 | Anal. Methods, 2011, 3, 653–658

3.7 Salting-out effect

In general, addition of a certain amount of salt can decrease the

solubility of analytes in the aqueous phase and enhance their

partitioning into the organic phase. However, much more salt

can change the physical properties of the Nernst diffusion film,

which reduced the rate of diffusion of the analytes into the

microdrop.24 In order to investigate the effect of salt addition,

a series of experiments over the NaCl concentration range of 5 �25% (w/v) were performed while keeping the other parameters

constant. The experimental data are shown in Fig. 6. It can be

seen that the peak area increased with the increase of the amount

of NaCl and reached its largest at 15% (w/v), and then decreased

when the NaCl concentration was over 15% (w/v). This result

Fig. 6 Salting-out effect. Conditions: volume of [C8MIM] [PF6], 50 mL;

sample volume, 10 mL; spiked concentration, 20 mg L�1 for 2-chlor-

ophenol, 2-naphthol, and 2,4-dinitrophenol, 30 mg L�1 for 2,4-dichlor-

ophenol; temperature, 60 �C; extraction time, 20 min; centrifugation

time, 20 min; ethanol addition, 7%; sample pH, 3; (-) 2-chlorophenol;

(C) 2-naphthol; (:) 2,4-dinitrophenol; (;) 2,4-dichlorophenol.

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was very like the results in the literature.7,36 So 15% NaCl was

used for achieving a better extraction performance.

3.8 Analytical performance

As far as the analytical method is concerned, linear ranges,

precisions and detection limits are very important. Under

optimal experimental conditions, a series of experiments were

performed for investigating such parameters. The experimental

results showed that they had good linear relationships over the

concentration ranges of 1.0 � 100 mg L�1 for 2-chlorophenol

2,4-dinitrophenol, and 2-naphthol, and 1.5 � 150 mg L�1 for 2,4-

dichlorophenol. The precisions were obtained by six reduplicate

extractions. The experimental results are summarized in Table 1.

The results indicated that this method was excellent with good

linearity with correlation coefficients over the range of 0.9995 �0.9998, the limits of detection (LODs), were in the range of 0.27

� 0.68 mg L�1 (S/N ¼ 3) and the intra day precisions were in the

range of 2.1 � 3.7% and inter day precisions were in the range of

5.1 � 7.2% (RSD, n ¼ 6). These merits indicated that the

proposed method would be a creative development in analytical

and environmental fields. Enrichment factors are important

parameters for the extraction and preconcentration method, and

which detemines the sensitivity and merits of the developed

method. The EF of the proposed method was calculated and

Table 1 Linear ranges, precisions, detection limits for the enrichment ofmicroextraction

Compounds

Linearrange(mg L�1) R2

Precision(RSD%, n ¼intra day)

2-chlorophenol 1–100 0.9998 3.72-naphthol 1–100 0.9998 2.12,4-dinitrophenol 1–100 0.9997 3.12,4-

dichlorophenol1.5–150 0.9995 2.8

a EF ¼ Csed/C0.

Table 2 Spiked recoveries obtained in samples by the proposed methoda,b

Compounds Concentrations (mg L�1) Melted snow water Do

2-chlorophenol 0 N.D. N.5 105.7 � 1.6 93.10 102.3 � 3.4 94.100 88.6 � 5.1 83.

2-naphthol 0 N.D. N.5 98.6 � 2.2 85.10 109.8 � 1.5 93.100 86.7 � 6.5 84.

2,4-dinitrophenol 0 N.D. N.5 111.5 � 2.6 97.10 110.6 � 1.9 100100 87.1 � 7.2 84.

2,4-dichlorophenol 0 N.D. N.7.5 109.7 � 2.8 99.15 112.0 � 2.4 103150 87.5 � 5.7 83.

a N.D.: not detected. b Spiked recovery, mean � standard deviation (%).

This journal is ª The Royal Society of Chemistry 2011

listed in Table 1. It can seen that the EFs were very good and the

sensitivity of proposed method was satisfied.

3.9 Real water sample analysis

The present method was evaluated by determining the concen-

tration of phenols in four real water samples. These samples were

directly analyzed and no target analytes were found. In order to

validate the applicability of the proposed method, the samples

were spiked with 5, 10, and 100 mg L�1 for 2-chlorophenol,

2-naphthol, and 2,4-dinitrophenol, 7.5, 15, 150 mg L�1 for 2,4-

dichlorophenol, respectively. The results are exhibited in Table 2

and the typical chromatogram of spiked real water sample is

shown in Fig. 7. The spiked recoveries were satisfied in the range

of 85.8 � 117%. Our research group had developed a headspace

liquid-phase microextraction based on a bell mouthed device

with a 3 mm PTFE tube.37 In the extraction procedure, 10 mL

[C4MIM][PF6] was suspended for extraction, a 10 mL aliquot of

the sample solution containing 35% NaCl was placed in a 45 mL

vial immersed in the recirculating water bath at a temperature of

80 �C, the magnetic stirrer was turned on at 1000 rpm, and

extraction time was 40 min. The results demonstrated that the

limits of detection for 2-nitrophenol, 4-chlorophenol, 2-naph-

thol, and 2,4-dichlorophenol were 0.5, 0.5, 0.3 and 0.3 mg L�1 and

the pricisions were in the range of 5.4–8.9% (RSD, n ¼ 6). In this

phenols by temperature controlled ionic liquid dispersive liquid-phase

6, Precision(RSD%,n ¼ 6, inter day)

Limits ofdetection (mg L�1) EFa

5.1 0.36 3396.2 0.27 3346.3 0.49 3577.2 0.68 371

nghu Lake water Shouxihu lake water Water from Xinfei factory

D. N.D. N.D.5 � 5.9 97.3 � 3.3 108.5 � 1.58 � 2.9 104.3 � 4.3 101.7 � 4.12 � 6.2 87.2 � 6.5 86.2 � 6.8D. N.D. N.D.8 � 5.6 91.6 � 1.1 100.9 � 6.67 � 2.8 103.8 � 2.8 101.4 � 3.15 � 5.9 84.9 � 5.4 83.9 � 7.5D. N.D. N.D.9 � 4.3 109.5 � 5.2 114.6 � 6.0.1 � 3.7 99.42 � 1.2 102.4 � 3.47 � 4.8 84.2 � 3.9 85.1 � 6.7D. N.D. N.D.7 � 8.1 109.4 � 3.4 117.0 � 7.5.9 � 3.2 107.5 � 2.7 109.9 � 3.48 � 5.4 87.8 � 6.8 87.4 � 7.8

Anal. Methods, 2011, 3, 653–658 | 657

Fig. 7 Typical water sample: Melted snow water: A: Blank; B: 5 mg L�1

for 2-chlorophenol, 2-naphthol, and 2,4-dinitrophenol, 7.5 mg L�1 for 2,4-

dichlorophenol; C: 10 mg L�1 for 2-chlorophenol, 2-naphthol, and 2,4-

dinitrophenol, 15 mg L�1 for 2,4-dichlorophenol; D: 100 mg L�1 for 2-

chlorophenol, 2-naphthol, and 2,4-dinitrophenol, 150 mg L�1 for 2,4-

dichlorophenol.

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procedure, the ionic liquid phase was completely injected into

HPLC for analysis, and the extraction time was obviously longer

than that of proposed method. However, the proposed method

was better than this method and provided a comparatively low

detection limit (0.27–0.68 mg L�1).

4. Conclusions

In this study, a new convenient, sensitive, simple determination

method based on temperature-controlled ionic liquid dispersive

liquid-phase microextraction prior to high performance liquid

phase chromatography was developed. The proposed method

has satisfied LODs and precisions which were in the range of

0.27 � 0.68 mg L�1, precisions were in the range of 2.1 � 3.7%

(intra day, RSD, n ¼ 6) and 5.1 � 7.2% (inter day, RSD, n ¼ 6).

The proposed method was also applied for the analysis of

phenols in real water samples and the spiked recoveries were in

the range of 85.8 � 117%. All these results indicated that the

proposed method had advantages such as good sensitivity,

simplicity, easy to operate, limited chance of exposure to the

toxic solvent, high enrichment factor which could be tuned by

changing the volume of ILs in a relatively wide range, etc.,

however, the toxicity of ionic liquid has also been studied and

paid more attention. In order to give reasonable results, the used

ionic liquid should be regenerated and reused for reducing the

possible secondary pollution. In other words, the developed

method was a good alternative and would be very competitive in

the analysis of phenols in the future.

Acknowledgements

This work was financially supported by the National Natural

Science Foundation of China (20877022), the Personal Innova-

tion Foundation of Universities in Henan Province ([2005]126),

658 | Anal. Methods, 2011, 3, 653–658

the Natural Science Foundation of Henan Province

(No. 082102350022), and the funds from the Henan Key Labo-

ratory for environmental pollution control.

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