ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus...

6
2514 J. Sep. Sci. 2012, 35, 2514–2519 Lijin Zhang 1,2 Fang Chen 2 Shaowen Liu 3 Biyu Chen 1 Canping Pan 3 1 Department of Food Science, Tianjin Agricultural University, Tianjin, China 2 College of Food Science & Nutritional Engineering, China Agricultural University, Beijing, China 3 College of Sciences, China Agricultural University, Beijing, China Received December 6, 2011 Revised February 18, 2012 Accepted February 20, 2012 Short Communication Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear An ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction (DLLME) was developed for the analysis of trace amounts of six organophosphorus pesticides (OPPs) in apple and pear coupled with high-performance liquid chromatography. During the DLLME, the effect of some experimental factors including extraction solvent and its volume, disper- sion solvent and its volume, vortex time, salt addition, and pH on the extraction procedure were investigated. Under the chosen extraction conditions, the analytes were enriched more than 300-fold and the limits of detections were greatly dropped to 0.061–0.73 g/kg. The linearity relationship was observed in the range of 2–100 g/kg with the correlation coef- ficients (R 2 ) ranging from 0.9967 to 0.9983. The relative standard deviations varied from 2.3 to 5.7% (n = 6). Mean recovery values of the OPPs were in the range of 69.8–109.1% with a relative standard deviation lower than 7.0%. Based on these above, it could be con- cluded that 1-octyl-3-methylimidazolium hexafluorophosphate ([C 8 MIM][PF 6 ]) was a good extraction solvent and the proposed [C 8 MIM][PF 6 ]-based vortex-assisted DLLME method was suitable for the effective extraction of the OPPs in apple and pear. Keywords: Dispersive liquid–liquid microextraction / High-performance liquid chromatography / Ionic liquid / Organophosphorus pesticides DOI 10.1002/jssc.201101060 1 Introduction Due to the low price and high efficiency, organophosphorus pesticides (OPPs) have been believed to own the largest mar- ket share in the agrochemical compounds. During the wide use of OPPs in the agricultural practices, some of them trans- fer to the foods to form the OPPs residues. As these OPPs residues have been proved to be acutely toxic to humans and organism [1], it is important to carry out the monitoring of these OPPs residues for the possible risks to human health. However, the OPPs residues always remain at trace or ultra- trace level, which are too low to be detected for the common gas chromatography (GC) or high-performance liquid chro- matography (HPLC). Therefore, it is necessary to develop some sample preparation methods for the extraction and en- richment of these OPPs residues. Correspondence: Prof. Canping Pan, College of Sciences, China Agricultural University, No.2 Yuanmingyuan West Road, Haidian District, Beijing 100193, China. E-mail: [email protected] Fax: 86-10-6273-3620 Abbreviations: [C 6 MIM][PF 6 ], 1-hexyl-3-methylimidazolium hexafluorophosphate; [C 8 MIM][PF 6 ], 1-octyl-3- methylimid- azolium hexafluorophosphate; LPME, liquid-phase microex- traction; LLE, liquid–liquid extraction; OPP, organophospho- rus pesticide Sample preparation plays an important role in the field of pesticide residues analysis [2]. Liquid–liquid extraction (LLE) may be the oldest and wildly used extraction method for the OPPs residues analysis [3]. However, it is laborious, time- consuming, and solvent-consuming. With the miniaturized, simplified, and automated demand of sample preparation methods, LLE has been developed into the liquid-phase mi- croextraction (LPME). However, single-drop microextraction, one kind of LPME, is difficult to practice because of the in- stability of extraction solvents shaped in a single drop or pro- tected in porous hollow fiber under stirring condition [4]. Re- cently, a new liquid–liquid microextraction method termed dispersive liquid–liquid microextraction (DLLME) was re- ported by Rezaee et al. [5]. Up to now, most DLLME has been mainly applied for the analysis of OPPs in water ex- cept for some food samples including tea [6], apple juice [7], maize [8], tomato [9], watermelon, and cucumber [1]. In these DLLME procedures, n-hexane, carbon tetrachloride, and chlorobenzene have been used as extraction solvents for the DLLME of OPPs. Besides these extraction solvents, ionic liquids [4,10–15] have also been served as extraction solvent in the different DLLME. To promote the dispersion of extraction solvent, temperature, ultrasound, and vortex-assisted DLLME have been introduced in the field, the latter of which proved to avoid the degradation of some analytes [16]. Based on these above, the ionic liquid-based vortex-assisted DLLME was de- veloped for the extraction of OPPs in apple and pear, which C 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Upload: lijin-zhang

Post on 16-Oct-2016

217 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

2514 J. Sep. Sci. 2012, 35, 2514–2519

Lijin Zhang1,2

Fang Chen2

Shaowen Liu3

Biyu Chen1

Canping Pan3

1Department of Food Science,Tianjin Agricultural University,Tianjin, China

2College of Food Science &Nutritional Engineering, ChinaAgricultural University, Beijing,China

3College of Sciences, ChinaAgricultural University, Beijing,China

Received December 6, 2011Revised February 18, 2012Accepted February 20, 2012

Short Communication

Ionic liquid-based vortex-assisted dispersiveliquid–liquid microextraction oforganophosphorus pesticides in appleand pear

An ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction (DLLME) wasdeveloped for the analysis of trace amounts of six organophosphorus pesticides (OPPs) inapple and pear coupled with high-performance liquid chromatography. During the DLLME,the effect of some experimental factors including extraction solvent and its volume, disper-sion solvent and its volume, vortex time, salt addition, and pH on the extraction procedurewere investigated. Under the chosen extraction conditions, the analytes were enriched morethan 300-fold and the limits of detections were greatly dropped to 0.061–0.73 �g/kg. Thelinearity relationship was observed in the range of 2–100 �g/kg with the correlation coef-ficients (R2) ranging from 0.9967 to 0.9983. The relative standard deviations varied from2.3 to 5.7% (n = 6). Mean recovery values of the OPPs were in the range of 69.8–109.1%with a relative standard deviation lower than 7.0%. Based on these above, it could be con-cluded that 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6]) was a goodextraction solvent and the proposed [C8MIM][PF6]-based vortex-assisted DLLME methodwas suitable for the effective extraction of the OPPs in apple and pear.

Keywords: Dispersive liquid–liquid microextraction / High-performance liquidchromatography / Ionic liquid / Organophosphorus pesticidesDOI 10.1002/jssc.201101060

1 Introduction

Due to the low price and high efficiency, organophosphoruspesticides (OPPs) have been believed to own the largest mar-ket share in the agrochemical compounds. During the wideuse of OPPs in the agricultural practices, some of them trans-fer to the foods to form the OPPs residues. As these OPPsresidues have been proved to be acutely toxic to humans andorganism [1], it is important to carry out the monitoring ofthese OPPs residues for the possible risks to human health.However, the OPPs residues always remain at trace or ultra-trace level, which are too low to be detected for the commongas chromatography (GC) or high-performance liquid chro-matography (HPLC). Therefore, it is necessary to developsome sample preparation methods for the extraction and en-richment of these OPPs residues.

Correspondence: Prof. Canping Pan, College of Sciences, ChinaAgricultural University, No.2 Yuanmingyuan West Road, HaidianDistrict, Beijing 100193, China.E-mail: [email protected]: 86-10-6273-3620

Abbreviations: [C6MIM][PF6], 1-hexyl-3-methylimidazoliumhexafluorophosphate; [C8MIM][PF6], 1-octyl-3- methylimid-azolium hexafluorophosphate; LPME, liquid-phase microex-traction; LLE, liquid–liquid extraction; OPP, organophospho-rus pesticide

Sample preparation plays an important role in the field ofpesticide residues analysis [2]. Liquid–liquid extraction (LLE)may be the oldest and wildly used extraction method for theOPPs residues analysis [3]. However, it is laborious, time-consuming, and solvent-consuming. With the miniaturized,simplified, and automated demand of sample preparationmethods, LLE has been developed into the liquid-phase mi-croextraction (LPME). However, single-drop microextraction,one kind of LPME, is difficult to practice because of the in-stability of extraction solvents shaped in a single drop or pro-tected in porous hollow fiber under stirring condition [4]. Re-cently, a new liquid–liquid microextraction method termeddispersive liquid–liquid microextraction (DLLME) was re-ported by Rezaee et al. [5]. Up to now, most DLLME hasbeen mainly applied for the analysis of OPPs in water ex-cept for some food samples including tea [6], apple juice[7], maize [8], tomato [9], watermelon, and cucumber [1].In these DLLME procedures, n-hexane, carbon tetrachloride,and chlorobenzene have been used as extraction solvents forthe DLLME of OPPs. Besides these extraction solvents, ionicliquids [4,10–15] have also been served as extraction solvent inthe different DLLME. To promote the dispersion of extractionsolvent, temperature, ultrasound, and vortex-assisted DLLMEhave been introduced in the field, the latter of which provedto avoid the degradation of some analytes [16]. Based on theseabove, the ionic liquid-based vortex-assisted DLLME was de-veloped for the extraction of OPPs in apple and pear, which

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Page 2: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

J. Sep. Sci. 2012, 35, 2514–2519 Sample Preparation 2515

are the most important fruits in China. Several factors in-fluencing the extraction efficiencies of the ionic liquid-basedvortex-assisted DLLME were studied and the performancesof the proposed extraction method were also evaluated.

2 Experimental

2.1 Reagents and standards

The OPPs of isocarbophos, phtalofos, triazophos, fen-thion, phoxim, and profenofos were purchased from Agro-Environmental Protection Institution (Beijing, China). Theirstock standard solutions were prepared by dissolving inmethanol at a concentration of 100 mg/L and then storedat 4�C. Each of stock standard solution was then dilutedwith doubly distilled water to get a series of working solu-tions. Two kinds of ionic liquids of 1-hexyl-3-methylimid-azolium hexafluorophosphate ([C6MIM][PF6]), 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6])were obtained from Shanghai Chengjie Chemical Co., Ltd.(Shanghai, China) and used without any further purifi-cations. Acetone, methanol, and acetonitrile of chromato-graphic grade were purchased from Tianjin Kermel Chemi-cal Reagent Co., Ltd. (Tianjin, China). Other reagents wereat least of analytical grade. Doubly distilled water was usedthroughout the whole experiment.

2.2 Instrumentation

HP1100 Series HPLC system (Agilent Technologies Co.,Ltd.), was equipped with a UV detector and a 20 �L sam-ple injection. The signal was recorded and processed by anHP1100 workstation (Agilent Technologies Co., Ltd.). Theseparation was performed on an Agilent Eclipse XDB-C18

column (Agilent Technologies Co., Ltd.) (150 mm × 4.6 mmI.D., 5 �m) at room temperature; the mobile phase was themixture of methanol and water (70:30, v/v). The flow-rate was1 mL/min, the injection volume was 20 �L, and the detectionwavelength was 254 nm.

2.3 Sample preparation

Apple and pear samples were bought in local markets(Hongqi vegetable market, Tianjin, China). A representativeportion of unwashed sample was chopped into pieces, ho-mogenized, and centrifuged to obtain the fruit juices. As thefruit juices were too viscous to carry out the DLLME, 5 gfruit juices and 5 mL water were mixed and then transferredinto 15 mL PTFE centrifuge tube. After 1 mL methanol and0.0623 g [C8MIM][PF6] (50 �L) was added to the fruit juices, acloudy solution appeared. The cloudy solution was vigorouslyshaken on a model WH-2 vortex agitator (Shanghai HuxiAnalysis Instrument Factory Co., Ltd., Shanghai, China) at2000 rpm for 1 min and then centrifuged at 4000 rpm for5 min to obtain the separation into two phases. After the up-

per aqueous phase was carefully taken out, the settled phaseof [C8MIM][PF6] was withdrawn using a 50 �L microsyringefor the subsequent HPLC analysis.

3 Results and discussion

3.1 Selection of extraction solvent and effect of its

volume

Although the prices of proposed ionic liquids are about RMB2000.00 ¥/kg in China, several tens microlitre of them approx-imately costs RMB 0.10 ¥, which can be neglected consideringthe experimental cost. As [C6MIM][PF6] and [C8MIM][PF6]possess the essential characters of the immiscibility in wa-ter, good extraction capability of target compounds and goodchromatographic behavior, they were selected as the extrac-tion solvent in the proposed DLLME. When [C8MIM][PF6]and the dispersion solvent were mixed and injected in the di-luted fruit juices, a cloudy solution appeared at once and therecoveries of six OPPs were satisfied. However, [C6MIM][PF6]just supplied a clear solution. Therefore, [C8MIM][PF6] wasbetter than [C6MIM][PF6], and it was used as the right extrac-tion solvent in the present work.

To ensure high recoveries of six OPPs, the volume of[C8MIM][PF6] was also tested in the range from 30 �L to60 �L. As shown in Fig. 1, the recoveries increased with theincrease of the [C8MIM][PF6] volume from 30 �L to 50 �Lwhile the recoveries almost kept constant with the furtherincrease of the [C8MIM][PF6] volume. According to these,50�L [C8MIM][PF6] was enough for the DLLME of the OPPsin the diluted fruit juices.

3.2 Selection of dispersion solvent and effect

of its volume

To promote the dispersion of extraction solvent, the disper-sion solvents must be soluble in both extraction solvent andthe aqueous solution. Compared to acetonitrile and acetone,methanol was better than them considering the recoveries,which was revealed in Fig. 2. So, methanol was adopted inthe following experiments.

As the dispersion solvent promotes the solubility of theextraction solvent into the sample solution, too large or toosmall volume of dispersion solvent may have some adverseimpact on the extraction efficiencies. Figure 3 shows thatthe recoveries slowly increased over the methanol volumerange of 0.6–1.0 mL and then decreased over the methanolvolume range of 1.0–1.2 mL. This can be explained that aexcess volume of dispersion solvent can lead to the decreaseof extraction solvent volume that actually works. However,the extraction solvent can not be well dispersed in the samplesolution if the volume of dispersion solvent is insufficient,which may reduce the contact area between the extractionsolvent and the analytes. Therefore, 1.0 mL methanol was thebest selection in the extraction procedure.

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Page 3: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

2516 L. Zhang et al. J. Sep. Sci. 2012, 35, 2514–2519

Figure 1. Effect of the volume ofextraction solvent on the DLLME.Extraction conditions: extraction sol-vent, [C8MIM][PF6]; dispersion sol-vent, 1.0 mL methanol; vortex mixing,2000 rmp, 1 min; salt addition, 0;pH, 7.

3.3 Vortex time

In order to achieve the full dispersion of extraction solventinto water phase, the vortex time was individually investi-gated because the highest rotational speed has been fixed

at 2000 rpm. Figure 4 reveals that the vortex time had noobvious effect on the final recoveries so that 1 min vor-tex time was adopted to save the experimental time. With50 �L [C8MIM][PF6] and 1.0 mL methanol, the DLLME usingthe vortex mixing still completed the extraction of OPPs in

Figure 2. Effect of dispersion solventon the DLLME. Extraction conditions:extraction solvent, 50 �L [C8MIM][PF6];dispersion solvent volume, 1.0 mL; vor-tex mixing, 2000 rmp, 1 min; salt addi-tion, 0; pH, 7.

Figure 3. Effect of the volume of dis-persion solvent on the DLLME. Ex-traction conditions: extraction solvent,50 �L [C8MIM][PF6]; dispersion solvent,methanol; vortex mixing, 2000 rmp,1 min; salt addition, 0; pH, 7.

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Page 4: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

J. Sep. Sci. 2012, 35, 2514–2519 Sample Preparation 2517

Figure 4. Effect of vortex time on theDLLME. Extraction conditions: extrac-tion solvent, 50 �L [C8MIM][PF6]; dis-persion solvent, 1.0 mL methanol; vor-tex mixing, 2000 rmp; salt addition, 0;pH, 7.

10 mL diluted fruit juices while the previous DLLME con-sumed 35 �L [C8MIM][PF6] and 1 mL methanol for theextraction of OPPs in 5.0 mL water sample [4]. Therefore,1 min vortex mixing not only relatively reduced the con-sumption of the extraction solvent of [C8MIM][PF6] and thedispersion solvent of methanol, but also promoted the dis-persion of extraction solvent and the increase of extractionefficiencies.

3.4 Salt addition

Although the addition of salt increases the ionic strengthof the sample solution to reduce the solubility of analytes inwater phase, it also excessively enhances the solubility of ionicliquids in the sample solution due to the salt effect [17, 18],which can greatly reduce the volume of extraction solventthat actually works. As a result of this, Fig. 5 shows that therecoveries faintly decreased with the addition of NaCl. Forthis reason, the diluted fruit juices no longer accepted theaddition of salt.

3.5 Effects of pH

The pH of water sample determines the existence of the ana-lytes and the distribution of analytes between the donor phaseand acceptor phase. To prevent the hydrolysis of OPPs un-der strong acidic or basic condition, the pH was tested in therange of 3–8. Figure 6 illustrates that the recoveries reachedtheir best levels at pH 6, so the pH of the diluted fruit juiceswere adjusted by the buffer solution to 6.0–7.0 before theDLLME.

3.6 Performance of the proposed ionic liquid-based

vortex-assisted DLLME method

To further evaluate the proposed ionic liquid-based vortex-assisted DLLME method, the matrix-matched calibration,limits of detections (LODs), and the reproducibility of OPPswere discussed. To prepare the matrix-matched calibration,the diluted fruit juices were pretreated by the blank fruit sam-ple and subjected to the DLLME processing as described in

Figure 5. Effect of salt on the DLLME.Extraction conditions: extraction sol-vent, 50 �L [C8MIM][PF6]; dispersionsolvent, 1.0 mL methanol; vortex time,2000 rmp, 1 min; pH, 7.

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Page 5: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

2518 L. Zhang et al. J. Sep. Sci. 2012, 35, 2514–2519

Figure 6. Effect of pH on the DLLME. Ex-traction conditions: extraction solvent,50 �L [C8MIM][PF6]; dispersion solvent,1.0 mL methanol; vortex time, 2000rmp, 1 min; salt addition, 0.

Table 1. Performances of the proposed method (n = 6)

OPPs Linearity (�g/kg) R2 LOD (�g/kg) RSD (%)range (�g/L)

Isocarbophos 2–100 0.9967 0.061 4.6Phtalofos 2–100 0.9974 0.077 2.3Triazophos 2–100 0.9969 0.091 5.7Fenthion 2–100 0.9978 0.19 4.3Phoxim 2–100 0.9975 0.22 3.5Profenofos 2–100 0.9983 0.73 4.9

the section 2.3, the settled phases of which were then spikedwith different concentration of OPPs before the chromato-graphic analysis. The linear ranges of OPPs in apple andpear samples were 2–100 �g/kg and the corresponding cor-relation coefficients were between 0.9967 and 0.9983, respec-tively. During the proposed ionic liquid-based vortex-assistedDLLME, the fruit solutions were concentrated from 10 mLto about 32.5 �L, the OPPs in the fruit samples were en-riched 307.7 folds and the LODs ranged from 0.061 �g/kgto 0.73 �g/kg, which can be seen in Table 1. Furthermore,the RSD (n = 6) of OPPs varied between 2.3 and 5.7%. Theseabove showed that the present extraction method was cred-ible as far as the LODs and reproducibility were concerned.Typical chromatogram of six OPPs in fruits after the ionicliquid-based vortex-assisted DLLME is shown in Fig. 7.

3.7 Application of the proposed method

In order to confirm the potential of the proposed method,apple and pear samples were collected from different regionsand carried out the chromatographic analysis. Under the opti-mal ionic liquid-based vortex-assisted DLLME condition, thefinal analysis showed that the concentration of six OPPs infruit samples were below the LODs of the proposed method.When all the fruits were spiked with six OPPs at three dif-ferent concentration levels of 5, 20, and 50 �g/kg, the rel-ative recoveries of the six OPPs in spiked samples rangedfrom 69.8 to 109.1% with the RSD of 2.1–7.0%, which areshown in Table 2. The results showed that the ionic liquid-based vortex-assisted DLLME was a simple, precise, and re-liable method, which can detect OPPs in real apple and pearsamples.

4 Conclusion

The present study has developed a new method for theanalysis of six OPPs in apple and pear samples by us-ing [C8MIM][PF6]-based vortex-assisted DLLME. Using theextraction solvent of 50 �L [C8MIM][PF6] and the dis-persion solvent of 1 mL methanol, six OPPs were per-formed with vortex-assisted DLLME from apple and pearsamples without the addition of salt. Due to the vortex

Figure 7. Chromatogram forspiked fruit samples at theconcentration level of 20�g/kg with the proposed ionicliquid-based vortex-assistedDLLME procedure. Peaks: (1)isocarbophos; (2) phtalofos;(3) triazophos; (4) fenthion; (5)phoxim; (6) profenofos.

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

Page 6: Ionic liquid-based vortex-assisted dispersive liquid–liquid microextraction of organophosphorus pesticides in apple and pear

J. Sep. Sci. 2012, 35, 2514–2519 Sample Preparation 2519

Table 2. Relative recovery and RSD values of six OPPs studied in spiked apple and pear samples (n = 3)

Sample Apple Pear

Spiked level (�g/kg) 5 20 50 5 20 50

Isocarbophos Recoveries (%) 70.3 84.6 103.8 69.8 97.5 108.4RSD (%) 2.3 3.1 2.6 2.9 3.5 4.0

Phtalofos Recoveries (%) 72.8 86.3 107.5 76.9 78.9 87.5RSD (%) 5.7 4.3 6.2 2.9 3.6 2.1

Triazophos Recoveries (%) 87.0 100.0 104.7 84.3 105.0 107.0RSD (%) 6.0 4.7 3.5 2.9 7.0 5.0

Fenthion Recoveries (%) 80.3 103.4 102.5 88.2 101.4 109.1RSD (%) 2.8 4.2 6.4 3.0 4.1 5.3

Phoxim Recoveries (%) 73.7 103.6 101.9 98.0 104.7 103.8RSD (%) 5.3 4.6 3.9 2.4 2.9 3.7

Profenofos Recoveries (%) 91.9 105.6 107.3 85.3 104.9 106.2RSD (%) 3.8 5.0 4.3 4.9 5.1 6.2

mixing, the proposed method relatively reduced the con-sumption of the extraction solvent of [C8MIM][PF6] andthe dispersion solvent of methanol compared to the similarDLLME. And the proposed method was validated by the lin-earity, reproducibility, and recoveries of six OPPs with goodresults. Based on these, the proposed approach is a simple,fast, efficient, and economical method for the effective extrac-tion of OPPs from apple and pear samples.

The authors have declared no conflicts of interest.

5 References

[1] Zhao, E. C., Zhao, W. T., Han, L. J., Jiang, S. R., Zhou, Z.Q., J. Chromatogr. A 2007, 1175, 137–140.

[2] Xiong, J., Hu, B., J. Chromatogr. A 2008, 1193, 7–18.

[3] Caldas, S. S., Costa, F. P., Primel, E. G., Anal. Chim. Acta2010, 665, 55–62.

[4] He, L. J., Luo, X. L., Xie, H. X., Wang, C. J., Jiang, X. M.,Lu, K., Anal. Chim. Acta 2009, 655, 52–59.

[5] Rezaee, M., Assadi, Y., Millani, M. R., Aghaee, E., Ahmadi,F., Berijani, S., J. Chromatogr. A 2006, 1116, 1–9.

[6] Moinfara, S., Hosseini, M. R. M., J. Hazard. Mater. 2009,169, 907–911.

[7] Cunha, S. C., Fernandes, J. O., Oliveira, M. B. P. P., J.Chromatogr. A 2009, 1216, 8835–8844.

[8] Cunha, S. C., Fernandes, J. O., J. Chromatogr. A 2011,1218, 7748–7757.

[9] Bidari, A., Ganjali, M. R., Norouzi, P., Hosseini, M. R. M.,Assadi, Y., Food Chem. 2011, 126, 1840–1844.

[10] Ravelo-Perez, L. M., Hernandez-Borges, J., Asensio-Ramos, M., Rodrıguez- Delgado, M. R., J. Chromatogr.A 2009, 1216, 7336–7345.

[11] Bai, H. H., Zhou, Q. X., Xie, G. H., Xiao, J. P., Anal. Chim.Acta 2009, 651, 64–68.

[12] He, L. J., Luo, X. L., Jiang, X. M., Qu, L. B., J. Chromatogr.A 2010, 1217, 5013–5020.

[13] Liu, Y., Zhao, E. C., Zhu, W. T., Gao, H. X., Zhou, Z. Q., J.Chromatogr. A 2009, 1216, 885–891.

[14] Wang, Y., You, J. Y., Ren, R. B., Xiao, Y., Gao, S. Q., Zhang,H. Q., Yu, A. M., J. Chromatogr. A 2010, 1217, 4241–4246.

[15] Zhou, Q. X., Zhang, X. G., Xiao, J. P., J. Chromatogr. A2009, 1216, 4361–4365.

[16] Jia, C. H., Zhu, X. D., Wang, J. H., Zhao, E. C., He, M.,Chen, L., Yu, P. Z., J. Chromatogr. A 2010, 1217, 5868–5871.

[17] Peng J. F., Liu J. F., Hu X. L., Jiang G. B., J. Chromatogr.A 2007, 1139, 165–170.

[18] Cruz-Vera M., Lucena R., Cardenas S., Valcarcel M., J.Chromatogr. A 2008, 1202, 1–7.

C© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com