Dt

Sa

b

a

ARRAA

KOGTD

1

papbaohdirmwctapbii

if

0d

Journal of Hazardous Materials 169 (2009) 907–911

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

evelopment of dispersive liquid–liquid microextraction method forhe analysis of organophosphorus pesticides in tea

oleyman Moinfara, Mohammad-Reza Milani Hosseinia,b,∗

Department of Analytical Chemistry, Faculty of Chemistry, Iran University of Science and Technology, Tehran, IranElectroanalytical Chemistry Research Center, Iran University of Science and Technology, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 2 February 2009eceived in revised form 8 April 2009ccepted 8 April 2009vailable online 16 April 2009

a b s t r a c t

In this article, a new method for the determination of organophosphorus pesticides (OPPs) in tea wasdeveloped by using dispersive liquid–liquid microextraction (DLLME) and gas chromatography–flamephotometric detection (GC-FPD). A mixture of acetonitrile and n-hexane was used as an extraction solventfor the extraction of OPPs from tea samples. When the extraction process was finished, the mixture of

eywords:rganophosphorus pesticidesC-FPDea sample analysis

solvents was rapidly dispersed in water; target analyte was extracted to a small volume of n-hexane,using DLLME. Recovery tests were performed for concentration 5.0 �g/kg. The recovery for each targetanalyte was in the range between 83.3 and 117.4%. The repeatability of the proposed method, expressedas relative standard deviation, varied between 3 and 7.8% (n = 3). The detection limit of the method for teawas found ranging from 0.030 to 1 �g/kg for all the target pesticides. Compared with the conventional

od, thfacto

ispersive liquid–liquid microextraction sample preparation methand has high-enrichment

. Introduction

Pesticides including organochlorine pesticides (OCPs), organo-hosphorus pesticides (OPPs), and nitrogen-containing herbicidesre types of well-known environmental contaminants. The use ofesticides provides benefits for increasing agricultural production,ut by bioaccumulation through the food web they can eventu-lly become a risk or threat to both animals and humans. Becausef their highly persistent properties and potential threat to humanealth, OCPs have been prohibited to be produced and used in mosteveloped countries. Instead OPPs are used as a substitute for OCPs

n many countries nowadays because they can degrade the envi-onment more easily [1]. Although OPPs as a whole are not theost toxic pollutants, they can be traced in a wide range of surfaceater, fruit, vegetable and foodstuff [2]. Many types of plants are

onsumed throughout the world as infusions for both pleasure andherapeutic purposes. Like other agricultural products, the materi-ls used for infusions must be subjected to control because of the

esticides used for their cultivation, in order to minimise possi-le risks to human health. Tea is one of the most popular drinks

n the world, so that such water-based drinks could represent anmportant share of total human exposure to pesticides [3]. Several

∗ Corresponding author at: Department of Analytical Chemistry, Faculty of Chem-stry, Iran University of Science and Technology, Tehran, Iran. Tel.: +98 21 73912750;ax: +98 21 77491204.

E-mail address: drmilani@iust.ac.ir (M.-R.M. Hosseini).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.04.030

e proposed method has the advantage of being quick and easy to operate,rs and low consumption of organic solvent.

© 2009 Elsevier B.V. All rights reserved.

countries have set their own maximum residue limits (MRLs) ofpesticides for tea and other plants consumed as infusions. Thus,for example, the European Community legislation established theMRLs for the pesticides considered in the present study at between0.02 �g/g for tetradifon and 5 �g/g for deltamethrin [4] in a widevariety of vegetables, although no MRL is specified in the case ofothers such as coumaphos. Different analytical procedures wereused for the determination of pesticides in medicinal plants andtea [3,5–11]. Owing to low concentration, the compounds of inter-est have to be separated from the matrix and concentrated to reachthe minimum level required for the particular detector used. Solid-phase extraction (SPE) [12], liquid–liquid extraction (LLE) [13], stirbar sorptive extraction (SBSE) [14] and solid-phase microextraction(SPME) [11,15] have been used for the separation and preconcentra-tion of pesticides from the infusion matrices. Actually, there are onlyfew studies based on modern sample preparation methods such asSPME, SBSE (stirring bar sorptive extraction) and SFE (supercriticalfluid extraction) focusing on the determination of pesticides in phy-tomedicines (herbal drugs, infusions, tinctures, dried extracts, etc.)[8,9,11,14–16]. Plant infusions are complex samples, containing sev-eral endogenous compounds extracted from the leaves by the hotwater during their preparation. Also, it is possible to find a widerange of different pesticide residues on these infusions, depend-

ing on the precedence of a particular sample; when present, theseanalytes are usually found on extremely low levels (�g/L or less).

Therefore, the procedures for the chromatographic detectionand quantitation of organochlorine (OCP) and organophosphorus(OPP) on Passiflora infusions must incorporate selective, robust and

9 of Haz

e(trrapDrtOtpn[p[hc

awolODGym

naTe

2

2

ipftaMsuwad4

2

it(sU3lwsc

08 S. Moinfar, M.-R.M. Hosseini / Journal

ffective clean-up and extraction steps [17]. Gas chromatographyGC) is a suitable technique for such a purpose [11–15] but, dueo low concentration, the compounds of interest have to be sepa-ated from the matrix and concentrated to reach the minimum levelequired for the particular detector used. In the previous research,new microextraction technique was demonstrated named dis-

ersive liquid–liquid microextraction (DLLME). The advantages ofLLME are simplicity of operation, rapidity, low time and cost, high

ecovery and enrichment factor [18]. This method has been used forhe determination of polycyclic aromatic hydrocarbons (PAHs) [18],PPs [19], chlorobenzenes [20], chlorophenols [21], phenols [22],

rihalomethanes [23], polychlorinated biphenyls [24], butyl andhenyltin [25] ionizable organic compounds [26], amitriptyline andortriptyline [27], polybrominated diphenyl ethers [28], anilines29], three phthalate esters [30], phthalate esters [31], organophos-horus and plastizicers [32], triazine herbicides [33], antioxidants34] and halogenated organic compounds [35] in liquid samples andas been used for the determination of organophosphorus pesti-ides [36] in watermelon and cucumber samples.

Because organophosphorus pesticides are the most widelypplied in mainland Iran to control agricultural crops insects,e selected 10 types of OPPs as the representative species of

rganophosphorus pesticides. However, to date, none of the pub-ished papers have reported the use of DLLME for the analysis ofPPs in tea sample by GC-FPD. The aim of this study is to assessLLME suitability for the determination of OPPs in tea sample byC-FPD. The effects of different experimental parameters on theield of the sample preparation step were also studied and opti-ized.In this study, we have developed a new method for the determi-

ation of OPPs in tea after extraction with a mixture of acetonitrilend n-hexane concentration with the developed DLLME method.his change in DLLME method was done to use any solvents inxtraction and decrease detection limit.

. Experimental

.1. Reagents and standards

All OPPs (phorate, diazinon, disolfotane, methyl parathion, sum-thion, malathion, fenthion, profenphose, ethion, phosalone) wereurchased from polyscience (Niles, USA). n-Hexane was obtainedrom Merck (Germany). This solvent was distillated at least twoimes and was used as an extraction solvent. Acetone, acetonitriles a solvent (suprasolv or gas chromatography) were obtained fromerck. Also sodium chloride and triphenylphosphate (as internal

tandard) were purchased from Merck. Doubly-distilled water wassed for the preparation of aqueous solution. Each OPP (0.01000 g)as dissolved in 10.0 mL acetone to obtain a standard solution withconcentration of 1000 mg/L. A tea from 20.0 mg/L of OPPs stan-ard solution was prepared in acetone every week and stored at◦C. The tea sample was obtained from local supermarkets.

.2. Instrumentation

A gas chromatograph (Shimadzu GC 2010) with a split/splitlessnjector system, and a flame photometric detector was used forhe separation and determination of OPPs. Ultra pure helium99.9999%, Air Products, UK), which passes through a molecularieve trap and oxygen trap, (Chromatography Research Supplies,SA) was used as the carrier gas at constant linear velocity of

5 cm/s. The injection port was held at 250 ◦C and used in the split-ess mode with splitless time 0.5 min. A deactivated glass lineras used in order to decrease the degradation of products. The

eparation was carried out on a BP-5, 28.5 m × 0.22 mm capillaryolumn with a 0.25-�m stationary film thickness, 5% phenyl/95%

ardous Materials 169 (2009) 907–911

dimethyl polysiloxane (Phenomenex, USA). The oven temperaturewas programmed as follows: initial 100 ◦C, from 100 ◦C (held 2 min)to 150 ◦C at the rate of 25 ◦C/min, from 150 to 175 ◦C at the rateof 5 ◦C/min, from 175 to 195 ◦C at the rate of 2 ◦C/min, from 195to 275 ◦C at the rate of 10 ◦C/min (held 5 min). The total time forone GC run was 32 min. The FPD temperature was maintainedat 300 ◦C, hydrogen gas was generated with hydrogen generator(OPGU-2200s, Shimadzu) for FPD at a flow of 80 mL/min. The flowof zero air (99.999%, Air Products) for FPD was 120 mL/min. Thescientific centurion centrifuge (model 2010D, UK) was used forcentrifuging. In order to investigate the temperature effect, lowtemperature incubator (LTI-601SD, EYELA, Japan) and also tempera-ture controlling centrifuge (K240R, Centurion Scientific) were used.All 5 mL screw cap glass test tubes with conic bottom (extractionvessel) were maintained at 500 ◦C in furnace (Carbolite, model CWF1200, UK) for the cleaning of any organic compounds and good sed-imentation of fine droplets of extraction solvent in the centrifugingstep.

2.3. Samples preparation and method development

In this study, 2.0 g of green tea which is produced on the fields ofGilan, was sprayed with 200 �L of standard solution OPPs and driedat room temperature. After 24 h, the spiked sample was powderedand 1.0 g of the powder sample was weighed in a capped vial. Then2 mL of acetonitrile and n-hexane mixture was added to the sam-ple with ratios of 250 and 3, respectively, and magnetically stirred(1000 rpm) for 45 min at 42 ◦C. Therefore, by adding the mixtureof solvents, the OPPs were extracted from tea. So there is no needto filter it or change the solvents ratio. After the tea being settled,0.5 mL of the mixture of solvents in the capped vial using a 1.00 mLsyringe (gastight, Hamilton, USA) was injected rapidly into a 5 mLscrew cap glass test tube with conic bottom containing 5.00 mL ofdoubly-distilled water. Then the mixture was gently shaken.

A cloudy solution (water/acetonitrile/n-hexane) was formed inthe test tube.

In fact, acetonitrile and n-hexane that were used in step 1(extraction) acted respectively as disperser and extraction solventsin step 2 (DLLME). These two steps were coupled together in sucha good manner that makes this method a suitable procedure toextract OPPs from the tea without changing solvents and neces-sity of filtration. After the extraction of OPPs (step 1), analytes wereconcentrated in the second step and ready to use for GC analysis.The cap of screw cap glass test tube was punched and a septumwas put on the test tube to seal it, then the punched screw cap wasfastened on the test tube. Next it was put upside down into cen-trifuge (3800 rpm) for 3 min. After this process the fine disperseddroplets of n-hexane were collected at the conic bottom of test tube– because the dense of n-hexane is less than that of water, it collectsat the conic bottom of the test tube – and OPPs were concentratedinto n-hexane and the tiny particles of tea were precipitated.

Saving time and energy and eliminating filtration step beforeDLLME are the advantages of using n-hexane. Besides, less OPPswere consumed compared with conventional methods.

After centrifugal process, the test tube was fixed by a clip inan upside down position. Then 0.50 �L of n-hexane, which is col-lected at conic bottom, was removed using 1.00 �L microsyringe(zero dead volume, cone tip needle, SGE, Australia) and was injectedinto GC. The volume of n-hexane collected at the conic bottomof test tube was about 5.0 mL which was determined by a 10 �Lmicrosyringe.

For recovery studies, 2.0 g of tea sample was spiked with200 �L of a working solution containing the pesticides from 100to 200 �g/L, depending on the compound, corresponding to forti-fication levels of approximately 0.01–20.0 ng/g, respectively. Threereplicates were analyzed in each case.

of Hazardous Materials 169 (2009) 907–911 909

3

fieabiasicsc

3

(wt(htiovtos

varF0ts

3f

v

Ffvi

S. Moinfar, M.-R.M. Hosseini / Journal

. Results and discussion

Since, in this study, acetonitrile was used as an extraction solventrom tea in step 1 and as a disperser solvent in the DLLME (step 2),t should have some properties such as: miscibility in water, goodxtractor and high boiling point. Among four solvents (acetonitrile,cetone, methanol and ethanol) which have the property of misci-ility in organic and aqueous phase, acetonitrile is chosen because

t has the highest boiling point as an extraction solvent from teand as a disperser solvent in DLLME. In developed DLLME the den-ity of the concentration solvent should be less than water, becausen DLLME step (step 2), concentration solvent is collected in theonic bottom of the test tube. Among solvents which have less den-ity than water, n-hexane is mostly used to extract OPPs and otherompounds [37].

.1. The optimization of the DLLME method (step 2)

In order to evaluate the effect of concentration solvent volumen-hexane), solutions containing different volumes of n-hexaneere examined by the same DLLME procedures. The experimen-

al conditions are fixed and include the use of 2.00 mL acetonitrileas extraction solvent from tea) containing different volumes of n-exane (20.0, 24.0, 28.0, 32.0, 36.0, 44.0 and 48.0 �L), when addedo 1 g tea. The curve of peak area versus volume of extraction solvents shown in Fig. 1. A high peak area was attained at a low volumef the concentration solvent (n-hexane), because by increasing theolume of the n-hexane, the volume of the collected n-hexane athe conic bottom increases. It is shown in Fig. 1. Thereby, 24.0 �Lf n-hexane was selected as the optimum volume of concentrationolvent in step 2.

From 2 mL of mixed solvent added to tea only about 1.2 mL of sol-ent remains and it can be injected to distilled water in which peakrea decreased by increasing injected solvent in DLLME step. As aesult, the optimum volume of mixed solvent 0.5 mL is selected.rom 2 mL acetonitrile added to tea as OPPs extraction solvent,.5 mL which contains n-hexane and extraction OPPs, is injectedo conic bottom of test tube containing distilled water by a 1.00 mLyringe.

.2. The effect of extraction solvent volume for extracting OPPsrom tea

The variation of the volume of acetonitrile (as disperser sol-ent in the second step and extraction solvent in the first step)

ig. 1. The effect of the volume of n-hexane on the peak area of OPPs obtainedrom DLLME. Extraction conditions: water sample volume, 5.00 mL; disperser sol-ent (acetonitrile) volume, 0.5 mL; temperature 42 ◦C. Concentration of each OPPsn tea, 2 �g/kg.

Fig. 2. The effect of a volume of acetonitrile on the peak area of OPPs obtained fromtea. The extraction conditions: as shown in Fig. 1 in 45 min.

causes change in the volume of upper residual phase. To avoidthis change and to achieve an optimum volume of the collectedn-hexane at the conic bottom the volume of acetonitrile waschanged. The experimental conditions are fixed and 24.0 �L n-hexane containing different volumes of acetonitrile (0.5, 1.5, 2.0,2.5, and 4.0 mL) is used. The curve of the peak area versus thevolume of acetonitrile solvent is shown in Fig. 2. According to thecurves, 2.0 mL has been chosen as the optimum volume of acetoni-trile.

3.3. The effect of the extraction time

In this method, the extraction time is defined as the time whentea contacts with mixture of solvents (acetonitrile and n-hexane),before starting to DLLME. After the extraction from the samplewith a 0.5 mL mixture of solvent (acetonitrile and n-hexane) DLLMEmethod was done. The effect of the extraction time was examinedin the range of 20–50 min with constant experimental conditions.Fig. 3 shows the peak area of OPPs versus the extraction time.According to the curves, 45 min has been chosen as the optimumtime.

The following parameters will influence the efficiency of theDLLME according to the study by Berijani et al. [19]: time extractionand salt effect.

Fig. 3. The effect of the extraction time on the peak area of OPPs obtained from theextraction of tea. Extraction conditions: as shown in Fig. 1; the volume of n-hexane,24.0 �L.

910 S. Moinfar, M.-R.M. Hosseini / Journal of Hazardous Materials 169 (2009) 907–911

Fe2

3

e

efbaBT

3

fl

Table 1Quantitative result of DLLME and GC-FPD from tea sample.a

OPPs RSD% (n = 3)b LOD (�g/kg)c Recovery (%)d

Phorate 6.2 0.5 98.6Diazinon 5.7 0.1 85.6Disolfotane 4.7 0.5 83.3Methylparathion 5.1 0.1 79.8Sumithion 4.2 0.2 90.4Malathion 5 0.03 90.7Fenthion 4 0.05 102.4Profenphose 4 0.05 93.1Ethion 3.2 1 116.3Phosalone 4.8 0.1 105.3

a Extraction condition: tea sample weight, 1 g; acetonitrile volume as dispersersolvent, 0.5 mL; as extraction solvent from tea samples, 2 mL; concentration solvent(n-hexane) volume, 24.0 �L; temperature, 42 ◦C.

b Relative standard deviation (RSD%). Concentration of 2.0 ng/g for each OPPs.

Fa(

ig. 4. The effect of the temperature on the peak area of OPPs obtained from thextraction of tea. Extraction conditions: as shown in Fig. 1. The volume of n-hexane,4.0 �L.

.4. The effect of the temperature

Temperature is another parameter that may affect the extractionfficiency from tea sample (step 1).

For defining the optimal extraction temperature (step 1), theffect of this parameter was evaluated using ranging temperaturesrom 35 to 50 ◦C. The results obtained from these tests show that,y increasing the temperature from 35 to 42 ◦C (Fig. 4), the peakrea increased as a result of increasing extraction from tea sample.ut in high temperature (50 ◦C), extraction recoveries decreased.herefore 42 ◦C is the optimum temperature.

.5. Analytical performance

In order to investigate the applicability of the proposed methodor determining OPPs in tea, several factors including recovery,imits of detection (LODs) and repeatability were evaluated. Tea

ig. 5. Chromatogram of real sample tea (A) and spiked at concentration level of 2 �g/kg ofs with Fig. 1; volume of n-hexane, 24.0 �L; concentration of triphenyl phosphate, 2 �g/kg5) sumithion; (6) malathion; (7) fenthion; (8) profenphose; (9) ethion; (10) phosalone; (

c The limits of detection concentration for a S/N ratio of approximately 3. Range0.01–20.0 ng/g.

d Concentration of 2.0 ng/g for each OPPs.

samples fortified with OPPs below the concentration 2 �g/kg werepreformed as described in Section 2.3. After the extraction of OPPsby acetonitrile, the extracted OPPs from the fortified samples areconcentrated by DLLME method. The performance of this techniqueis shown in Table 1. Acceptable recoveries and repeatability couldbe obtained. The calculated detection limits of development DLLMEwere between 0.03 and 1 �g/kg.

3.6. Real sample analysis

Real tea sample was collected from Gilan (Iran), and was ana-lyzed by developing DLLME combined with GC-FPD. In the real

sample, fenthion, ethion and phosalone were detected and theywere confirmed by spiking OPPs into the real tea sample. Fig. 5shows the obtained chromatograms for sample tea and spiked teaat the concentration level of 2 �g/kg of each OPPs.

each OPPs (B) obtained using DLLME combined with GC-FPD. Extraction conditions:. Peak identification: (1) phorate; (2) diazinon; (3) disolfotan; (4) methyl parathion;I.S.) triphenyl phosphate.

of Haz

4

cpido

A

oY

R

[

[

[

[

[

[

[[[

[

[

[

[[

[

[

[[[[[[

[[

S. Moinfar, M.-R.M. Hosseini / Journal

. Conclusion

The results of study demonstrate that a very sensitive methodould be achieved by the development of DLLME method. Under theroposed method, the sensitivity of the method could be greatly

ncreased. The comparison of DLLME with conventional methodemonstrated DLLME is very rapid, simple, lower consumption ofrganic solvent and has low limits of detection.

cknowledgements

Financial support from Iran University of Science and Technol-gy is gratefully acknowledged. The authors thank Dr. Professoraghoub Assadi and Ms. Sedighehe Basiri.

eferences

[1] F. Ahmadi, Y. Assadi, S.M.R. Milani Hosseini, M. Rezaee, J. Chromatogr. A 1101(2006) 307.

[2] M.R. Driss, M.C. Hennion, M.L. Bouquerra, J. Chromatogr. 639 (1993) 352.[3] N. Campillo, R. Penalver, M. Hernández-Córdoba, Talanta 71 (2007) 1417.[4] http://www.mapya.es, accessed in February 2006.[5] H. Fiedler, C.K. Cheung, M.H. Wong, Chemosphere 46 (2002) 1429.[6] C. Matos Lino, M.I. Norohna da Silveira, J. Chromatogr. A 769 (1997) 275.[7] W. Ho, S. Hsieh, Anal. Chem. Acta 428 (2001) 111.[8] V.G. Zuin, J.H. Yariwake, C. Bicchi, J. Chromatogr. A 985 (2003) 159.[9] L. Cai, J. Xing, L. Dong, C. Wu, J. Chromatogr. A 1015 (2003) 11.

10] N. Ochiai, K. Sasamoto, H. Kanda, T. Yamagami, F. David, B. Tiempont, P. Sandra,

J. Sep. Sci. 28 (2005) 1083.11] V. Gomes Zuin, A. Leite Lopes, J. Harumi Yariwake, F. Augusto, J. Chromatogr. A

1056 (2004) 21.12] J.C. Molto, B. Lejeune, P. Prognon, D. Pradeau, Int. J. Environ. Anal. Chem. 4 (1994)

81.

[[[[

ardous Materials 169 (2009) 907–911 911

13] S. Jaggi, C. Sood, V. Kumar, S.D. Ravindranth, A. Shanker, J. Agric. Food Chem. 49(2001) 5479.

14] C. Bicchi, C. Cordero, C. Iori, P. Rubiolo, P. Sandra, J.H. Yariwake, V.G. Zuin, J. Agric.Food Chem. 51 (2003) 27.

15] M.V.N. Rodrigues, F.G.R. Reyes, V.L.G. Rehder, S. Rath, Chromatographia 61(2005) 291.

16] V.G. Zuin, J.H.Y. Vilegas, Phytother. Res. 14 (2000) 73.17] A. Sartoratto, F. Augusto, Chromatographia 57 (2003) 351.18] M. Rezaee, Y. Assadi, M.R. Millani, E. Aghaee, F. Ahmadi, S. Berijani, J. Chromatogr.

A 1116 (2006) 1.19] S. Berijani, Y. Assadi, M. Anbia, M.R. Milani Hosseini, E. Aghaee, J. Chromatogr.

A 1123 (2006) 1.20] R. Rahnama Kozani, Y. Assadi, F. Shemirani, M.R. Milani Hosseini, M.R. Jamali,

Talanta 72 (2007) 387.21] N. Fattahi, Y. Assadi, M.R. Milani Hosseini, E. Zeini Jahromi, J. Chromatogr. A 1157

(2007) 23.22] L. Farina, E. Boido, F. Carrau, E. Dellacassa, J. Chromatogr. A 1157 (2007) 46.23] R. Rahnama Kozani, Y. Assadi, F. Shemirani, M.R. Milani Hosseini, M.R. Jamali,

Chromatographia 66 (2007) 81.24] F. Rezaei, Y. Assadi, A. Bidari, A.P. Birjandi, M.R. Milani Hosseini, J. Hazard. Mater.

158 (2008) 621.25] A.P. Birjandi, A. Bidaria, F. Rezaeia, M.R. Milani Hosseinia, Y. Assadi, J. Chro-

matogr. A 1193 (2008) 19.26] M.B. Melwanki, M-Ren Fuh, J. Chromatogr. A 1198–1199 (2008) 1.27] A.S. Yazdi, N. Razavi, S.R. Yazdinejad, Talanta 75 (2008) 1293.28] Y. Li, G. Wei, J. Hu, X. Liu, X. Zhao, X. Wang, Anal. Chem. 615 (2008) 96.29] J.-S. Chiang, S.-D. Huang, Talanta 75 (2008) 70.30] P. Liang, J. Xu, Q. Li, Anal. Chem. 609 (2008) 53.31] H. Farahani, P. Norouzi, R. Dinarvand, M.R. Ganjali, J. Chromatogr. A 1172 (2007)

105.32] M. García-López, I. Rodríguez, R. Cela, J. Chromatogr. A 1166 (2007) 9.33] D. Nagaraju, S.-D. Huang, J. Chromatogr. A 1161 (2007) 89.

34] M.A. Farajzadeh, M. Bahram, J.A. Jönsson, Anal. Chem. 591 (2007) 69.35] M.-I. Leong, S.-D. Huang, J. Chromatogr. A 1211 (2008) 8.36] E. Zhao, W. Zhao, L. Han, S. Jiang, Z. Zhou, J. Chromatogr. A 1175 (2007) 137.37] R. Rial-Otero, E.M. Gaspar, I. Moura, J.L. Capelo, Chromatographic-based meth-

ods for pesticide determination in honey: an overview, Talanta 71 (2007)503–514.