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Talanta 71 (2007) 1068–1074

Microwave-assisted steam distillation for the determination oforganochlorine pesticides and pyrethroids in Chinese teas

Jie Ji, Chunhui Deng, Huiqin Zhang, Yunyun Wu, Xiangmin Zhang ∗Department of Chemistry, Fudan University, Shanghai 200433, China

Received 10 April 2006; received in revised form 27 May 2006; accepted 30 May 2006Available online 7 July 2006

bstract

In this work, microwave-assisted steam distillation (MASD) extraction method followed by gas chromatography/electron capture detectionGC/ECD) was developed for the determination of organochlorine pesticides (OCPs) and pyrethroids in the Chinese teas. MASD is a combinationf microwave-assisted extraction (MAE) and steam distillation techniques. Water vapor generated by microwave irradiation is used to accelerateesorption of the analytes from the sample, and the nonpolar organic solvent used for trapping the analytes is kept from direct contact with theample by the water. Therefore, relatively clean extracts were obtained compared to the method directly using organic solvent as extraction solvent,uch as ultrasonic extraction (USE). Microwave power of 200 W and irradiation time of 2 min was found to be the optimum conditions for the

ASD process, and n-heptane was chosen as the analyte-trapping solvent in the study. Five OCPs (�-HCH, �-HCH, dicofol, p,p′-DDE, p,p′-DDT)

nd two pyrethroids (bifenthrin, fenvalate) were determined using this extraction method in the tea samples. The relative standard deviation (R.S.D.)f the analytes varied from 2.2 to 8.4%, and the method detection limits (MDLs) found were lower than 0.23 �g/kg. The recoveries of the sevenompounds in the Jasmine tea sample were between 84.04 and 110.1%. Comparative results obtained by MASD and USE were also discussed inhe study.

2006 Elsevier B.V. All rights reserved.

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eywords: Microwave-assisted steam distillation; Organochlorine pesticides; P

. Introduction

Organic pollutants have been shown to exhibit potentiallyarmful effects to the environment and human beings. As anxample pesticides used in agricultural as well as industrialhould be considered as hazardous chemicals with great concernor the general population. Organochlorine pesticides (OCPs)nd pyrethroids are two kinds of the most widely used pesti-ides, for they have been effective in the control of pests andiseases. Due to their low biodegradability and high persis-ence, they are ubiquitous among environmental samples, suchs food, air, water, soil, sediments and biological tissues [1–4].oreover, these chemicals are known to induce cancer and be

ndocrine disrupters in several organisms for their toxic char-

cteristic. Although the use of most OCPs and pyrethroids haseen banned or restricted in industrialized countries, they aretill detected in the environment [5–8].

∗ Corresponding author. Fax: +86 21 65641740.E-mail address: xmzhang@fudan.edu.cn (X. Zhang).

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039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.talanta.2006.05.087

roids; Analyte-trapping solvent; Chinese teas

OCPs and pyrethroids, like dicofol and fenvalate, are widelysed in the teas, mainly for the purpose of eliminating the mitend other pests. Tea has been the most favourite drink sincehousands of years ago in China, and nowadays it is also veryopular in the other countries of the world for its characteris-ic aroma, flavor and health benefits [9]. But only a few studiesere published in the area of pesticides research for the Chinese

eas [10,11]. The complexity of the matrix made the identifi-ation and quantification of pesticides in Chinese teas difficult.oxhlet extraction and ultrasonic extraction (USE), followed bycomplicated clean-up process, are the conventional sample

reparation method before gas chromatography/electron cap-ure detection (GC/ECD) for the determination of OCPs andyrethroids in the tea.

Microwave-assisted extraction (MAE) of organic pollutantsas first introduced in 1986 by Ganzler et al. [12]. Since that

ime, this extraction technique has been successfully applied to

xtract organic compounds, such as PAHs, pesticides, PCBs andhenols, from various solid and liquid matrices, such as sedi-ents, soils, plant materials and water samples [13–17]. MAEas more effective and reduced the sample preparation time and

71 (2007) 1068–1074 1069

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olvent volumes substantially in comparison with conventionalxtraction techniques, as the heating during MAE is based on theirect effect of microwaves on molecules by ionic conductionnd dipole rotation. MAE solvents for the extraction of organicollutant are usually iso-octane, acetonitrile, dichloromethaneDCM) [18–20], acetone [21,22] and mixtures of low relativeermittivity solvents with acetone, as n-hexane–acetone21–26]. Water also acts as an efficient extraction solvent of

AE process, because water molecules with a high dielec-ric constant were characterized by a high ability to absorb

icrowave energy [27]. But other extraction methods, likeolid phase microextraction (SPME) and liquid phase microex-raction (LPME), are usually combined with MAE as a furtherreparation for the sample, when the extraction solvent is water28–30].

In this work, a novel microwave-assisted steam distillationMASD) extraction method was applied for the determinationf OCPs and pyrethroids in Chinese teas. MASD technique,hich uses water as the extraction solvent, is a combinationf MAE and steam distillation method. The samples of Chi-ese teas, which were immersed in the water solvent, wereeated by microwave irradiation. Under this condition, the ana-ytes (OCPs and pyrethroids) were desorbed from the matrixith water vapor, and absorbed by the nonpolar organic sol-ent covered on the water. Then the extracts with very littlelean-up were analyzed by GC/ECD. This method gives rel-tively clean extracts in comparison with conventional MAEsing organic solvent. In MAE, direct contact between organicolvent and sample matrix often causes low selectivity for thextracts, but mainly hydrophobic and volatile or semi-volatileompounds are recovered by the combination with steam distil-ation [32–34], in some cases, even clean-up is unnecessary forC analysis [31]. In addition, MAE and steam distillation are

he coinstantaneous process in MASD, so the total extractionime of MASD is relatively short compared to the MAE–SPME35–37] or MAE–LPME technique. Jasmine tea, Longjing teand Maofeng tea were analyzed by MASD–GC/ECD in thiseport, and the effects of the extraction conditions on effi-iency and a comparison between this method and USE are alsoresented.

. Experimental

.1. Reagent and samples

Distilled water was purified by a Milli-Q system (Milford,A, USA). Analytical grade acetone, n-hexane, n-heptane, n-

ctane, cyclohexane, Florisil and activated carbon were fromuoyao (Shanghai, China). Pesticide standards of OCPs (�-CH, �-HCH, dicofol, p,p′-DDE, p,p′-DDT) and pyrethroids

bifenthrin, fenvalate) were obtained from Supelco (Bellefonte,A, USA). Stock solutions were prepared for each compoundespectively in hexane with a concentration of 100 �g/ml. Hep-

achlor, also obtained from Supelco, was used as internal stan-ard (IS) in the study, for it was not detected in the samples ofhinese teas. Prepared stock solution of IS was 10 �g/ml. All

he stock solutions were stored in a refrigerator at 4 ◦C. StandardCh

Fig. 1. Extraction apparatus of the microwave-assisted steam distillation.

olutions of analytes at the concentration level of interest wererepared daily from the stock solution, and 100 ng of IS wasdded in each solution.

Three kinds of Chinese teas, Jasmine tea, Longjing tea andaofeng tea, were purchased from local supermarket (Shanghai,hina). All of them are classified into green tea according to theegree of fermentation (unfermented).

.2. Microwave-assisted steam distillation procedure

The apparatus of MASD used in this study is shown in Fig. 1.fter addition of 200 ng heptachlor, 2 g Jasmine tea sample wasut into a 100 ml round-bottom flask containing 25 ml distilledater. After the sample soaked up the water in a few minutes,0 ml of nonpolar organic solvent used for analyte-trapping sol-ent, such as n-hexane, n-heptane, n-octane and cyclohexane,as added to the flask to cover the distilled water. Then theask was put into a MO-2270M1 model microwave oven (Haier,ingdao, China). The flask with the tea sample was heated bymicrowave at the power of 100–400 W for 2–6 min, and a

ondenser with a continuous flow of freezing water was usedo condense the water vapour, so that the water could take partn the extraction process repeatedly. Upon the termination ofhe microwave irradiation, the flask was cooled in the air. Afterooling, the organic solvent was removed and concentrated toml using a rotary evaporator without heating. Then, the con-entrated extract was passed through a glass extraction cartridge8 mm i.d.) packed with 1.0 g anhydrous sodium sulfate, 0.4 gctivated carbon and 0.5 g activated Florisil, which was rinsedith n-hexane after packing. The analytes were eluted from

he column with 10 ml acetone–n-hexane solvent (1:1, v/v) andollected in a 50 ml pear-shaped flask. And the eluate was con-entrated to 1 ml by a rotary evaporator.

.3. Ultrasonic extraction procedure

An ultrasonator model DS-5510 (Shangchao, Shanghai,hina) was used in USE procedure. After addition of 200 ngeptachlor, 2 g Jasmine tea sample was put into a 50 ml vial

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Chromacol, Herts, UK), and the vial was sealed with an alu-inum cap with PTFE-silicon septa. About 20 ml acetone–n-

exane (1:1, v/v) was then put into the vial. One hour latter, theial with sample completely immersed in the solvent was soni-ated with continuous power for 30 min. After a 15 min coolingtep, the raw extract was transferred to a pear-shaped flask. Theemained teas were washed three times with 1 ml portions of n-exane. The combined solution of raw extract and wash solutionas filtered and concentrated to 1 ml using a rotary evapora-

or without heating. Then the concentrated extract was passedhrough a solid phase extraction (SPE) cartridge (15 mm i.d.)acked with 1.0 g anhydrous sodium sulfate, 3.0 g activated car-on and 4.0 g activated Florisil, which was rinsed with n-hexanefter packing. The analytes were eluted from the column with0 ml acetone–n-hexane solvent (1:1, v/v) and collected in a0 ml pear-shaped flask. And the eluate was concentrated toml by a rotary evaporator.

.4. Instruments

Analyses of OCPs and pyrethroids in the extracts were per-ormed using a gas chromatograph system CP-3800 (Varian,alo Alto, USA) with electron capture detector. The GC/ECDystem was equipped with a CP-Sil 5 CB fused silica cap-llary column (15 m × 0.25 mm i.d., 0.25 �m film thickness),nd nitrogen (99.999%, purity) was used as the carrier gas atml/min flow rate. The injection port temperature was set at80 ◦C, and detector temperature was 330 ◦C. The injector wasperated in splitless mode, and an amount of 1 �l concentratedxtract was injected. Analytes were separated an oven temper-ture program built as follows: initial temperature 75 ◦C (holdt 2 min), increased at 25–200 ◦C/min (hold 2 min) and finallyncreased at 6–280 ◦C/min (hold 8 min).

.5. Analytical data

Linear range for the analytes was studied by replicate analy-is of the standard solutions (from 0.1 to 2000 ng/ml) spikedith 100.0 ng/ml heptachlor (IS), and 1 �l standard solutionas directly injected and analyzed by GC/ECD system. The

inear regression values were calculated with the average peakreas of three replicate injections for each analyte. Recoveries ofhe analytes were studied with Jasmine tea spiked with 63.4 ng-HCH, 80.2 ng �-HCH, 913.6 ng dicofol, 78.0 ng p,p′-DDE,49.2 ng p,p′-DDT, 861.0 ng bifenthrin, 739.2 ng fenvalate and00.0 ng heptachlor (IS). And apparent recoveries of all the ana-ytes using MASD were compared with the recoveries obtainedy USE. Relative standard deviation (R.S.D.) was studied by thenalysis of spiked Jasmine tea sample. Also, the peak height ofach compound of the spiked Jasmine tea sample was used toalculate the detection limit (S/N = 3) of this work.

. Results and discussion

.1. The extraction device

Fig. 1 shows the extraction device of MASD used in thistudy. Because microwave was not absorbed by the nonpolar

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2007) 1068–1074

rganic solvent, it penetrated the nonpolar solvent and directlyeated the water solvent with the tea sample immersed in. Then,he analytes (OCPs and pyrethroids) in the sample were des-rbed from the matrix, and absorbed by the nonpolar organicolvent covered on the water. It was supposed that the waterapor accelerated vaporization of the volatile and semi-volatileolvent, including OCPs and pyrethroids. Then the water wasondensed on the inner face of the condenser and took partn the extraction repeatedly. Through this process, OCPs andyrethroid molecules carried by water vapor were trapped inhe nonpolar organic solvent without direct contact between therganic solvent and the tea sample. After extraction, the greenolor of the organic solvent was much lighter than that obtainedy the extraction with direct contact between organic solventnd the sample, such as USE and MAE. And the distilled watersed as extraction solvent was dark yellow and turbid, with smallediment and particles in it. So the MASD is a relatively selec-ive extraction method, and mainly hydrophobic and volatile oremi-volatile compounds are recovered by the extraction pro-ess.

Enough amount of water was supposed to be used in thisASD procedure in order to keep the tea sample from direct

ontact with the organic solvent. And a comparatively shorticrowave irradiation time was used to prevent the excessive

aporization of water for the same reason. In this work, 25 mlater was used and it proved to be a proper amount to the extrac-

ion in the subsequent experiment, for it reached the equilibriumetween vaporization and condensation during the extractionrocess. The extraction time used in MASD was from 2 to 6 min,nd the effect of extraction time will be discussed later.

The vaporization of a part of nonpolar organic solvent coveredn the water was also accelerated by the water vapor during thextraction process, and it resulted in poor extraction efficiencyf the ratio of vaporized solvent was too high. In the work, 20 mlonpolar organic solvent, a relatively large amount of solvent,as used to avoid this influence. Also, a continuous flow of

reezing water was used for the condenser to accelerate the speedf condensation.

.2. Optimization of MASD procedure

The effects of organic extraction solvent, microwave powernd microwave extraction time of MASD were investigated inhe study. Jasmine tea sample spiked with 200.0 ng heptachlorIS) were treated through MASD method described above. Thextraction efficiency of dicofol, p,p′-DDE, bifenthrin and fen-alate in the sample were investigated and the analytical resultsbtained by GC/ECD measurement under different conditionsere compared to obtain the optimum extraction conditions.

.2.1. Selection of extraction solventIn the proposed method, the extraction solvent should have

ittle capacity of microwave absorption, and good extraction

fficiencies for the analytes. Nonpolar solvent can absorb lessicrowave energy than polar solvent, so, the nonpolar organic

olvent was considered. Effects of the organic extractionolvent on dicofol, p,p′-DDE (OCPs) and bifenthrin, fenvalate

J. Ji et al. / Talanta 71 (2007) 1068–1074 1071

Fig. 2. Effect of different analyte-trapping solvent for the MASD process of theJCp

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asmine tea. Solvent: cyclohexane (a), n-hexane (b), n-octane (c), n-heptane (d).ompounds: dicofol (1), p,p′-DDE (2), bifenthrin (3), fenvalate (4). Microwaveower: 230 W, microwave extraction time: 2 min.

pyrethroids) analysis in Jasmine tea are shown in Fig. 2.our kinds of different nonpolar organic solvent (cyclohexane,-hexane, n-octane and n-heptane) were used as the analyte-rapping solvent of the MASD (microwave power: 230 W,

icrowave extraction time: 2 min). Results obtained by theSE–GC/ECD measurement were compared, and the solvent

hat gave the best extraction efficiencies of the four pesticidesas select. Aromatic hydrocarbon such as toluene was not

nvestigated in this study because of the low recovery yields.t might be caused by the absorption of microwave irradiation31]. Observed concentrations in Fig. 2 are the mean valuesf triplicate measurements (error bars, S.D.), and they arehown as the relative value normalized to the values obtainedy n-hexane. Although it gave the best extraction efficiencyf dicofol while n-octane was used as the trapping solvent, thextraction efficiency of pyrethroids was poor compared to thether solvents. Among the four solvents, n-heptane was selecteds the organic trapping solvent for the further study, because itelatively gave the best extraction efficiency of the four analytestudied. But the reason for the difference in the extractionfficiency of the organic solvent tested was not clear yet.

.2.2. Microwave extraction time and powerThe effects of microwave extraction time and microwave

ower on the analytical results are shown in Fig. 3. n-Heptaneas used for MASD, and the analytical results (peak areas nor-alized to the IS) of dicofol, p,p′-DDE, bifenthrin and fenvalate

n Jasmine tea obtained by GC/ECD measurement depending

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able 1nalytical performance characteristics of combined MASD–GC/ECD measurement

nalytes R2 Intraday R.S.D. (%) Interday R.S.D. (%)

-HCH 0.9921 8.4 10.9-HCH 0.9974 6.9 9.7icofol 0.9939 4.7 6.6,p′-DDE 0.9922 6.5 8.6,p′-DDT 0.9960 5.1 7.1ifenthrin 0.9948 3.9 5.9envalate 0.9988 2.2 3.7

ig. 3. Effect of microwave power and irradiation time on detector response areaor the MASD process of the Jasmine tea. Compounds: dicofol (a), p,p′-DDEb), bifenthrin (c), fenvalate (d). Analyte-trapping solvent: n-heptane.

n microwave extraction time and microwave power were com-ared. When the microwave power was 100 W, extraction timef 4 min gave the best extraction efficiency. But the peak areasould reach a relative maximum at the extraction time of 2 minhen the microwave power was 200 and 400 W. Then a decreaseas observed for longer exposure times, possibly due to degra-ation of the compounds resulting from higher temperature.eanwhile, response signals observed mostly decreased when

he microwave power increased at the same irradiation time,lso for reasons of the degradation of the analytes. As seen fromig. 3, the best extraction efficiency was obtained at microwaveower of 200 W and extraction time of 2 min. The MASD con-itions were then used for the followed study.

.3. Method validation

Several levels of standard solutions (from 0.1 to 2000 ng/ml)piked with 100.0 ng/ml heptachlor (IS) were directly analyzedy GC/ECD system. The linear range and linear regression

alues were obtained by the analytical results of GC/ECD mea-urement. The accuracy of the determination of OCPs andyrethroids was quantified by the recovery of 2 g Jasmine teaample spiked with standard analytes and IS. The precision

for OCPs and pyrethroids determinations in Jasmine tea sample

LOD (�g/kg) Linear range (ng/ml) Recovery yields (%)

0.025 0.1–100 96.840.049 0.2–200 84.040.21 1–2000 110.10.039 0.2–100 88.200.19 1–1000 98.800.17 1–1000 97.470.23 1–1600 97.79

1072 J. Ji et al. / Talanta 71 (2007) 1068–1074

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ig. 4. GC/ECD chromatograms of the Jasmine tea sample. (a) The Jasmine tea sytes using MASD extraction. Peaks: �-HCH (1), �-HCH (2), dicofol (3), p,p′-Donditions: microwave power: 230 W, microwave extraction time: 2 min. Analy

as expressed by the relative standard deviation (R.S.D.) val-es for both interday and intraday, and the R.S.D. values wereiven in Table 1. LOD was also calculated from the analyti-al results (Table 1). The correlation coefficients of the linearalibration curves for all the analytes were above 0.9921. Therecision of the method evaluated by three replicated determi-ations was less than 9%, and the limits of detection (LODs)alculated via three times the background noise level were below.23 �g/kg.

Fig. 4a and b are the chromatograms of Jasmine teaample and Jasmine tea sample spiked with standard ana-

ytes (as discussed in Section 2.5) respectively obtained by

ASD–GC/ECD measurement. Fenvalate has four isomers, andives two peaks on a CP-Sil 5 CB fused silica capillary column,nd the peak area sum of the two peaks was used to calcu-

tats

e using MASD extraction. (b) The Jasmine tea sample spiked with standard ana-), p,p′-DDT (5), bifenthrin (6), fenvalate (7), heptachlor (IS). Chromatographic

pping solvent: n-heptane.

ate the fenvalate concentration. The method was then appliedo the determination of OCPs and pyrethroids in Longjing teand Maofeng tea separately. The concentrations of OCPs andyrethroids determined in the three kinds of Chinese tea samplesre summarized in Table 2. The existences of these compoundsere identified by the retention time. Dicofol, bifenthrin and fen-alate were the three compounds detected in all the tea samples,nd they were proved to be three kinds of the most widely usedesticides in the teas, for the purpose of eliminating the mite andther pests. On the other hand, HCHs that used to be the OCPsften used in the vegetables and fruits were not found in either

ea sample. The concentrations of dicofol, p,p′-DDE, bifenthrinnd fenvalate in the Jasmine tea sample were the highest in thehree kinds of teas, and it could be seen that the Jasmine teaample was seriously polluted.

J. Ji et al. / Talanta 71 (2007) 1068–1074 1073

Table 2The concentration of OCPs and pyrethroids in three kinds of Chinese tea sample

Analytes Retention timea (min) Jasmine tea (�g/kg) Longjing tea (�g/kg) Maofeng tea (�g/kg)

�-HCH 7.156 ndb nd nd�-HCH 7.469 nd nd ndDicofol 9.170 991.8 ± 22.4 53.4 ± 2.4 157.3 ± 6.9p,p′-DDE 11.186 11.1 ± 0.9 nd ndp,p′-DDT 12.022 nd 4.1 ± 0.2 ndBifenthrin 14.662 203.0 ± 6.4 17.6 ± 1.1 86.8 ± 4.3Fenvalate 19.675, 20.063c 316.8 ± 10.7 73.8 ± 2.7 8.2 ± 0.3

a As retention time of IS was 8.597 min.b nd, lower than detection limits (S/N = 3).c Fenvalate has four isomers, and gives two peaks on a CP-Sil 5 CB fused silica capillary column.

Table 3Comparative study of OCPs and pyrethroids concentrations and recoveries using different extraction methods (Jasmine tea sample)

Analytes MASD USE

Concentrations (�g/kg) Recovery yields (%) Concentrations (�g/kg) Recovery yields (%)

�-HCH nda 96.84 1.5 75.39�-HCH nd 84.04 nd 63.84Dicofol 991.8 110.1 846.0 81.15p,p′-DDE 11.1 88.20 12.6 44.36p,p′-DDT nd 98.80 nd 63.37BF

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a nd, lower than detection limits (S/N = 3).

.4. Comparison with ultrasonic extraction

The chromatogram of Jasmine tea sample obtained by theSE–GC/ECD measurement was also studied. Response signalsecreased obviously compared to the chromatogram obtained byhe MASD for the relatively poor extraction efficiency. The com-arison of OCPs and pyrethroids concentrations and recoveriesetween MASD and USE is in Table 3. Although the concen-rations of OCPs showed nearly no difference between the twoxtraction methods, it resulted in comparatively low concentra-ions of pyrethroids when the USE was used as the extraction

ethod. The recovery yields of all the analytes by the USEere also lower than the yields by the MASD as described inable 3.

. Conclusions

In the study, the MASD technique followed by GC/ECDeasurement had been successfully applied to the determina-

ion of OCPs and pyrethroids in the Chinese teas. Because ofhe relatively greater sample throughput, lower method detectionimit, less extraction time and simplification of the clean-up pro-ess (the lack of necessity of solid–liquid separation and smallmounts of sorbents required for clean-up), the MASD tech-ique has the advantage of the conventional extraction methods

irectly using organic solvent as extraction solvent, such as theSE. Therefore, a simple technique for determination of pesti-

ides like OCPs and pyrethroids can be performed by the MASDethod.

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cknowledgement

This work was supported by Shanghai Research Projectf Science and Technology on Scientific Instrument, Natu-al Science Foundation of China (Project No. 39870451) andational Basic Research Priorities Programme (Project No.001CB510202).

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