Determination of endosulfan isomers and their metabolites in tap water and commercial samples using microextraction by packed sorbent and GC-MS

966 J. Sep. Sci. 2014, 37, 966–973

Ramandeep KaurSusheela RaniAshok Kumar MalikJatinder Singh Aulakh

Department of Chemistry,Punjabi University, Patiala, India

Received October 22, 2013Revised January 22, 2014Accepted February 3, 2014

Research Article

Determination of endosulfan isomers andtheir metabolites in tap water andcommercial samples using microextractionby packed sorbent and GC–MS

A simple, rapid, accurate and sensitive method using microextraction by packed sorbent(MEPS) followed by GC–MS has been pursued for the determination of organochlorineinsecticide endosulfan isomers (� and �) and their metabolites (ether, lactone and sulfate).MEPS is a miniaturised version of SPE employing C18 packing material. It is very efficienttechnique as it employs as low as 10 �L of sample volume. The distinct feature of MEPS isthe magnitude of the elution volume that could be directly injected to GC system. Variousparameters such as extraction cycles, washing solvent, elution solvent, elution volume andpH, which influenced the MEPS performance, were tested and optimised. The calibrationcurves were obtained in the concentration range 1–500 ng/mL. The results showed a closecorrelation coefficient (R2 > 0.991) for all analytes in the calibration range studied. TheLOD and LOQ obtained for GC–MS under selected ion monitoring acquisition are between0.0038–0.01 and 0.0125–0.033 ng/mL, respectively. The developed method is applicablefor the quantification of these compounds in tap water and commercial samples. Thismethod has been shown to be selective as no interferences from endogenous substanceswere detected by analysis. This method not only decreases sample preparation time but ischeaper, eco-friendly and easier to perform compared to traditional techniques.

Keywords: Endosulfan / GC–MS / Microextraction by packed sorbent /Organochlorine insecticidesDOI 10.1002/jssc.201301154

� Additional supporting information may be found in the online version of this articleat the publisher’s web-site

1 Introduction

Pesticides have become an indispensible part of mod-ern agriculture. Endosulfan [1,4,5,6,7,7-hexachloro-8, 9,10-trinorborn-5-en-2,3-ylenebismethylenes sulfite] is a broad-spectrum chlorinated cyclodiene insecticide widely appliedto fruits, vegetables, cereals and beverages [1]. Commercialendosulfan is composed of two isomers (� and �) in theratio of 7:3. It undergoes oxidation to form metabolites en-dosulfan ether, endosulfan lactone and endosulfan sulfate [2](Fig. 1). Endosulfan is commercially available as thiodan, cy-clodan, thimol, thiofer and malix [3]. It is of environmentaland public concern due to its liposolubility, persistence, toxic-ity, bioaccumulation, endocrine disruption and carcinogenicnature [1,4,5]. As it is a primary pollutant and has pernicious

Correspondence: Dr. Ashok Kumar Malik, Department of Chem-istry, Punjabi University, Patiala 147002, IndiaE-mail: [email protected]

Abbreviations: BIN, barrel insert and needle; MEPS, microex-traction by packed sorbent; SPME, solid-phase microextrac-tion

nature, ban on its use has been enforced by the StockholmConvention [6]. However, it is still found in the environmentdue to its persistence and lipophilic character [7]. Therefore,it is important to analyse them in edible and environmentalsamples and to develop sensitive and accurate methods fortheir detection. This study has been focused on the detectionof endosulfan isomers � and � and their metabolites ether,lactone and sulfate.

Sampling and sample preparation consuming 80% ofthe time determine the success of the analysed compoundsof interest in the matrices. Therefore, choice of sample prepa-ration method has great influence on the reliability and accu-racy [8]. During the last few years, several methods have beenadopted for the determination of endosulfan and its metabo-lites by GC–MS and GC with electron capture detection pre-ceded by various preconcentration methods such as Soxh-let extraction [9, 10], ultrasound-assisted extraction [11–13],pressurised liquid extraction or accelerated solvent extrac-tion [1, 14–20], supercritical fluid extraction [21], quick, easy,cheap, effective, rugged and safe method [7, 22–25], liquid-phase microextraction [26, 27], SPE [28, 29], and solid-phasemicroextraction (SPME) [30–35] in different matrices. Al-though these techniques are very useful, the current need

C© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

J. Sep. Sci. 2014, 37, 966–973 Gas Chromatography 967

Figure 1. Structures of endosulfan isomers and metabolites.

is to develop a more selective sorbent that consumes lesssolvent for sample clean-up and enrichment.

Microextraction by packed sorbent (MEPS) is an excellentalternative. MEPS is a miniaturised version of conventionalSPE, which can be fully automated without any modifica-tion of chromatographic apparatus [36–40]. MEPS has beentermed as “LC column in a syringe” [41]. It is constructed as abarrel insert and needle (BIN) containing a SPE sorbent bed(4 mg) on which sample pretreatment takes place. The BIN inturn is fixed and sealed to a 100 or 250 �L syringe. The com-monly used sorbents in MEPS are silica based (C2, C8, C18),strong cation exchanger using sulfonic acid bonded silica,restricted access materials, carbon, polystyrene-divinyl ben-zene copolymer or molecularly imprinted polymers [41–43].Even nanofibres have been used as a sorbent in MEPS for thedetermination of organophosphorus pesticides [44]. MEPScan be used for multiple extractions 100 times for plasmaand urine samples and more than 400 times for water. It re-duces preparation time and solvent volume from millilitresto microlitres. It is very accurate, fast (2 min) and gives goodrecoveries (>70%) even at very low concentration. As MEPSuses a very low volume of the eluting solvent, it can be di-rectly injected to the instrument [45–48]. We have coupledthis method with GC–MS for the analysis of endosulfan, asit not only minimises the consumption of solvent but is eco-friendly and improves the sensitivity of analytical protocols.

2 Materials and methods

2.1 Analytical reference standards and reagents

Pesticides standards of endosulfan isomers � and � andmetabolites ether, lactone and sulfate have been obtainedfrom Reidel de haёn (Germany). LC-grade methanol, acetoni-trile, chloroform, toluene and ethyl acetate were purchasedfrom Rankem (New Delhi, India). Water to be used for experi-mentation was deionised (Riviera, SCHOTT DURAN, Mainz,Germany). Solvents to be used were filtered with 0.45 �m Ny-lon 66 membranes (Rankem).

Individual stock solutions having concentration 1 mg/mLwere prepared in acetonitrile and stored in refrigerator at−4�C. Standard solutions were obtained by diluting stocksolution with acetonitrile. Stock solutions were stable for atleast six months when stored at −4�C.

2.2 GC–MS instrumentation and conditions

GC–MS system with model GC–MS-QP2010 Plus (ShimadzuCorporation, Kyoto, Japan) was used for the analysis. The cap-illary column used in the GC was Rtx-1MS (30 m × 0.2 mm ×0.25 �m) supplied by Restek U.S. (Bellefonte, PA, USA).Chromatographic data were collected and recorded by GC–MS real-time analysis software. Sample injection was done insplit mode (split ratio 5:1). Helium was used as the carrier gasat a flow rate of 1 mL/min. The GC injector temperature wasset at 270�C. The column oven temperature was optimisedto hold at 120�C for 1 min and then to increase by 25�C/minup to 230�C holding for 2 min, then increase by 15�C/minup to 280�C holding for 4 min and then finally increased by3�C/min to 300�C holding it for 2 min.

MS conditions were as follows: electron ionisation sourceset at 70 eV, MS source temperature 230�C and solvent cuttime was 3.0 min. The mass spectrometer was run in full scanmode (m/z 50–500) and in selected ion monitoring mode. Thecharacteristic ions selected for qualitative and quantitativestudies are listed in Table 1. The quantitation of samples wasdone by using the selected ion monitoring mode. The totalruntime was 23.40 min.

Table 1. Retention time and ions selected for monitoring of en-dosulfan isomers and metabolites

Analyte Acronym Retentiontime (min)

Monitoringions m/z

Endosulfan ether EE 6.4 241a), 272, 207Endosulfan lactone EL 7.8 241a), 275, 321Endosulfan-� E-� 8.4 207a), 195, 241Endosulfan-� E-� 9.4 207a), 195, 241Endosulfan sulfate ES 10.0 272a), 229, 387

a) Most abundant ion.


968 R. Kaur et al. J. Sep. Sci. 2014, 37, 966–973

Figure 2. GC–MS fragmentation of (A) endosulfan ether, (B) endosulfan lactone, (C) endosulfan-� and endosulfan-� and (D) endosulfansulfate.



Figure 3. MEPS–GC–MS chromatogram of a standard solutioncontaining 1 ng/mL of endosulfan ether (EE), E-�, E-�, endosul-fan lactone (EL) and endosulfan sulfate (ES). Pump cycles: 15 ×50 �L; washing solvent: water; eluent: chloroform; eluent volume:3 × 10 �L

2.3 Sample preparation

2.3.1 Tap water samples

Tap water samples needed for experimentation were takenfrom our lab in Pyrex borosilicate amber glass containers,which had been rinsed with triply distilled water. It was thenfiltered through 0.45 �m pore size Nylon-66 membrane filter,degassed for 15 min with an ultrasonic bath and stored at−4�C. Prior to MEPS–GC–MS studies, spiked samples ofdifferent concentration were prepared in 1 mL of water.

2.3.2 Commercial samples

A commercial sample was prepared in tap water, which hadbeen already filtered having concentration 1 mg/mL. It wasthen sonicated for 15 min for degassing and the requiredconcentration was then prepared from this stock solution.

2.4 MEPS conditions

MEPS was performed on a BIN assembly containing 4 mgof solid-phase silica-C18 material, inserted into a 250 �L gas-tight syringe from SGE Analytical Science (Melbourne, Aus-tralia). This sorbent has irregular particles with an averagesize of 45 �m and nominal porosity 60 Å. Before being usedfor the first time, the sorbent was manually conditioned with100 �L acetonitrile followed by 100 �L water. The volumes ofacetonitrile and water were drawn up and then discardedevery time at an approximate flow rate of about 20 �L/s(±5 �L/s).

The tap water and commercial sample (50 �L each) weredrawn through the syringe 15 times manually. It is importantthat samples are drawn slowly (approximately 20 ± 5 �L/s)and with caution to obtain good percolation between sampleand solid support. The sorbent was then washed once with50 �L of water to remove impurities and other interferences.The analytes were then eluted with 3 × 10 �L of chloroforminto a vial and then 1 �L of it was injected into GC injector.The sorbent was used for multiple extractions. The multiplepulling/pushing of the sample by the syringe increases theextraction recovery. To increase the sorbent lifetime, the C18

adsorbent in the BIN assembly was washed with acetonitrile(4 × 50 �L) and water (4 × 50 �L). This step serves not onlyas conditioning step but also decreases memory effects. Thesame packing bed can also be used for 400 extractions beforebeing discarded.

3 Results and discussion

3.1 GC–MS analysis

A GC–MS method has been developed and optimised forextraction recovery of endosulfan isomers and metabolitesin spiked tap water and commercial sample. The developedGC–MS method was optimised for column temperature pro-gram, flow rate of carrier gas and temperatures for injection,ion source and interface. Separation of analytes was accom-plished within 10 min though the total program runtimewas 23.40 min. A compound can be easily identified with itsunique molecular fragment ion even if they co-elute.

All the major fragments for endosulfan isomers and itsmetabolites are shown in Fig. 2. Also the mass spectrum

Table 2. Linearity parameters of endosulfan isomers and its metabolites

Analyte Linearity range (ng/mL) Equation Regression coefficient R2 LODa) (ng/mL) LOQa) (ng/mL)

EE 1–500 Y = 250.4x+4758 0.998 0.0038 0.0125EL 1–500 Y = 229.2x+9800 0.991 0.004 0.0132E-� 1–500 Y = 212.5x+2926 0.998 0.0045 0.0148E-� 1–500 Y = 98.17x+401.1 0.990 0.01 0.033ES 1–500 Y = 91.10x+1277 0.995 0.0105 0.0346

a) Each result is an average of three replicate determinations.



Table 3. Recovery and repeatability of the method in analysis oftap water and commercial sample

Analyte Amountadded(ng/mL)

Extractionyield (%)a)

Intra-dayRSD (%)a)

Inter-dayRSD (%)a)

EE 1 90.72 3.6 4.210 93.62 3.5 4.0

100 97.61 3.1 3.9250 97.84 2.8 3.5

EL 1 88.45 3.8 4.410 90.90 3.7 4.2

100 91.49 2.9 4.0250 93.27 2.8 3.7

E-� 1 90.24 (90.22) 3.6 (3.7) 4.2 (4.3)10 91.32 (91.34) 3.3 (3.3) 4.1 (4.4)

100 94.43 (93.25) 2.7 (2.9) 3.9 (4.1)250 96.38 (94.78) 2.5 (2.8) 3.3 (3.5)

E-� 1 91.26 (89.32) 4.1 (4.4) 5.1 (5.3)10 89.47 (88.48) 3.9 (4.1) 4.8 (4.9)

100 92.32 (91.32) 3.3 (3.5) 4.3 (4.5)250 93.32 (90.21) 3.1 (3.3) 4.1 (4.4)

ES 1 88.42 3.8 4.110 91.57 3.6 3.9

100 90.13 2.9 3.5250 91.47 2.6 3.1

a) Each value is a mean of six independent assays.Commercial sample values were given in parentheses.The extraction yield was calculated from the analyte peak areafrom spiked tap water and commercial samples compared withthose obtained from the same analyte concentration in standardsolution.

obtained for these analytes are shown in Supporting Infor-mation Fig. S1. The program developed for analysis gavewell-separated peaks for the pesticide within 10 min runtime.Figure 3 shows a GC–MS chromatogram for endosulfan andits metabolites at 1 ng/mL.

3.2 MEPS method development

While performing microextraction in packed syringe,C18 (4 mg) was used as sorbent. Recoveries were evaluated bycomparing the peak area of calibration curve of pure standardand spiked samples. The factors influencing the absolute re-covery are draw/eject speed, sample loading amount, sort ofeluents, elution volume and carryover effects.

In MEPS, number of extraction cycles and the flow ratehave a great influence on the retention of analyte to the sor-bent phase. A little volume of sample can be drawn up anddown through the syringe many times without being dis-carded. The multiple extraction in this experimentation hasbeen carried out by drawing the sample and discarding it. Theeffect of the number of extraction cycles on the extraction ofthe pesticide is clearly illustrated in Supporting InformationFig. S2a. Sample recovery increased as the number of cyclesincreased to 15 after which it decreased as BIN was saturated

Figure 4. MEPS–GC–MS chromatograms of EE, E-�, E-�, endo-sulfan lactone (EL) and endosulfan sulfate (ES) in real samples:(A) blank tap water, (B) spiked with 1 ng/mL of tap water sam-ple by MEPS and (C) commercial sample having concentration of1 ng/mL.



Table 4. Comparison of LODs of endosulfan isomers and metabolites between present study and earlier study

Analyte Method used Sample analysed Sample preparation LOD (ng/mL) Reference

E-�, E-�, EE, ES GC–MS Urine SPE 0.006–0.013 [4]E-�, E-�, ES GC–MS–MS Aqueous SPME 0.3–0.45 [5]E-�, E-� GC–ECD Environmental SPE 0.5 [29]E-�, E-� GC–ECD Aqueous SPME 0.01–0.02 [34]E-�, E-�, EE, ES GC–MS Aqueous MEPS 0.0038–0.01 Present method

ECD, electron capture detection.

and analytes eluted with the loaded sample. So the numberof extraction cycles optimised for these analytes is 15.

Similarly, effect of washing solvent was investigated onthe absolute recovery of the analytes. Acetonitrile, methanol,chloroform and toluene were tried but analytes eluted withthese solvents as it is clear from Supporting InformationFig. S2b. So, 50 �L of water, drawn once at a flow rate ofabout 20 �L/s could meet the required demands as recoveryof the analytes was ≥88%.

Next, effect of desorbing solvent and its volume were op-timised for the effective elution of trapped analytes from thesorbent. The elution solvent should be the one that coulddisplace the target analytes from the sorbent in the lowestvolume. Organic solvents with different polarity and func-tionality were tried, out of which chloroform is the best asshown in Supporting Information Fig. S2c. This is becausethe polarity of chloroform and endosulfan match best whileother solvents have either lower or higher polarity. As chlo-roform showed highest desorption efficiency, so chosen bestdesorption solvent for further analysis. The extraction wascarried out using 1 mL of aqueous sample spiked with tar-get analytes having concentration 10 ng/mL and 30 �L ofdesorption solvent used.

Effect of desorption solvent volume was also studied andit was noticed that analyte response was enhanced with in-creasing desorption solvent volume up to 30 �L after whichit remained almost constant. Out of 30 and 3 × 10 �L, thelatter showed enhanced effect compared with the former. So,3 × 10 �L is chosen as elution volume for further analysisSupporting Information Fig. S2d.

Another factor that plays a significant role is the pH ofthe aqueous solution. The influence of pH on the extractionefficiency of selected analytes from aqueous solution was eval-uated in the pH range 2–10. Initially 10 ng/mL solution had5.7 pH. For MEPS experimentation, pure water spiked withthese pesticides was analysed with the C18 BIN at differentpH values: pH: 2, pH: 4 (both adjusted with 0.1 M HCl)and pH 8 and pH 10 (both adjusted with 0.1 M NaOH solu-tion). From experimentation, it is clear that 5.7 is the best pH(Supporting Information Fig. S2e) where the pesticideshowed maximum peak area. It may be due to the rea-son that endosulfan is stable at this pH only. Underboth acidic and basic pH, it undergoes hydrolysis to formits metabolite. So the quantity of endosulfan isomers de-creases and its metabolites increases at both higher andlower pH.

3.3 Method validation

Calibration curves for spiked tap water and commercial sam-ple were performed in the range 1–500 ng/mL on GC–MSwith seven concentration levels separately. The calibrationcurves were described by the linear regression equation:

y = mx + c (1)

where y is the peak area, x is the concentration, m is the slopeand c is the intercept. Good linearity R2 > 0.991 was obtainedin the selected range (Table 2). LODs were calculated basedon S/N = 3 from spiked samples at low concentrations. LODsof these pesticides ranged between 0.0038 and 0.001 ng/mL.LOQs (S/N = 10) for the analytes studied were in the rangeof 0.0125–0.033 ng/mL.

3.4 Recovery

Extraction yield and precision assays were made at four dif-ferent concentration levels of analytes, i.e. 1, 10, 100 and250 ng/mL. The results of these assays are reported inTable 3. The results are satisfactory with extraction yield val-ues in the range 88–98%. The intra-day precision was alsosatisfactory with RSD values being ≤ 4.4 for all analytes. Theexperiments were conducted six times during the same day toobtain repeatability (intra-day precision) and inter-day preci-sion was obtained by repeating the experiment six times oversix different days. Both the values are expressed as RSD%.

3.5 Carryover and matrix effects

The carryover effect was analysed on the column by com-paring three successive aliquots of standard mixture havinghigh concentration of analyte with three successive aliquotsof extracted blank tap water. A non-significant carryover effect(<0.1%) was evident. Furthermore, assays were examined tomake sure that small quantity of sorbent phase (4 mg) inthe MEPS could be easily and effectively washed before sub-sequent pretreatment by reducing possibility of carryover.MEPS was washed four times with eluting solvent (4 × 50 �Lof chloroform) after each extraction of tap water spiked withhigh concentration of analytes and then injected to the GC–MS system. The chromatograms obtained showed minimalmatrix effect (Fig. 4).



3.6 Comparison of method

The results of the present method were compared with re-sults from other sample preparation technique such as SPEand SPME (Table 4) and LOD is very low as compared to ear-lier published data. This method showed comparable value ofaccuracy and precision with the other methods for the anal-ysis of endosulfan isomers and metabolites. The unbeatableadvantage of this method is reduction in time as comparedwith other methods [4,5,29,34]. Although SPE gives high re-covery, but it uses large volume of sample and eluting solventas compared to MEPS. On the other hand, SPME has lowsensitivity and frequent inability of the fibre to withstand acomplete run. Compared with the methods in the literature,this method presented here is more rapid, simple and robust(Table 4). The key aspect of this method is that it uses as lowas 1 mL of sample volume as compared to 10–30 mL solutionused in other conventional methods.

3.7 Concluding remarks

A new sensitive, accurate, selective, simple MEPS method hasbeen developed for the analysis of the pesticide endosulfan.As it is quick and uses less washing and eluting solvent, sothis method is easily employed for the analysis of pesticides.Also it is a cost-effective method, as the MEPS BIN barrelcan be used for more than 400 times. This method, whichis rapid and simple, can be used to monitor the presenceof this pesticide and its metabolites in environmental andtoxicological studies in order to establish its real effect on theenvironment.

R.K. is thankful to the UGC, New Delhi for financial supportfor this research.

The authors have declared no conflict of interest.

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Determination of endosulfan isomers and their metabolites in tap water and commercial samples using microextraction by packed sorbent and GC-MS