An effective configuration for automated magnetic micro solid-phase extraction of phenylurea herbicides from water samples followed by high-performance liquid chromatography

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An effective configuration for automated magnetic micro solid-phase extraction of phenylurea herbicides from water samples followed by high-performance liquid chromatography Mohammad Saraji , Leila Mohammadipour , Narges Mehrafza PII: DOI: Reference:

S0021-9673(19)31300-7 https://doi.org/10.1016/j.chroma.2019.460829 CHROMA 460829

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Journal of Chromatography A

Received date: Revised date: Accepted date:

20 September 2019 25 December 2019 25 December 2019

Please cite this article as: Mohammad Saraji , Leila Mohammadipour , Narges Mehrafza , An effective configuration for automated magnetic micro solid-phase extraction of phenylurea herbicides from water samples followed by high-performance liquid chromatography, Journal of Chromatography A (2019), doi: https://doi.org/10.1016/j.chroma.2019.460829

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ighlights 

A new automated MSPE method was introduced.



Sorbent was placed inside of a tube located near the pole of a cylindrical magnet.



No frit was used for immobilization of sorbent inside of the extraction tube.



The MSPE method was applied for the determination of phenylurea herbicides.



The method was applicable for environmental water samples analysis.

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An effective configuration for automated magnetic micro solid-phase extraction of phenylurea herbicides from water samples followed by high-performance liquid chromatography

Mohammad Saraji*, Leila Mohammadipour and Narges Mehrafza

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran.

Correspondence: Mohammad Saraji, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran. Phone: +98 31 33913248. Fax: +98 31 33912350. E-mail: [email protected], [email protected]

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Abstract In this study, a new automated magnetic micro solid-phase extraction was introduced. A Tygon tube was folded and fixed around the pole of a cylindrical magnet. Nanosized magnetic sorbents modified with diphenyldichlorosilane were uniformly immobilized on one side of the inner wall of the tube. Sample solution and desorption solvent were passed through the tube using a peristaltic pump. Four phenylurea herbicides (tebuthiuron, monolinuron, isoproturon, and monuron) were used as the model compounds to evaluate the method performance. HPLC-UV was used to separate and quantify the analytes. The effective parameters influencing the performance of the extraction process (i.e., extraction and desorption flow rates, eluent and sample volumes, type of eluent and sample ionic strength) were investigated. The limit of detection was 0.04 μg L-1 for all studied compounds. Calibration curves were linear in the range of 0.1-500 μg L-1 with a determination coefficient between 0.9988 and 0.9999. Intra-day, inter-day and batch-to-batch precisions expressed as relative standard deviation were less than 7%. Several environmental water samples were investigated to assess the applicability of the method for real sample analysis.

Keywords: automated micro solid-phase extraction, nanosized magnetic sorbent, phenylurea herbicides

3

1. Introduction Matrix effects and trace concentration of analytes hinder the direct analysis of real samples. Sample preparation is an important and inseparable part of an analytical method that helps to reduce the adverse effect of matrix components on the analysis and analyte enrichment [1]. Magnetic solid-phase extraction (MSPE) using nanosized sorbent provides a simple and efficient extraction from a large sample volume in a short time. Another advantage of MSPE is recycling and reusing sorbent in subsequent extractions [2]. Compared with SPE, MSPE resolves the problems related to the sorbent packing including bed clogging and back pressure. Magnetic properties of sorbent eliminate the centrifugation step in extraction process [3-6]. Automation of MSPE increases analysis throughput, improves reproducibility and provides the direct coupling of the extraction process with detection systems. In addition, automation minimizes the operator’s errors that occurred during the experiments. Sample contamination and analyte adsorption on glass vial are also reduced in the automated systems [7,8]. Two general approaches have been used for the automation of MSPE. In a few reports [9-11], magnetic nanoparticles (MNPs) were dispersed in the sample solution by loading the sample into an extraction vial at a high flow rate while the sample solution was stirring. Due to enhancing contact surface between sample and sorbent, extraction is performed in a short time. The method requires a complex experimental setup to perform the extraction. Another approach is passing sample solution through MNPs packed in a microcolumn or tube using a peristaltic or syringe pump. An external magnet is placed around the minicolumn to prevent the outflow of MNPs. Compared to the first approach (i.e., sorbent dispersion-based method), the extraction system has a simpler setup. Also, the low eluent volume used in this technique provided a higher analyte enrichment. Due to negligible sorbent loss during repetitive extractions, the sorbent can be reused many times. The agglomeration of the sorbent packed

4

in the minicolumn, bed clogging and column back pressure are the main disadvantages of this technique [12-15]. In some studies, only one magnet was used to retain the sorbent inside the minicolumn [11,13]. In these cases, two frits should be used to immobilize the sorbent inside the column. Moreover, the distribution of magnetic particles inside of the column was not uniform, resulting in poor reproducibility and low recovery of the analytes [13]. To provide a more uniform magnetic field around the minicolumn, two permanent magnets were placed on each side of the packed column [13,15]. With this configuration, no need to use frits or plugs to hold the magnetic particles into the minicolumn. However, the preferential flow-through channels created due to the agglomeration of magnetic nanoparticles, resulted in low extraction efficiency [16]. Very recently, an effort was made to use an external magnetic ring outside of a stainless tube [17]. However, the magnetic field inside the ring magnet is not uniform, and sorbent materials would not be uniformly distributed in the tube [18]. In this work, an automated MSPE method using a Tygon tube folded near the pole of a cylindrical magnet was introduced. Due to the high strength of the magnetic field at the poles, the tube was fixed around the pole. In this configuration, a uniform high strength magnetic field was applied to the tube which led to the uniform immobilization of the sorbent on one side of the inner wall of the tube. The high strength field reduced the loss of sorbent during the extraction/desorption process. Moreover, agglomeration of particles and other disadvantages of a packed column such as creating flow-through channels are avoided. Fe3O4 magnetic nanoparticles modified with diphenyldichlorosilane (Fe3O4@SiO2-DPDCS MNPs) were synthesized and used as a sorbent. The sorbent was loaded in the tube and sample solution was passed through the extraction tube using a peristaltic pump. Four phenylurea herbicides (PUHs) including tebuthiuron, monolinuron, isoproturon, and monuron were used as the model analytes. The eluted analytes from the extraction tube were detected by HPLC-UV. The effective extraction parameters (extraction and desorption flow rate,

5

eluent and sample volume, type of eluent and sample ionic strength) were studied and optimized. The applicability of the method was assessed by analyzing environmental water samples.

2. Experimental 2.1. Materials Ferric chloride hexahydrate (FeCl3.6H2O, >99%), ferrous chloride tetrahydrate (FeCl2.4H2O), ammonia solution (25% w/w), hydrochloric acid and diphenyldichlorosilane (DPDCS) were purchased from Merck (Darmstadt, Germany). HPLC grade methanol, acetonitrile, acetone, and 2-propanol were obtained from Daejung (Shiheung, Korea). Sodium hydroxide (>99%) and sodium metasilicate (Na2SiO3) were purchased from Sigma-Aldrich (Buchs, Switzerland). Tebuthiuron, monolinuron, isoproturon, and monuron were supplied by Dr. Ehrenstorfer GmbH (Augsburg, Germany).

2.2. Apparatus and chromatographic conditions An Ismatec peristaltic pump (model ISM444, Wertheim, Germany) was used to pass sample solution and desorption solvent through the sorbent. A Tygon MHSL 2001 tube (Ismatec Co., Wertheim, Germany) with 0.64 mm I.D. was used to deliver solvents and samples. The sorbent was loaded on a Tygon tube (Tygon MHSL 2001, I.D., 1.52 mm and O.D., 3.24 mm). Fourier transform infrared spectroscopy (Jasco-FTIR-350, Japan) was used to record IR spectra. To study the morphology and surface characteristic of MNPs, a field emission scanning electron microscope (FE-SEM, model MIRA3TESCAN-XMU from TESCAN FESEM instrument, Kohoutovice, Czech Republic) was applied. A high-performance liquid chromatograph (RIGOL, China) included an online degasser, a quaternary pump, and a UVVis detector was used for the separation and detection of analytes. PUHs were separated 6

using an analytical column (LiChrospher 100RP-18, 5-μm, 125 × 4-mm) purchased from Merck (Darmstadt, Germany). A guard column (10×4 mm) with the same packing material was connected to the analytical column. Water:acetonitrile (1:1) was used as mobile phase at a flow rate of 1 mL min-1. The detector wavelength was set at 245 nm for tebuthiuron and monuron, and 250 nm for isoproturon and monolinuron.

2.3. Sample preparation A stock solution containing the analytes at the concentration of 400 mg L-1 was prepared in methanol and kept in the refrigerator. Diluted standard solutions at known concentrations were daily prepared in water. River water samples were collected from Zarjob river (Rasht, Iran) and Zayandeh-rood river (Isfahan, Iran). Agricultural wastewater was collected from an area around Isfahan city (Isfahan, Iran). Samples were filtered before use.

2.4. Synthesis of organosilane coated Fe3O4/SiO2 nanoparticles For the synthesis of magnetic nanoparticles, 2.00 g of ferric chloride hexahydrate and 0.844 g of ferrous chloride tetrahydrate were added to 75 mL of water (85 ºC). Then, 5.6 mL of ammonia solution (25%, w/w) was rapidly added to the Fe2+/Fe3+ solution and stirred (800 rpm) under nitrogen atmosphere for 0.5 h. The Fe3O4 nanoparticles were washed several times with water. The MNPs were added to 60 mL solution containing 0.2 g of NaOH, and 0.9785 g of Na2SiO3 and the mixture was sonicated for 5 min. The pH of the suspension was adjusted to 6 with 2 M HCl, and Fe3O4@SiO2 nanoparticles were washed several times with pure water and once with methanol [19]. Fabricated Fe3O4@SiO2 nanoparticles were dried at room temperature. For the preparation of Fe3O4@SiO2-DPDCS MNPs, 0.5 g of Fe3O4@SiO2 MNPs were ultrasonically suspended in 10 mL of toluene, and then 1.0 mL of the DPDCS was added into the solution. The mixture was shaken for 12 h and Fe3O4 @SiO2-DPDCS

7

MNPs were then washed several times with toluene and methanol and dried at room temperature.

2.5. Extraction setup The extraction setup is schematically shown in Fig. 1. As shown, a peristaltic pump tube was folded around a cylindrical magnet (power 1.4 Tesla; length, 40 mm and O.D., 20 mm) and fixed using a fast curing glue (Alfa fix, Istanbul, Turkey). An amount of 50 mg of Fe3O4@SiO2-DPDCS MNPs was added to 20 mL of water in a test tube and sonicated for 2 min. Using a 1-mL polypropylene syringe the sorbent was introduced into the tube by very slowly pushing the plunger of the syringe. The loading procedure was repeated until complete transferring the sorbent into the extraction tube. The sorbent was uniformly loaded on the inner wall of the tube.

2.6. Automated MSPE procedure The pump tube was connected to the sorbent loaded tube (extraction tube) using a piece of Teflon tube. To perform extraction, a volume of 20 mL of an aqueous sample containing analytes and 4 g of sodium sulfate was added into a 25-mL test tube. After dissolving sodium sulfate, the sample was passed through the extraction tube at the flow rate of 680 µL min-1. Then, air was passed through the tube for 5 min. Analyte desorption was performed using 0.5 mL of methanol at the flow rate of 50 µL min-1. The solvent was evaporated to the volume of 250 µL using a mild stream of nitrogen gas. Finally, 20 µL of the extract was injected into the HPLC system. Before each extraction, the sorbent was washed by passing 500 µL of methanol at a flow rate of 50 µL min-1.

3. Results and discussion 8

3.1. Characteristics of MNPs The FT-IR spectrum of Fe3O4@SiO2-DPDCS MNPs is shown in Fig. S1 (supplementary data). The peaks appeared at 580 and 634 cm-1 are attributed to Fe-O bond. The broad band at 3446 cm-1 is due to the vibration of hydroxyl groups of Fe3O4 surface. The band at 1080 cm-1 is related to the asymmetric stretching vibration of Si-O-Si. The absorption band at 1640 cm-1 is assigned to H-O-H stretching mode (adsorbed water on the surface of the sorbent). The peak at 1136 cm-1 corresponds to C-Si-C stretching mode in the structure of Ph-Si-Ph. The sharp peak at 1431 cm-1 is attributed to the stretching mode of the C=C in the diphenyl ring. Two peaks around 2856 to 2925 cm-1 are related to C-H stretching mode in the phenyl structure [20,21]. The morphology of the nanoparticles and size of the synthesized Fe3O4@SiO2-DPDCS MNPs were determined by FE-SEM. FE-SEM image shown in Fig. 2, indicated that the distribution of the particles was uniform, and the MNPs had spherical shapes with an average diameter of about 21-27 nm. Brunauer-Emmett-Teller (BET) surface area of the sorbent was 113.28 m2/g. The mean pore diameter of the sorbent was 12.485 nm.

3.2. Optimization of extraction conditions The important experimental variables affecting the automated MSPE procedure, including extraction and desorption flow rates, eluent and sample volumes, type of eluent and sample ionic strength were studied in order to attain the maximum extraction efficiency.

3.2.1. Extraction flow rate To evaluate the effect of sample flow rate on the extraction efficiency, sample was passed through the sorbent at various flow rates (120, 180, 400, 680 and 1100 µL min-1). As seen in

9

Fig. 3, by increasing the flow rates, the peak height of the analytes gradually decreased. In addition, at higher flow rates, the sorbent particles were washed out from the tube. Although the extraction efficiency was slightly lower at higher flow rates, to avoid lengthy extraction times, 680 µL min-1 was selected as the sample flow rate.

3.2.2. Salt addition effect To examine the salt effect on the analyte extraction, sodium sulfate in the range of 0-25% (w/v) was added to the sample solution. The results in Fig. S2 (supplementary data) demonstrated that increasing sodium sulfate up to 20% led to an increase in the peak height due to decreasing the solubility of analytes. Decreasing extraction efficiency at the higher amount of sodium sulfate is probably due to covering the active adsorption sites of the sorbent surface with salt and thus the extraction efficiency was declined [22]. Therefore, 20% (w/v) sodium sulfate was chosen as the optimum point for further experiments.

3.2.3. The volume of sample loading To evaluate the effect of sample volume, different volumes from 5 to 25 mL were passed through the extraction tube. By increasing sample volume, extraction efficiency was also increased (Fig. S3, supplementary data). For the sake of analysis time, a 20-mL volume was selected for sample loading.

3.2.4. Type and volume of eluent To select the type of eluent, three common solvents, i.e. acetonitrile, methanol, and isopropanol were used. Because the eluent and its volume are dependent parameters, the effect of each solvent on the extraction efficiency was studied at different volumes. A volume of 20 mL of the standard solution of the analytes at two concentration levels (50 and 500 µg

10

L-1) was passed through the sorbent. For the elution of the analytes from the loaded sorbent, 4 × 250 µL of the studied solvents were applied. Each fraction was individually injected into HPLC. Considering the data in Fig. 4, for the concentration of 500 µg L-1, the major amount of the analytes was eluted from the sorbent using the first 250 µL fraction of the eluents. Among the studied eluents, methanol and acetonitrile showed higher efficiency for the desorption of analytes. About 90% of the loaded analytes were eluted from the sorbent by passing 250 µL of methanol and acetonitrile through the sorbent. Methanol showed slightly better desorption for tebuthiuron. All of the analytes retained on the sorbent were eluted using the second fraction of solvents. None of the analytes were found in the third and fourth fractions of the eluents. For the sample with the analyte concentration of 50 µg L-1, all analytes were eluted from the sorbent using the first 250 µL fraction of the eluents (Fig. S4, supplementary data). None of the analytes were found in the second to fourth fractions. According to the results, 500 µL of methanol was chosen for the desorption of analytes.

3.2.5. Desorption flow rate To study the effect of eluent flow rate on the desorption efficiency, methanol at different flow rates (30, 50, 100 and 200 µL min-1) was passed through the loaded sorbent. Considering the data in Fig. S5 (supplementary data), with decreasing flow rate from 200 to 50 µL min-1, the efficiency of desorption increased and lower desorption rate had no significant effect on the desorption yield. According to the results, 50 µL min-1 was selected as the optimum flow rate for the desorption of analytes.

3.3. Method evaluation

11

To assess the performance of the method, under the optimum condition linearity range, the limit of detection (LOD), precision and enrichment factor (EF) were calculated. The results were shown in Table 1. The calibration plots were linear in the range of 0.1-500 μg L-1 with the determination coefficient (r2) between 0.9988 and 0.9999. The LOD of the method (based on a signal-to-noise ratio of 3) was 0.04 μg L-1. EF was calculated by dividing the concentration of analyte after extraction to its initial concentration in the sample. To study the precision of the method, both within-day and day-to-day reproducibility (n=3) were investigated. According to the data in Table 2, the relative standard deviation (RSD) for within-day and day-to-day reproducibility was 1-3 and 3-6%, respectively. In addition, the reproducibility of the method using different sorbents (batch-to-batch reproducibility) assessed by investigating three extraction tubes was between 4 and 7%.

3.4. Comparison with other methods To compare the present method with other HPLC-based techniques used for the determination of phenylurea herbicides, some analytical parameters of the methods were listed in Table 3. Compared with other microextraction-based methods, the present method showed lower LODs. The LODs obtained by the method are comparable to those obtained by SPE. However, higher sample and desorption volumes are required in SPE technique. Also, the present method showed good reproducibility mainly because of the automation of the procedure.

3.5. Real water samples To evaluate the applicability and versatility of the present method, three real samples including Zarjob river, Zayandeh-rood river and wastewater samples were analyzed. The standard addition method was used for analyte quantification. In addition, to investigate the 12

precision and accuracy of the method, RSD and relative recovery (RR) were calculated. The results are shown in Table 4. Monuron, isoproturon, and monolinuron were found in wastewater sample at the concentration of 0.32, 0.11 and 0.30 μg L-1, respectively. In Zarjob and Zayandeh-rood river water samples only monuron was detected at 0.53 and 0.29 μg L-1, respectively. Tebuthiuron was not detected in the samples. The chromatograms of the nonspiked and spiked wastewater samples are shown in Fig. 5. Relative recovery was calculated using the following equation: RR%=

× 100

Cfound and Creal are the concentration of analyte after and before spiking, respectively. Cadded is the analyte concentration added to the sample. Relative recovery was assessed by spiking the samples at the concentrations of 1.0 and 100 μg L-1. RR was found to be in the range of 81-116%. The RSDs of the data were in the range of 1-7 and 1-12% for the samples spiked at 100 and 1.0 μg L-1, respectively. According to the data, it was concluded that the matrix effect had no significant effect on the analyte determination and the present method was reliable for quantifying the analytes in water samples.

4. Conclusions An automated MSPE followed by HPLC was developed for the pre-concentration and quantification of trace amounts of PUHs in water samples. Magnetic Fe3O4 nanoparticles modified with diphenyldichlorosilane were used as the sorbent for MSPE. The magnetic nanoparticles were simply immobilized on the inner wall of the extraction tube through an external magnetic field without the use of frits or plugs. By folding the extraction tube around the pole of a cylindrical magnet, a high strength and uniform magnetic field was exerted and the sorbent particles were uniformly distributed in the inner wall of the tube. This 13

configuration avoids sorbent agglomeration, creating flow-through channels, bed clogging and loss of sorbent during the extraction/desorption process. Under the optimized condition, the LOD of the method was in sub-ppb range. High recoveries obtained by the method in the analysis of real water samples showed that the sample matrix has no significant effect on the quantification of PUHs. Credit Author Statement Mohammad Saraji: Conceptualization, Supervision, Writing - Review & Editing, Funding acquisition Leila Mohammadipour: Methodology, Visualization, Investigation, Resources, Data curation, Writing - Original Draft Narges Mehrafza: Writing - Review & Editing, Project administration, Validation

eclaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments The authors thank the Research Council of Isfahan University of Technology (IUT) and the Center of Excellence in Sensor and Green Chemistry, for their financial support of this work.

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[24] H.H. Lin, Y.H. Sung, S.D. Huang, Solid-phase microextraction coupled with highperformance liquid chromatography for the determination of phenylurea herbicides in aqueous samples, J. Chromatogr. A 1012 (2003) 57-66. [25] M. Li, J. Wang, C. Jiao, C. Wang, Q. Wu, Z. Wang, Graphene oxide framework: an adsorbent for solid phase extraction of phenylurea herbicides from water and celery samples, J. Chromatogr. A 1469 (2016) 17-24. [26] G.L Scheel, C.R.T. Tarley, Feasibility of supramolecular solvent-based microextraction for simultaneous preconcentration of herbicides from natural waters with posterior determination by HPLC-DAD, Microchem. J. 133 (2017) 650-657. [27] M. Saraji, N. Mehrafza, Phenyl carbamate functionalized zinc oxide nanorods for paperbased thin film microextraction, RSC Adv. 7 (2017) 50210-50215. [28] J. Wu, R. Ma, L. Hao, C. Wang, Q. Wu, Z. Wang, Triphenylamine-based hypercrosslinked organic polymer as adsorbent for the extraction of phenylurea herbicides, J. Chromatogr. A 1520 (2017) 48-57.

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Figures caption

Fig. 1. The schematic diagram of the extraction setup.

Fig. 2. Scanning electron microscopy image of modified magnetic nanoparticles.

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Fig. 3. Effect of extraction flow rate on the extraction efficiency (volume of sample loading: 5 mL; volume and type of eluent: 500 µL of methanol; desorption flow rate: 30 µL min -1).

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Fig. 4. Effect of type and volume of eluent on the desorption efficiency (extraction flow rate: 680 µL min-1 ; salt concentration: 20%; volume of sample loading: 20 mL; desorption flow rate: 30 µL min-1).

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Fig. 5. The chromatograms of wastewater sample; A: non-spiked wastewater sample and B: wastewater sample spiked with 1 µg L-1 of 1) tebuthiuron, 2) monuron, 3) isoproturon and 4) monolinuron. Detection wavelength was 245 nm (0-4 min), and 250 nm (4-7 min).

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Table 1 Analytical performance of the proposed method. Analyte Linear LOD range (μg L-1) (μg L-1) Tebuthiuron 0.10-500 0.04 Monuron 0.10-500 0.04 Isoproturon 0.10-500 0.04 Monolinuron 0.10-500 0.04

Enrichment factor 57 40 62 43

Table 2 Reproducibility of the method at the concentration levels of 2 and 200 μg L-1. RSD%, (n = 3) RSD%, (n = 3) Analytes Within-day reproducibility Day-to-day reproducibility

Tebuthiuron Monuron Isoproturon Monolinuron

2 μg L-1 1 3 1 1

200 μg L-1 1 2 1 2.4

23

2 μg L-1 6 6 5 6

200 μg L-1 4 4 3 4

RSD%, (n =3( Batch-to batch reproducibility 200 μg L-1 4 5 5 7

Table 3 Comparison of the proposed method with some other extraction and microextraction methods for the determination of phenylureas. Technique

Analyte

Sample volume (mL) 5

Extraction time (min) -

Desorption volume (mL) -

LOD (μg L-1)

RSD%

0.10-0.12

Calibration range (μg L-1) 0.5-100

1.5-5.9

[23]

Ref.

PDLLMEa /HPLC-DAD

Monuron Monolinuron Isoproturon

SPME /HPLC-UV

Monuron Isoproturon Monolinuron

3

40

-

0.5-3.8

5-1000

2.3-5.9

[24]

SPE /HPLC-DAD

Monuron Isoproturon

100

-

2

0.02

0.1-40

3.2-4.6

[25]

SUPRASsb /HPLC-DAD

Tebuthiuron

10

10.5

0.15

0.24

1-400

2.6-4.5

[26]

TFME /HPLC-DAD

Monuron

15

25

0.2

0.13

0.5-50

2.1

[27]

SPE /HPLCDAD

Monuron Isoproturon Monolinron

150

50

0.2

0.008 0.01 0.01

0.05-40

2.6-5.4

[28]

This method

Tebuthiuron Monuron Monolinuron Isoproturon

20

30

0.5

0.04 0.04 0.04 0.04

0.1-500

1-3

a

b

Partitioned dispersive liquid–liquid microextraction. Supramolecular solvent-based microextraction.

24

Table 4 Analytical results for the determination of phenylureas in real samples. Sample

a

Analyte

Amount measured in the real sample (μg L-1)

Spiked at 100 μg L-1

Spiked at 1 μg L-1

RSD%

RR%

RSD%

RR%

Wastewater

Tebuthiuron Monuron Isoproturon Monolinuron

NDa 0.32 0.11 0.30

3 1 2 2

94 88 94 89

4 4 3 2

96 81 80 98

Zarjob river water

Tebuthiuron Monuron Isoproturon Monolinuron

ND 0.53 ND ND

1 1 1 1

91 92 96 97

1 2 1 3

116 106 97 83

Zayandehrood river water

Tebuthiuron Monuron Isoproturon Monolinuron

ND 0.29 ND ND

6 6 6 7

92 91 99 90

7 9 12 12

114 83 94 88

Not detected.

25