Temperature‐controlled ultrasound‐ and vortex‐assisted liquid–liquid microextraction combined with GC for the determination of the concentrations of organophosphorus pesticides in beverage samples

The aim of this work was to develop temperature‐controlled ultrasound‐ and vortex‐assisted liquid–liquid microextraction as a fast and efficient approach for the extraction of nine organophosphorus pesticides in beverage samples followed by GC with flame photometric detection analysis. The combination of ultrasonication and vortexing were used to assist the microextraction, and the use of a dispersion solvent was avoided. Several variables that could potentially affect the extraction efficiency, namely, the type and volume of extraction solvent, sequence, and time of ultrasonication and vortexing, ultrasonication bath temperature and ionic strength were optimized. Under optimum conditions, the calibration graphs were linear over the range of 0.5–200 μg/L. The LOD (S/N = 3) was between 0.01 and 0.05. The optimized method exhibited a good precision level with RSD values between 4.5 and 9.8%. The enrichment factors for the nine organophosphorus pesticides were between 224 and 339. Four beverage samples were successfully analyzed using the proposed method.     

Vortex‐assisted liquid–liquid microextraction using a low‐toxicity solvent for the determination of five organophosphorus pesticides in water samples by high‐performance liquid chromatography

Vortex‐assisted liquid–liquid microextraction followed by high‐performance liquid chromatography with UV detection was applied to determine Isocarbophos, Parathion‐methyl, Triazophos, Phoxim and Chlorpyrifos‐methyl in water samples. 1‐Bromobutane was used as the extraction solvent, which has a higher density than water and low toxicity. Centrifugation and disperser solvent were not required in this microextraction procedure. The optimum extraction conditions for 15 mL water sample were: pH of the sample solution, 5; volume of the extraction solvent, 80 μL; vortex time, 2 min; salt addition, 0.5 g. Under the optimum conditions, enrichment factors ranging from 196 to 237 and limits of detection below 0.38 μg/L were obtained for the determination of target pesticides in water. Good linearities (r > 0.9992) were obtained within the range of 1–500 μg/L for all the compounds. The relative standard deviations were in the range of 1.62–2.86% and the recoveries of spiked samples ranged from 89.80 to 104.20%. The whole proposed methodology is simple, rapid, sensitive and environmentally friendly for determining traces of organophosphorus pesticides in the water samples.     

Novel solvent‐free microwave‐assisted extraction coupled with low‐density solvent‐based in‐tube ultrasound‐assisted emulsification microextraction for the fast analysis of organophosphorus pesticides in soils

A novel and rapid solventless microwave‐assisted extraction coupled with low‐density solvent‐based in‐tube ultrasound‐assisted emulsification microextraction has been developed for the efficient determination of nine organophosphorus pesticides in soils by GC analysis with microelectron capture detection. A specially designed, homemade glass tube inbuilt with a scaled capillary tube was used as an extraction device to collect and measure the separated extractant phase easily. Parameters affecting the efficiencies of the developed method were thoroughly investigated. From experimental results, the following conditions were selected for the extraction of organophosphorus pesticides from 1.0 g of soil sample to 5 mL of aqueous solution under 226 W of microwave irradiation for 2.5 min followed by ultrasound‐assisted emulsification microextraction with 20 μL toluene for 30 s and then centrifugation at 3200 rpm for 3 min. Detections were linear in the range of 0.25–10 ng/g with detection limits between 0.04 and 0.13 ng/g for all target analytes. The applicability of the method to real samples was assessed on agricultural contaminated soils and the recoveries ranged between 91.4 and 101.3%. Compared to other methods, the present method was shown to be highly competitive in terms of sensitivity, cost, eco‐friendly nature, and analysis speed.     

Coprecipitation‐assisted coacervative extraction coupled to high‐performance liquid chromatography: An approach for determining organophosphorus pesticides in water samples

An analytical methodology based on coprecipitation‐assisted coacervative extraction coupled to HPLC‐UV was developed for determination of five organophosphorus pesticides (OPPs), including fenitrothion, guthion, parathion, methidathion, and chlorpyrifos, in water samples. It involves a green technique leading to an efficient and simple analytical methodology suitable for high‐throughput analysis. Relevant physicochemical variables were studied and optimized on the analytical response of each OPP. Under optimized conditions, the resulting methodology was as follows: an aliquot of 9 mL of water sample was placed into a centrifuge tube and 0.5 mL sodium citrate 0.1 M, pH 4; 0.08 mL Al₂(SO₄)₃ 0.1 M; and 0.7 mL SDS 0.1 M were added and homogenized. After centrifugation the supernatant was discarded. A 700 μL aliquot of the coacervate‐rich phase obtained was dissolved with 300 μL of methanol and 20 μL of the resulting solution was analyzed by HPLC‐UV. The resulting LODs ranged within 0.7–2.5 ng/mL and the achieved RSD and recovery values were <8% (n = 3) and >81%, respectively. The proposed analytical methodology was successfully applied for the analysis of five OPPs in water samples for human consumption of different locations of Mendoza.     

Comparison of air‐agitated liquid–liquid microextraction and ultrasound‐assisted emulsification microextraction for polycyclic aromatic hydrocarbons determination in hookah water

In this work, two disperser‐free microextraction methods, namely, air‐agitated liquid–liquid microextraction and ultrasound‐assisted emulsification microextraction are compared for the determination of a number of polycyclic aromatic hydrocarbons in aqueous samples, followed by gas chromatography with flame ionization detection. The effects of various experimental parameters upon the extraction efficiencies of both methods are investigated. Under the optimal conditions, the enrichment factors and limits of detection were found to be in the ranges of 327–773 and 0.015–0.05 ng/mL for air‐agitated liquid–liquid microextraction and 406–670 and 0.015–0.05 ng/mL for ultrasound‐assisted emulsification microextraction, respectively. The linear dynamic ranges and extraction recoveries were obtained to be in the range of 0.05–120 ng/mL (R² ≥ 0.995) and 33–77% for air‐agitated liquid–liquid microextraction and 0.05–110 ng/mL (R² ≥ 0.994) and 41–67% for ultrasound‐assisted emulsification microextraction, respectively. To investigate this common view among some people that smoking hookah is healthy due to the passage of smoke through the hookah water, samples of both the hookah water and hookah smoke were analyzed.     

Ultrasound‐assisted dispersive liquid–liquid microextraction coupled with capillary gas chromatography for simultaneous analysis of nine pyrethroids in domestic wastewaters

A simple and rapid ultrasound‐assisted dispersive liquid–liquid microextraction method coupled with GC‐flame ionization detection was developed for simultaneous determination of nine pyrethroids in domestic wastewater samples. An ultrasound‐assisted process was applied to accelerate the formation of the fine cloudy solution using small volume of disperser solvent, which markedly increased the extraction efficiency and reduced the equilibrium time. Various parameters affecting the extraction efficiency were investigated, including the type and volume of extraction solvent and disperser solvent, extraction and ultrasonic time. Good linearity was obtained for all analytes in the range of 0.8–100 μg/L with the correlation coefficient (r²)≥0.998. The recoveries at three spiking levels ranged from 75.3 to 101.2% with the RSD less than 8.7% (n=5). Under the optimum condition, the enrichment factors for the nine pyrethroids ranged from 728‐ to 1725‐fold. This method offered a good alternative for routine analysis due to its simplicity and reliability.     

Application of continual injection liquid‐phase microextraction method coupled with liquid chromatography to the analysis of organophosphorus pesticides

A liquid‐phase microextraction coupled with LC method has been developed for the determination of organophosphorus pesticides (methidation, quinalphos and profenofos) in drinking water samples. In this method, a small amount (3 μL) of isooctane as the acceptor phase was introduced continually to fill‐up the channel of a 1.5 cm polypropylene hollow fiber using a microsyringe while the hollow fiber was immersed in an aqueous donor solution. A portion of the acceptor phase (ca. 0.4 μL) was first introduced into the hollow fiber and additional amounts (ca. 0.2 μL) of the acceptor phase were introduced to replenish at intervals of 3 min until set end of extraction (40 min). After extraction, the acceptor phase was withdrawn and transferred into a 2 mL vial for a drying step prior to injection into a LC system. Parameters that affect the extraction efficiency were studied including the organic solvent, length of fiber, volume of acceptor and donor phase, stirring rate, extraction time, and effect of salting out. The proposed method provided good enrichment factors of up to 189.50, with RSD ranging from 0.10 to 0.29%, analyte recoveries of over 79.80% and good linearity ranging from 10.0 to 1.25 mg/L. The LOD ranged from 2.86 to 82.66 μg/L. This method was applied successfully to the determination of organophosphorus pesticides in selected drinking water samples.     

Optimization of a single-drop microextraction procedure for the determination of organophosphorus pesticides in water and fruit juice with gas chromatography-flame photometric detection

A single-drop microextraction (SDME) procedure was developed for the analysis of organophosphorus pesticides (OPPs) in water and fruit juice by gas chromatography (GC) with flame photometric detection (GC-FPD). The significant parameters affecting the SDME performance such as selection of microextraction solvent, solvent volume, extraction time, stirring rate, sample pH and temperature, and ionic strength were studied and optimized. Two types of SDME mode, static and cycle-flow SDME, were evaluated. The static SDME procedure provided more sensitive analysis of the target analytes. Therefore, static SDME with tributyl phosphate (TBP) as internal standard was selected for the real sample analysis. The limits of detection (LODs) in water for the six studied compounds were between 0.21 and 0.56 ng/mL with the relative standard deviations ranging from 1.7 to 10.0%. Linear response data was obtained in the concentration range of 0.5-50 ng/mL (except for dichlorvos 1.0-50 ng/mL) with correlation coefficients from 0.9995 to 0.9999. Environmental water sample collected from East Lake and fruit juice samples were successfully analyzed using the proposed method, but none of the analytes in both lake water and fruit juice were detected. The recoveries for the spiked water and juice samples were from 77.7 to 113.6%. Compared with the conventional methods, the proposed method enabled a rapid and simple determination of organophosphorus pesticides in water and fruit juice with minimal solvent consumption and a higher concentration capability.     

Trace analysis of some organophosphorus pesticides in rice samples using ultrasound‐assisted dispersive liquid–liquid microextraction and high‐performance liquid chromatography

An ultrasound‐assisted dispersive liquid–liquid microextraction based on solidification of a floating organic drop method followed by high‐performance liquid chromatography was developed for the extraction, preconcentration, and determination of trace amounts of organophosphorus pesticides in rice samples. Variables affecting the performance of both steps were thoroughly investigated. Some effective parameters on extraction were studied and optimized. Under the optimum conditions, recoveries for rice sample are in the range of 58.0–66.0%. The calibration graphs are linear in the range of 4–800 μg/kg and, limits of detection and limits of quantification are in the range of 1.5–3 and 4.2–8.5 μg/kg, respectively. The relative standard deviation for 50.0 μg/kg of organophosphorus pesticides in rice sample are in the range of 4.4–5.1% (n = 5). The obtained results show that proposed method is a fast and simple method for the determination of pesticides in cereals.     

On-line determination of organophosphorus pesticides in water by solid-phase microextraction and gas chromatography with thermionic-selective detection

This paper describes the extraction of 49 organophosphorus pesti-cides (OPPs) from water samples using solid-phase microextraction (SPME). Three fibers, including a 15-μm XAD-coated fiber, a 85-μm polyacrylate-coated fiber, and a 30-μm polydimethylsilox-ane-coated fiber (PDMS), were evaluated here. The effects of stirring and the addition of NaCl to the sample were examined for the polyacrylate-coated fiber. The precision of the technique was examined for all three fibers and the extraction kinetics were investigated using the XAD- and polyacrylate-coated fibers. With some exceptions, the XAD- and polyacrylate-coated fibers performed better than the PDMS-coated fiber. The superiority of the XAD-nd polyacrylate-coated fiber. The superiority of the XAD- and polyacrylate-coated fibers over the PDMS-coated fibers can be attribuibuted to the aromatic functionalities of the XAD and the polar functionalities in the polyacrylate. The relatively high percent RSDs indicate that the SPME technique needs to be further refined before it can be used for anything other than screening. A more effective form of agitation than mechanical stirring may be neccessary to reduce variability and achieve a faster equilibrium between the sample and the SPME fiber.     

Ultrapreconcentration and determination of organophosphorus pesticides in water by solid‐phase extraction combined with dispersive liquid–liquid microextraction and high‐performance liquid chromatography

Solid‐phase extraction coupled with dispersive liquid–liquid microextraction was developed as an ultra‐preconcentration method for the determination of four organophosphorus pesticides (isocarbophos, parathion‐methyl, triazophos and fenitrothion) in water samples. The analytes considered in this study were rapidly extracted and concentrated from large volumes of aqueous solutions (100 mL) by solid‐phase extraction coupled with dispersive liquid–liquid microextraction and then analyzed using high performance liquid chromatography. Experimental variables including type and volume of elution solvent, volume and flow rate of sample solution, salt concentration, type and volume of extraction solvent and sample solution pH were investigated for the solid‐phase extraction coupled with dispersive liquid–liquid microextraction with these analytes, and the best results were obtained using methanol as eluent and ethylene chloride as extraction solvent. Under the optimal conditions, an exhaustive extraction for four analytes (recoveries >86.9%) and high enrichment factors were attained. The limits of detection were between 0.021 and 0.15 μg/L. The relative standard deviations for 0.5 μg/L of the pesticides in water were in the range of 1.9–6.8% (n = 5). The proposed strategy offered the advantages of simple operation, high enrichment factor and sensitivity and was successfully applied to the determination of four organophosphorus pesticides in water samples.     

Dispersive liquid–liquid microextraction using extraction solvent lighter than water

For the first time a dispersive liquid–liquid microextraction method on the basis of an extraction solvent lighter than water was presented in this study. Three organophosphorus pesticides (OPPs) were selected as model compounds and the proposed method was carried out for their preconcentration from water samples. In this extraction method, a mixture of cyclohexane (extraction solvent) and acetone (disperser) is rapidly injected into the aqueous sample in a special vessel (see experimental section) by syringe. Thereby, a cloudy solution is formed. In this step, the OPPs are extracted into the fine droplets of cyclohexane dispersed into aqueous phase. After centrifuging the fine droplets of cyclohexane are collected on the upper of the extraction vessel. The upper phase (0.40 μL) is injected into the gas chromatograph (GC) for separation. Analytes were detected by a flame ionization detector (FID) (for high concentrations) or MS (for low concentrations). Some important parameters, such as the kind of extraction and dispersive solvents and volume of them, extraction time, temperature, and salt amount were investigated. Under the optimum conditions, the enrichment factors (EFs) ranged from 100 to 150 and extraction recoveries varied between 68 and 105%, both of which are relatively high over those of published methods. The linear ranges were wide (10–100 000 μg/L for GC‐FID and 0.01–1 μg/L for GC‐MS) and LODs were low (3–4 μg/L for GC‐FID and 0.003 μg/L for GC‐MS). The RSDs for 100.0 μg/L of each OPP in water were in the range of 5.3–7.8% (n = 5).     

Combining ultrasound‐assisted extraction and vortex‐assisted liquid–liquid microextraction for the sensitive assessment of aflatoxins in aquaculture fish species

Although aflatoxins contamination in feedstuff is a well‐known problem, and hence these residues are controlled in poultry products, there is scarce information regarding the presence of these toxic substances in aquaculture fish, facilities that use several feedstuff for fish breeding. A simple, rapid, and sensitive method has been therefore developed for aflatoxins (B1, B2, G1, and G2) assessment in aquaculture products by combining ultrasound probe‐assisted extraction and vortex‐assisted liquid–liquid microextraction as a sample pretreatment, and high‐performance liquid chromatography‐tandem mass spectrometry as a separation/detection system. Aflatoxins were extracted from fish flesh/liver with a 60:40 acetonitrile/aqueous phosphate buffer (pH 7.0) mixture before preconcentration and clean‐up by vortex‐assisted liquid–liquid microextraction under the following optimized conditions: 5.0 mL of fish extract at pH 7.0 and NaCl at 0.5% (w/v), 400 μL of chloroform as extracting solvent, and vortex shaking at 2000 rpm for 1 min. The proposed method is shown to be precise and accurate, and the limit of quantitations (from 0.20 to 1.10 μg kg^?1) were lower than the value established by the European Commission Regulation for aflatoxins in foodstuff. Results have shown that fish flesh is free of aflatoxins, but aflatoxins B2 and G1 were quantified in fish liver.     

Sol–gel polydimethylsiloxane/poly(vinylalcohol)‐coated stir bar sorptive extraction of organophosphorus pesticides in honey and their determination by large volume injection GC

A PDMS/poly(vinylalcohol) (PDMS/PVA) film prepared through a sol–gel process was coated on stir bars for sorptive extraction, followed by liquid desorption and large volume injection–GC–flame photometric detector (LVI–GC–FPD) for the determination of five organophosphorus pesticides (OPPs) (phorate, fenitrothion, malathion, parathion, and quinalphos) in honey. The preparation reproducibility of PDMS/PVA‐coated stir bar ranged from 4.3 to 13.4% (n = 4) in one batch, and from 6.0 to 12.6% (n = 4) in batch to batch. And one prepared stir bar can be used for more than 50 times without apparent coating loss. The significant parameters affecting stir bar sorptive extraction (SBSE) were investigated and optimized. The LODs for five OPPs ranged from 0.013 (parathion) to 0.081 μg/L (phorate) with the RSDs ranging from 5.3 to 14.2% (c = 1 μg/L, n = 6). The proposed method was successfully applied to the analysis of five OPPs in honey.     

Ionic‐liquid‐based,manual‐shaking‐ and ultrasound‐assisted,surfactant‐enhanced emulsification microextraction for the determination of three fungicide residues in juice samples

A novel manual‐shaking‐ and ultrasound‐assisted surfactant‐enhanced emulsification microextraction method was developed for the determination of three fungicides in juice samples. In this method, the ionic liquid, 1‐ethyl‐3‐methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, instead of a volatile organic solvent was used as the extraction solvent. The surfactant, NP‐10, was used as an emulsifier to enhance the dispersion of the water‐immiscible ionic liquid into an aqueous phase, which accelerated the mass transfer of the analytes. Organic dispersive solvent typically required in common dispersive liquid–liquid microextraction methods was not necessary. In addition, manual shaking for 15 s before ultrasound to preliminarily mix the extraction solvent and the aqueous sample could greatly shorten the time for dispersing the ionic liquid into aqueous solution by ultrasound irradiation. Several experimental parameters affecting the extraction efficiency, including type and volume of extraction solvent, type and concentration of surfactant, extraction time, and pH, were optimized. Under the optimized conditions, good linearity with the correlation coefficients (γ) higher than 0.9986 and high sensitivity with the limit of detection ranging from 0.4 to 1.6 μg/L were obtained. The average recoveries ranged from 61.4 to 86.0% for spiked juice, with relative standard deviations from 1.8 to 9.7%. The proposed method was demonstrated to be a simple, fast, and efficient method for the analysis of the target fungicides in juice samples.     

Combination of dispersive solid‐phase extraction and salting‐out homogeneous liquid–liquid extraction for the determination of organophosphorus pesticides in cereal grains

A new analytical method for the determination of organophosphorus pesticides in cereal samples was developed by combining dispersive SPE (d‐SPE) and salting‐out homogeneous liquid–liquid extraction (SHLLE). The pesticides were first extracted from cereal grains with acetonitrile, followed by d‐SPE cleanup. A 2 mL aliquot of the extract was then added to a centrifuge tube containing 9.2 mL water and 3.3 g NaCl for SHLLE. Analysis of the extract was carried out by gas chromatography coupled with flame photometric detection. The d‐SPE procedure effectively provides the necessary cleanup of the extract while SHLLE is used as an efficient concentration technique. Experimental parameters influencing the extraction efficiency including amounts of added water and salt were investigated. Recovery studies were carried out at three fortification levels, yielding recoveries in the range of 57.7–98.1% with the RSD from 3.7 to 10.9%. The reported limits of determination obtained from this study were 1 μg/kg, which is better than the conventional methods. In the analysis of 40 wheat and corn samples taken from Beijing suburbs, only two wheat samples have chlorpyrifos residue over the limits of determination.     

Application of pneumatic nebulization single‐drop microextraction for the determination of organophosphorous pesticides by gas chromatography–mass spectrometry

Organophosphorous pesticides (OPPs) including dichlorvos, diazinon, malathion, phenamiphos and chlorpyrifos, in water samples were extracted by pneumatic nebulization single‐drop microextraction (PN‐SDME) and then determined by gas chromatography–mass spectrometry (GC‐MS). Experimental parameters affecting the performances of PN‐SDME, such as flow rate of carrier gas, extraction time and microdrop volume, were examined and optimized. The limits of detection for the analytes were in the range of 0.0014–0.0019 μg/mL. The linear range was 0.0050–0.50 μg/mL, except dichlorvos (0.0070–0.50 μg/mL). Water samples were analyzed and the recoveries of the analytes in the spiked water samples were from 75.2 to 105.3%. The relative standard deviations were lower than 12.7%.     

Extraction and determination of organophosphorus pesticides in water samples by a new liquid phase microextraction-gas chromatography-flame photometric detection

A rapid, sensitive and efficient liquid phase microextraction (LPME) method was developed to determine trace concentrations of some organophosphorus pesticides in water samples. This method combines liquid phase microextraction with gas chromatographic (GC) analysis in a simple and inexpensive apparatus involving very little organic solvent consumption. It involves exposing a floated drop of an organic solvent on the surface of aqueous solution in a sealed vial. Experimental parameters which control the performance of LPME such as type of organic solvent, organic solvent and sample volumes, sample stirring rate, sample solution temperature, salt addition and exposure time were investigated and optimized. Finally, the enrichment factor, dynamic linear range (DLR), limit of detection (LOD) and precision of the method were evaluated by the water samples spiked with organophosphorus pesticides. Using optimum extraction conditions, very low detection limits (0.01-0.04 μg L⁻¹) and good linearities (0.9983 < r² < 0.9999) were achieved. The LPME was performed for determination of organophosphorus pesticides in different types of natural water samples and acceptable recoveries (96-104%) and precisions (3.5 < R.S.D.% < 8.9) were obtained. The results suggested that the newly proposed LPME method is a rapid, accurate and effective sample preparation method and could be successfully applied for extraction and determination of organophosphorus pesticides in water samples.     

Polyol‐enhanced dispersive liquid–liquid microextraction coupled with gas chromatography and nitrogen phosphorous detection for the determination of organophosphorus pesticides from aqueous samples,fruit juices,and vegetables

Polyol‐enhanced dispersive liquid–liquid microextraction has been proposed for the extraction and preconcentration of some organophosphorus pesticides from different samples. In the present study, a high volume of an aqueous phase containing a polyol (sorbitol) is prepared and then a disperser solvent along with an extraction solvent is rapidly injected into it. Sorbitol showed the best results and it was more effective on the extraction recoveries of the analytes than inorganic salts such as sodium chloride, potassium chloride, and sodium sulfate. Under the optimum extraction conditions, the method showed low limits of detection and quantification within the ranges of 12–56 and 44–162 pg/mL, respectively. Enrichment factors and extraction recoveries were in the ranges of 2799–3033 and 84–92%, respectively. The method precision was evaluated at a concentration of 10 ng/mL of each analyte, and relative standard deviations were found to be less than 5.9% for intraday (n = 6) and less than 7.8% for interday (n = 4). Finally, some aqueous samples were successfully analyzed using the proposed method and four analytes (diazinon, dimethoate, chlorpyrifos, and phosalone) were determined, some of them at ng/mL level.     

Determination of some organophosphorus pesticides in water and watermelon samples by microextraction prior to high-performance liquid chromatography

A novel method based on simultaneous liquid-liquid microextraction and carbon nanotube reinforced hollow fiber microporous membrane solid-liquid phase microextraction has been developed for the determination of six organophosphorus pesticides, i.e. isocarbophos, phosmet, parathion-methyl, triazophos, fonofos and phoxim, in water and watermelon samples prior to high-performance liquid chromatography (HPLC). Under the optimum conditions, the method shows a good linearity within a range of 1-200 ng/mL for water samples and 5-200 ng/g for watermelon samples, with the correlation coefficients (r) varying from 0.9990 to 0.9997 and 0.9986 to 0.9995, respectively. The limits of detection (LODs) were in the range between 0.1 and 0.3 ng/mL for water samples and between 1.0 and 1.5 ng/g for watermelon samples. The recoveries of the method at spiking levels of 5.0 and 50.0 ng/mL for water samples were between 85.4 and 100.8%, and at spiking levels of 5.0 and 50.0 ng/g for watermelon samples, they were between 82.6 and 92.4%, with the relative standard deviations (RSDs) varying from 4.5-6.9% and 5.2-7.4%, respectively. The results suggested that the developed method represents a simple, low-cost, high analytes preconcentration and excellent sample cleanup procedure for the determination of organophosphorus pesticides in water and watermelon samples.