Rapid, low level determination of silver(I) in drinking water by colorimetric-solid-phase extraction

A rapid, highly sensitive two-step procedure for the trace analysis of silver(I) is described. The method is based on: (1) the solid-phase extraction (SPE) of silver(I) from a water sample onto a disk impregnated with a silver-selective colorimetric reagent, and (2) the determination of the amount of complexed analyte extracted by the disk by diffuse reflectance spectroscopy (DRS). This method, called colorimetric-solid-phase extraction (C-SPE), was recently shown effective in determining low concentrations (0.1-5.0 mg/ml) of iodine and iodide in drinking water. This report extends C-SPE to the trace (∼4 μg/l) level monitoring of silver(I) which is a biocide used on the International Space Station (ISS). The determination relies on the manually driven passage of a water sample through a polystyrene-divinylbenzene disk that has been impregnated with the colorimetric reagent 5-(p-dimethylaminobenzylidene) rhodanine (DMABR) and with an additive such as a semi-volatile alcohol (1,2-decanediol) or nonionic surfactant (Brij 30). The amount of concentrated silver(I) is then determined in a few seconds by using a hand-held diffuse reflectance spectrometer, with a total sample workup and readout time of ∼60 s. Importantly, the additive induces the uptake of water by the disk, which creates a local environment conducive to silver(I) complexation at an extremely high concentration factor (∼800). There is no detectable reaction between silver(I) and impregnated DMABR in the absence of the additive. This strategy represents an intriguing new dimension for C-SPE in which additives, directly loaded in the disk material, provide a means to manipulate the reactivity of the impregnated reagent.     

Spectrofluorimetric determination of copper(II) by its static quenching effect on the fluorescence of 4,5-dihydroxy-1,3-benzenedisulfonic acid

A spectrofluorimetric method has been developed for the determination of trace Cu(II) in real samples with 4,5-dihydroxy-l,3-benzenedisulfonic acid (Tiron) as a fluorimetric reporter. Tiron is very soluble in water and is a good fluorimetric reagent. However, as Tiron was complexed with Cu(II), the fluorescence intensity decreased proportionally to the concentration of Cu(II) by a static quenching effect. The excitation wavelength and the fluorescence wavelength of Tiron were 294 and 350 nm, respectively, as it was caused by a quenching effect from Cu(II) at pH 8.0. The highest sensitivity was shown at Tiron concentration of 5.0x10(-5) M. To enhance the quenching effect, the Cu(II)-Tiron complex solution was heated up to 80 degrees C for 90 min. As for Cu(II), the interference by Co(II) was very serious, which was eliminated by oxalate ion. The linear response to Cu(II) was shown at the concentration range between 5.0x10(-7) and 1.0x10(-5) M. With this proposed method, the detection limit of Cu(II) was 3.83(+/-0.09)x10(-7) M. Recoveries of Cu(II) in the diluted brass samples and the stream water samples were almost 100%. Based on results from the experiment, this proposed technique could be applied to the practical determination of Cu(II) in real samples.     

Sorbent extraction of Pb(II), Cu(II), Ni(II), and Fe(III) ions as 2-(5-bromo-2-pyridylazo)-5-diethylamino-phenol chelates on single-walled carbon nanotube disks prior to their flame atomic absorption spectrometric determinations in animal feeds and natural water samples

A sorbent extraction procedure for Pb(II), Cu(II), Ni(II), and Fe(III) ions on single-walled carbon nanotube disks has been established. Analyte ions were converted to 2-(5-bromo-2-pyridylazo)-5-diethylamino-phenol chelates, then adsorbed on the disk. Adsorbed chelates were easily desorbed from the disk by using 10 mL 2 M HNO3. The various analytical parameters, including pH and reagent amounts that were effective for the recoveries of the analytes on nanotube disks, were optimized. The influence of matrix ions was also studied. The LOD values based on 3sigma were in the 0.3-4.6 microg/L range. Validation of the proposed SPE procedure was carried out by the determination of analytes in certified reference materials (TMDA-54.4 fortified lake water and HR-1 Humber River sediment). Spiking and recovery experiments for the analyte ions in real samples gave good results. Application of the procedure was illustrated by the determination of analyte contents in some animal feeds and water samples from Turkey.     

Thioacetamide chemically immobilized on silica gel as a solid phase extractant for the extraction and preconcentration of copper(II), lead(II), and cadmium(II)

Thioacetamide immobilized on silica gel was prepared via the Mannich reaction. The extraction and enrichment of copper(II), lead(II), and cadmium(II) ions from aqueous solutions has been investigated. Conditions for effective extraction are optimized with respect to different experimental parameters in both batch and column processes prior to their determination by flame atomic absorption spectrometry (FAAS). The optimum pH ranges for quantitative adsorption are 4.0-8.0, 2.0-7.0, and 5.0-10.0 for Pb(II), Cu(II), and Cd(II), respectively. Pb(II) and Cd(II) can be desorbed with 3 mol/L and 0.1 mol/L HCl/HNO3, and Cu(II) can be desorbed with 2.5% thiourea. The adsorption capacity of the matrix has been found to be 19.76, 16.35, and 12.50 mg/g for Pb(II), Cu(II), and Cd(II), respectively, with the preconcentration factor of approximately equal to 300 for Pb(II) and approximately equal to 200 for Cu(II) and Cd(II). Analytical utility is illustrated in real aqueous samples generated from distilled water, tap water, and river water samples.     

Determination of nickel(II) as the nickel dimethylglyoxime complex using colorimetric solid phase extraction

Colorimetric solid phase extraction (C-SPE) is an analytical technique in which analytes in water samples are extracted onto a solid adsorbent matrix impregnated with a colorimetric reagent and then quantified directly on the adsorbent surface using diffuse reflectance spectroscopy. This paper presents a further development in C-SPE. In this case, the reagent employed to detect the analyte is not impregnated on the extraction medium. Instead, the reagent is weakly immobilized on a solid support (i.e., filter paper) and released into the sample as it flows through the support. The reagent complexes the analyte in solution, forming a highly colored precipitate that is collected on the surface of an extraction membrane. The concentration of analyte is determined using the Kubelka-Munk function calculated from the diffuse reflectance spectrum of the precipitate on the membrane surface. This precipitation-spectrophotometric platform is extensively evaluated by determining nickel(II) using dimethylglyoxime (DMG) as the precipitating reagent. The ability to optimize reaction conditions with immobilized reagents by in-line buffering is also demonstrated. Specifically, borax buffer was utilized to adjust the pH of nickel(II) samples prepared in deionized water. This combination of immobilized buffer and reagent allows C-SPE to operate in a solid-phase mode in which all the reagents requisite for optimal analyte determination are immobilized on solid supports. Using this method, nickel(II) was determined in a single processing step over the concentration range 0.50-5.0 ppm in ∼40 s with 1.0 ml sample volumes.     

Determination of copper(II) by anodic stripping voltammetry at a 2,5-dimercapto-1,3,4-thiadiazol self-assembled monolayer-based gold electrode.

2,5-Dimercapto-1,3,4-thiadiazol (DMTD) can bind on the surface of a gold electrode through the strong gold-sulfur interaction. The fabrication and electrochemical characteristics of the DMTD self-assembled monolayer (SAM)-modified gold electrode were investigated. The DMTD SAM electrode exhibited a significantly increased sensitivity. Cu(II) was accumulated in phosphate buffer (pH 4.6) at a potential of -0.6 V (vs. Ag/AgCl) for 40 s and then determined by anodic stripping voltammetry (ASV) in copper-free phosphate buffer (pH 5.0). The effects of various parameters, such as the pH values of the preconcentration solution and measurement solution, the accumulation potential, and the accumulation time, were investigated. Under the optimum conditions, a linear calibration graph was obtained in the concentration range of 8.0 x 10(-6) to 8.0 x 10(-5) mol l(-1) with a correlation coefficient of 0.9978. The relative standard deviations for eight successive determinations were 4.3 and 2.9% for 1.0 x 10(-5) and 2.0 x 10(-5) mol l(-1) Cu(II), respectively. The detection limit (three times signal to noise) was 4.0 x 10(-7) mol l(-1). The proposed voltammetric method was utilized successfully to detect the concentration of Cu(II) ions in tap water samples.     

C_(18)膜萃取/液相色谱-质谱联用法测定极地水体中有机磷酸酯

建立了C_(18)膜萃取/液相色谱-质谱联用技术测定极地水体中10种有机磷酸酯(OPEs)的方法。根据优化后的样品前处理及仪器方法,利用C_(18)膜富集4 L水体中的10种OPEs,经二氯甲烷超声提取,在电喷雾正离子模式下,采用选择反应监测(SRM)模式进行分析,线性相关系数为0.994 4~0.999 9。10种OPEs的加标回收率为64.1%~115%,方法检出限为0.08~0.55 ng/L。该方法适用于极地水体中10种OPEs的分析,利用该方法对北极水体样品中的10种OPEs进行检测,测得冰川融水中∑OPEs的质量浓度为0.64~6.64 ng/L,海水中∑OPEs的质量浓度为0.09~2.03 ng/L。     

Porous metal membranes for solid-phase extraction of polycyclic aromatic hydrocarbons

Solid-phase extraction (SPE) is one of the most important techniques for sample preparation, purification, concentration and cleanup. Membranes made from synthetic organic polymers, cellulose, or glass fibers are used for sample pretreatment. In this work, we report that a porous metal membrane, the metal filter in HPLC, was used as a novel kind of solid-phase extraction adsorbent material. To evaluate the performance of the porous metal membrane for the SPE, naphthalene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, chrysene, perylene and benzo(a)pyrene were selected as analytes. Several parameters that affected the extraction efficiency such as the extraction time, the concentration of NaCl, the extraction temperature and the agitation speed were optimized. The experimental result indicates that the porous metal membrane possesses high adsorption ability to the tested polycyclic aromatic hydrocarbons (PAHs). Under the optimum conditions, the detection limits of the developed method were in the range of 0.03-0.082 μg L(-1) (S/N = 5), and excellent linear correlations between peak area and concentration of PAHs were found over the range of 0.1-60 μg L(-1). The precisions (RSD) for five replicate extractions of the PAHs from sample solutions were in the range of 2.6-5.0%. The recoveries of the PAHs from tap water and river water samples spiked with 9 PAHs (20 μg L(-1) of each individual PAH) ranged from 83.0% to 112.5%. The porous metal membrane is durable, simple, inexpensive, reproducible and has a high adsorption ability for use in SPE of PAHs.     

Separation and Determination of Diol Pollutants Come from Demulsifier in the Produced Water of Oil Fields

This paper introduced a novel test method for determination of the main demulsifier‐based pollutants in the produced water of oil wells, before draining to seawater. The type, concentration and distribution of diols depend on the demulsifier of interest; however, the main chemical species are the same. The novelty of this work is selective extraction and preconcentration of low chain diols in the produced water of oil fields. In this extraction method, dispersive solvent (1.2 mL, acetonitrile) containing extraction solvent (10.0 μL, carbon tetrachloride) was rapidly injected into the water sample containing analytes, and a cloudy solution was formed. After centrifugation (2 min at 3,000 rpm), these droplets were sedimented in the bottom of the conical test tube. Then 2.0 mL of sedimented phase containing preconcentrated analytes was injected into the gas chromatograph with flame ionization detector. The parameters affecting the extraction efficiency were evaluated and optimized. Factors such as the kind and volume of both extraction and disperser solvents, extraction and centrifugation times, pH and temperature, and salt effect were studied and optimized. The method exhibited enrichment factors and recoveries ranging from 39.0 to 44.4 and 78.9 to 92.2%, respectively, within very short extraction time. The linearity (and limit of detection) of the method ranged 4.0‐100.0 (2.0) ng/mL for 1,2‐ethandiol, 6.0‐80.0 (4.0) ng/mL for 1,3‐propandiol, 2.0‐90.0 (1.0) ng/mL ng/mL for 1,4‐butanediol, 5.0‐120.0 (2.0) ng/mL for 1,5‐pentandiol. The relative standard deviations (RSD) for the concentration of diols, 5.0 ng/mL in water by using the internal standard were in the range of 1.5–4.5% (n = 5) and without the internal standard was in the range of 2.6–9.0% (n = 5). It is concluded that this method is successful for determination of diols in produced water samples.     

Determination of palladium(II) with alpha-(2-benzimidazolyl)-alpha',alpha'-(N-5-nitro-2-pyridylhydrazone)-toluene by adsorptive cathodic stripping voltammetry

The determination of palladium(II) complexed with alpha-(2-benzimidazolyl)-alpha',alpha'-(N-5-nitro-2-pyridylhydrazone)-Toluene (BINPHT) was investigated by adsorptive cathodic stripping voltammetry using hanging mercury drop electrode. Palladium(II) in the sample solution can be determined in BINPHT and ethylenediaminetetraacetic acid (EDTA). Accumulation is achieved by adsorption of Pd(II)-BINPHT complex on a hanging mercury drop electrode. Optimal conditions were found to be: supporting electrolyte; 0.01 M sodium acetate buffer at pH 5.0, accumulation potential; -590 mV versus Ag/AgCl, accumulation time; 180 s, scan rate; 50 mV s(-1), concentration of BINPHT; 2x10(-5) M. The linear range of Pd(II) was observed over the concentration range 20-100 ng ml(-1) The detection limit (S/N=3) is 2 ng ml(-1). A good reproductivity shows RSD of 2.0% (n=7). This procedure offers high selectivity with the presence of EDTA masking some metallic ions. River water sample spiking with palladium was determined.     

Use of Saccharomyces cerevisiae immobilized in agarose gel as a binding agent for diffusive gradients in thin films

A new binding agent, consisting of the yeast Saccharomyces cerevisiae immobilized in agarose, is proposed for use in diffusive gradients in thin films (DGT). Different gel compositions, containing from 4.5% to 20% (m/v) of S. cerevisiae and 1.5-5.0% (m/v) of agarose, were prepared and tested for uptake of Cd(II). For gels containing 20% (m/v) of S. cerevisiae, a mass of 14,900 ng has been attributed as the uptake limit of Cd for each disk. Determination of the Cd retained in the binding agent was readily carried out using a slurry of the agarose-yeast disk introduced directly into the inductively coupled plasma optical emission spectrometer. The performance characteristics of the DGT samplers, which were assembled with the proposed binding agent (25 mm disk containing 20% of S. cerevisiae and 1.5% of agarose) and a diffusive layer of cellulose (chromatographic paper 3MM Chr of 25 mm diameter), were evaluated by measuring the Cd(II) uptake at various pH values and ionic strengths. Very consistent results were found within the pH range 4.5-7.5 and at ionic strengths ≥0.005 mol L⁻¹. The precision of DGT measurements was characterized by relative standard deviations of <8%. No changes in the uptake of Cd(II) were observed in the samplers that were assembled with recently prepared disks or 35-day-old stored disks. The proposed material has been applied to the analyses of river and sea water samples. For determination of Cd(II), excellent agreement between the results obtained from devices assembled with the proposed material and those assembled with conventional material (Chelex-100 resin) were obtained, strongly validating the use of the agarose-yeast gel disk as a new binding agent for DGT.     

Bis(2-hydroxyacetophenone)ethylenediimine as a neutral carrier in a coated-wire membrane electrode for lead(II).

A coated-wire ion-selective electrode (CWISE), based on a Schiff base as a neutral carrier, was successfully developed for the detection of Pb(II) in aqueous solution. CWISE exhibited a linear response with a Nernstian slope of 29.4 +/- 0.5 mV/decade within the concentration range of 1.0 x 10(-5) - 1.0 x 10(-1) M lead ion. CWISE has shown detection limits of 5.0 x 10(-6) M. The electrode exhibited good selectivity over a number of alkali, alkaline earth, transition and heavy metal ions. This sensor yielded a steady potential within 10 to 20 s at a linear dynamic range. The electrode was suitable for use in aqueous solutions in a pH range of 2.0 to 5.0. Applications of this electrode for the determination of lead in real samples and as indicator electrode for potentiometric titration of Pb2+ ion using K2CrO4 are reported.     

Study on the solid phase extraction of Co(II)-QADEAB chelate with C(18) disk and its application to the determination of trace cobalt

A sensitive, selective and rapid method has been developed for the determination mug l(-1) level of cobalt based on the rapid reaction of cobalt(II) with 2-(2-quinolylazo)-5-diethylaminobenzoic acid (QADEAB) and the solid phase extraction (SPE) of the colored chelate with Waters Porapak(R) Sep-Park C(18) disk. The QADEAB can react with Co(II) in the presence of pH 3.8 acetic acid-sodium acetate buffer solution and cetyl trimethylammonium bromide (CTMAB) medium to form a violet chelate of a molar ratio 1:2 (cobalt to QADEAB). This chelate can retained on Waters Porapak(R) Sep-Park C(18) disk quantitatively when they passed the disk as aqueous solution. After the enrichment finished, the retained chelate can be eluted from disk by 2.5 ml of ethanol (contain 5% acetic acid). In the measured solution, the molar absorptivity of the chelate is 1.58x10(5) l mol(-1) cm(-1)at 635 nm, and Beer's law is obeyed in the range of 0.01-0.4 mug ml(-1). The relative standard deviation for 11 replicate sample of 0.01 mug ml(-1) level is 2.23%. The detection limit is 0.01 mug l(-1) (in original samples). This method can be applied to the determination of mug l(-1) level of cobalt in drinking water with satisfactory results.     

Preconcentration and separation of iron, zinc, cadmium and mercury, from waste water using Nile blue a grafted polyurethane foam

The present work describes a novel method for the incorporation of Nile blue A into polyurethane foam matrix. This foam material was found to be very suitable for the extraction of metal ions from aqueous solutions. The characterization of Nile blue A grafted foam and the effect of halide concentration, pH, shaking time, extraction isotherm and capacity have been investigated. This foam material was found to be suitable for the separation and preconcentration of iron (III), zinc (II), cadmium (II) and mercury (II) from waste water. The extraction was accomplished in (15-20) minutes. Iron was separated from acid medium (2-4 M HCl), zinc from (3-5 M HCl), cadmium from (4-6 M HCl) as thiocyanate complexes and mercury was separated from (1-2 M HCl) as chloride.     

Selective extraction of melamine using 11-mercaptoundecanoic acid-capped gold nanoparticles followed by capillary electrophoresis

This study describes the use of 11-mercaptoundecanoic acid-capped gold nanoparticles (MUA-AuNPs) for selective extraction of melamine prior to analysis by capillary electrophoresis with UV detection. The highest degree of melamine-induced aggregation of MUA-AuNPs was found to occur at pH 5.0, indicating that the NP aggregation is mainly because of hydrogen bonding between the carboxylate groups of MUA and the amine groups of melamine. Moreover, the degree of melamine-induced NP aggregation gradually increased when the chain length of the mercaptoalkanoic acid was increased from two to 12 carbon atoms. At pH 5.0, the extraction efficiency of melamine was highly dependent on the concentration of MUA-AuNPs, the concentration of dithiothreitol (DTT), the extraction time between MUA-AuNPs and melamine, and the incubation time between melamine-adsorbed AuNPs and DTT. The separation of the extracted melamine and DTT (releasing agent) was accomplished using a solution of 10 mM phosphate (pH 6.0) containing 1.6% (v/v) poly(diallyldimethylammonium chloride). Under the optimum extraction and separation conditions, the limit of detection at a signal-to-noise ratio of 3 was estimated to be 77 pM for melamine, with linear range of 1-1000 nM. The proposed method was successfully applied to the determination of melamine in tap water and in milk.     

Selective extraction of gold(III) in the presence of Pd(II) and Pt(IV) by salting-out of the mixture of 2-propanol and water

The mixture of 2-propanol with water has been employed to extract Au(III) along with other precious metals such as Pd(II) and Pt(IV) by using NaCl in the concentration range of 2.5-4.0 mol dm(-3). Upon the addition of NaCl within this concentration range (2.5-4.0 mol dm(-3)) phase separation was attained. Gold(III) in aqueous phase was quantitatively extracted into the 2-propanol phase at 2.5-4.0 mol dm(-3) of NaCl. The extraction of the other metals such as Pd(II) and Pt(IV) was much lower than for that of Au(III). Thus a maximal selective separation of Au(III) from these metals could be attained using the mixture of 2-propanol with water. A reaction mechanism involving the ion-pair of Na(+) and [AuCl(4)](-) has been proposed to explain this extraction.     

Determination of four aromatic amines in water samples using dispersive liquid-liquid microextraction combined with HPLC

A new method for the determination of four aromatic amines in water samples was developed by using dispersive liquid-liquid microextraction (DLLME) technique combined with HPLC-variable wavelength detection (HPLC-VWD). In this extraction method, 0.50 mL methanol (as dispersive solvent) containing 25.0 microL tetrachloroethane (as extraction solvent) was rapidly injected by a syringe into 5.00 mL water sample. Accordingly, a cloudy solution was formed. After centrifugation for 2 min at 4000 rpm, the fine droplets of the tetrachloroethane containing the analytes were sedimented in the bottom of the conical test tube (7+/-0.2 microL). Then, 5.0 microL of the settled phase was determined by HPLC-VWD. Parameters such as the kind and volume of extraction solvent and dispersive solvent, extraction time, and salt concentration were optimized. Under the optimum conditions, the enrichment factors ranged from 41.3 to 94.5. Linearity was observed in the range of 5-5000 ng/mL. The LODs based on S/N of 3 ranged from 0.8 to 1.8 ng/mL. The RSDs (for 400 ng/mL of p-toluidine and o-chloroaniline, 100 ng/mL of p-chloroaniline and p-bromoaniline) varied from 4.1 to 5.3% (n=6). The water samples collected from rivers and lakes were successfully analyzed by the proposed method and the relative recoveries were in the range of 85.4-111.7% and 90.2-101.3%, respectively.     

Rapid automated in-situ monitoring of total dissolved iron and total dissolved manganese in underground water by reverse-flow injection with chemiluminescence detection during the process of water treatment

Measurement of iron and manganese is very important in evaluating the quality of natural waters. We have constructed an automated Fe(II), total dissolved iron(TDI), Mn(II), and total dissolved manganese(TDM) analysis system for the quality control of underground drinking water by reverse flow injection analysis and chemiluminescence detection(rFIA-CL). The method is based on the measurement of the metal-catalyzed light emission from luminol oxidation by potassium periodate. The typical signal is a narrow peak, in which the height is proportional to light emitted and hence to the concentration of metal ions. The detection limits were 3 x 10(-6)mug ml(-1) for Fe(II) and the linear range extents up to 1.0 x 10(-4) and 5 x 10(-6)mug ml(-1) for Mn(II) cover a linear range to 1.0 x 10(-4)mug ml(-1). This method was used for automated in-situ monitoring of total dissolved iron and total dissolved manganese in underground water during water treatment.     

Head-column field-amplified sample stacking in capillary electrophoresis for the determination of cimetidine, famotidine, nizatidine, and ranitidine-HCl in plasma.

In this study, low concentrations of histamine2-receptor (H2-)antagonists were effected across a water plug, with separation taking place in a binary buffer comprising ethylene glycol and NaH2PO4 (pH 5.0), and detection at 214 nm. Liquid-liquid extraction with ethyl acetate- isopropanol is shown to provide extracts that are sufficiently clean. The calibration curves were linear over a concentration range of 0.1-2.00 microg/mL cimetidine, 0.2-5.0 microg/mL ranitidine-HCl, 0.3-5.0 microg/mL nizatidine, and 0.1-3.0 microg/mL famotidine. Mean recoveries were > 82%, while the intra- and interday relative standard deviations (RSDs) and relative errors (REs) were all < 13%. The method is sensitive with a detection limit of 3 ng/mL cimetidine, 30 ng/mL ranitidine HCl, 50 ng/mL nizatidine and 10 ng/mL famotidine (S/N = 3, electric-driven injection 90 s). This newly developed capillary electrophoresis (CE) method was applied for the determination of analytes extracted from plasma taken from a volunteer dosing a cimetidine, ranitidine, and nizatidine tablet simultaneously. These three H2-antagonists can be detected in real samples by this method, excluding the low dosing of famotidine tablet.     

Application of dispersive liquid-liquid microextraction combined with high-performance liquid chromatography for the determination of methomyl in natural waters

A new method was developed for determination of methomyl in water samples by combining a dispersive liquid-liquid microextraction (DLLME) technique with HPLC-variable wavelength detection (VWD). In this extraction method, 0.50 mL of methanol (as dispersive solvent) containing 20.0 microL of tetrachloroethane (as extraction solvent) was rapidly injected by syringe into a 5.00-mL water sample containing the analyte, thereby forming a cloudy solution. After phase separation by centrifugation for 2 min at 4000 rpm, the enriched analyte in the settled phase (8 +/- 0.2 microL) was at the bottom of the conical test tube. A 5.0-microL volume of the settled phase was analyzed by HPLC-VWD. Parameters such as the nature and volume of the extraction solvent and the dispersive solvent, extraction time, and the salt concentration were optimized. Under the optimum conditions, the enrichment factor could reach 70.7 for a 5.00-mL water sample and the linear range, detection limit (S/N = 3), and precision (RSD, n = 6) were 3-5000 ng/mL, 1.0 ng/mL, and 2.6%, respectively. River and lake water samples were successfully analyzed by the proposed method. Comparison of this method with solid-phase extraction, solid-phase microextraction, and single-drop microextraction, indicates that DLLME combined with HPLC-VWD is a simple, fast, and low-cost method for the determination of methomyl, and thus has tremendous potential in trace analysis of methomyl in natural waters.