Liquid–Liquid Extraction of Organic Compounds into a Single Drop of the Extractant: Overview of Reviews

Single-drop microextraction (SDME) and hollow-fiber membrane microextraction (HFME) belong to methods of the liquid-phase microextraction preconcentration of organic compounds. These methods are characterized by the low consumption of organic solvents, high preconcentration factors, simplicity, low cost, ease of combination with various chromatographic methods; processes of preconcentration and sample injection are combined in a single device. Since the emergence of SDME (1996) and HFME (1999), a large number of versions have been developed that differ in the preconcentration technique, nature of the extractants used, and combinations with methods for the subsequent determination of the preconcentrated substances. The popularity of these methods among the analysts is evidenced by many reviews that we have summarized in this publication.

     

Measurement of the Bulk Modulus of a Liquid Using a Pump–Probe Laser Technique

We describe an undergraduate laboratory experiment that uses a time-resolved laser technique. Using pump-probe in a novel way, students determined the bulk modulus of a liquid. Employing the fourth harmonic from a pulsed Nd:YAG laser, an elastic wave is generated in an aqueous solution of N-acetyltryptophan, and the wave propagation is probed by a He-Ne laser. This experiment serves as a rare example of how a bulk property of a condensed phase can be measured using time-resolved optical measurements having relevance to undergraduate physical chemistry or material science laboratory.     

Viscosities for Ionic Liquid Binary Mixtures with a Common Ion

At present, there is a considerable amount of work devoted to the study of the thermophysical properties of pure ionic liquids, which contrasts with the few data available for their mixtures. One of the most appealing characteristics of ionic liquids is the capability of subtly changing the chemical structure of the cation and anion in order to design appropriate solvents for specific applications. Mixtures of ionic liquids increase enormously this specificity, due to the unlimited combinations that arise from mixing two or more ionic liquids. In this context, the study of the thermophysical properties of these mixtures is revealed as a fundamental task. In this work the viscosities of the ionic liquid binary mixtures with a common ion ([C₆mim] + [C₂mim])[BF₄], ([C₆mim] + [C₄mim])[BF₄], [C₄mim]([BF₄] + [MeSO₄]) and [C₄mim]([PF₆] + [BF₄]) were determined within the temperature range (298.15–308.15) K. The temperature dependence of the viscosity for pure liquids is analyzed by means of the Vogel-Tammann-Fulcher equation and several mixing rules are applied for the mixtures.     

Analysis of Mechanisms at the Plasma–Liquid Interface in a Gas–Liquid Discharge Reactor Used for Treatment of Polluted Water

Aqueous solutions polluted by contaminants different from those generally studied (phenol and chlorophenols) were treated in a falling film gas–liquid dielectric barrier discharge reactor. The lower was the Henry’s law constant of a molecule, the better was its removal percentage, regardless of its other chemical properties. In the case of saturated molecules, the removal mechanism is the transfer of pollutants from the liquid phase to the gas phase where they react with the active species of the discharge. For phenol, the reaction with ozone in the liquid phase was estimated to be responsible of about 30% of the removal. A computational fluid dynamic modelling provided a better understanding of the phenomena, indicating that mass transfer of pollutants from liquid to gas is accelerated due to (1) the intense mixing in the liquid film and (2) the reaction of the pollutant with the active species in the gaseous phase.     

Spectrophotometric Determination of Copper in Waste Water Using Liquid–Liquid Extraction in a Flow-Injection System

A flow-injection procedure for the determination of copper in waste water is described. The procedure is based on the formation of an ion-pair between tetrabutylammonium and the tetrathiocyanatocuprate(II) anion. This ion-pair is extracted with chloroform, and the absorbance of the organic phase is measured at 485nm. Concentrations of copper 0.4mg L^–1 can be detected with a linear application range of between 0.5 and 25mg L^–1, a relative standard deviation of 2.3% (0.8mg L^–1, n=10) and a sampling frequency of 30h^–1. Under the proposed working conditions, only Ni(II) and Pd(II) interfere at molar ratios Ni(II)/Cu(II) 50 and Pd(II)/Cu(II) 100.     

Determination of Cadmium in Human Serum and Blood Samples after Dispersive Liquid–Liquid Microextraction Using a Task-Specific Ionic Liquid

Task-specific ionic liquid dispersive liquid–liquid microextraction (TSIL-DLLME) is a simple and rapid preconcentration approach for the measurement of cadmium in serum and blood samples of human subjects. In this method a novel task-specific ionic liquid, trioctylmethyl ammonium thiosalicylate (TOMATS), which has dual characteristics as a chelating agent and extractive solvent, was investigated. TOMATS complexes with Cd due to the chelating effect of the ortho-positioned carboxylate relative to the thiol functionality. The assessment of the optimum values of variables including the pH, amount of reagents (TOMATS, diluents, Triton X114, and back extracting acid solution), temperature, and incubation time, which affect the recoveries of analyte by TSIL-DLLME method were studied. After enrichment experiments, acidic solution was used to back extract the metal ions from the ionic liquid rich phase and with determination by electrothermal atomic absorption spectrometry. Using the optimal experimental conditions, the limit of detection (3?s), precision (relative standard deviation), preconcentration, and enhancement factors of developed method for Cd were found to be 0.05?µg/L, greater than 5%, 62.5, and 52.8, respectively. To check the accuracy of the developed method, certified reference material of serum and blood were analyzed by the developed method, and the measured values of Cd were in good agreement with the certified values. The developed method was applied successfully to determine Cd in blood and serum samples of lymphatic cancer patients relative to healthy controls.     

A Dissipative Reaction Network Drives Transient Solid–Liquid and Liquid–Liquid Phase Cycling of Nanoparticles

Transient states maintained by energy dissipation are an essential feature of dynamic systems where structures and functions are regulated by fluxes of energy and matter through chemical reaction networks. Perfected in biology, chemically fueled dissipative networks incorporating nanoscale components allow the unique properties of nanomaterials to be bestowed with spatiotemporal adaptability and chemical responsiveness. We report the transient dispersion of gold nanoparticles in water, powered by dissipation of a chemical fuel. A dispersed state that is generated under non-equilibrium conditions permits fully reversible solid–liquid or liquid–liquid phase transfer. The molecular basis of the out-of-equilibrium process is reversible covalent modification of nanoparticle-bound ligands by a simple inorganic activator. Activator consumption by a coupled dissipative reaction network leads to autonomous cycling between phases. The out-of-equilibrium lifetime is tunable by adjusting the pH value, and reversible phase cycling is reproducible over several cycles.     

Determination of Iron in Tap and Waste Water Using Liquid–Liquid Extraction in a Flow-Injection System

A flow-injection procedure for the determination of iron(III) in water is described. The procedure is based on the formation of an ion pair between the tetraphenylarsonium (Ph₄As⁺) (TPA) or tetrabutylammonium (But₄N⁺) (TBA) cations and the tetrathiocyanatoferrate(III) complex (TTF). This ion pair is extracted with chloroform, and the absorbance of the organic phase is measured at 503nm (for Ph₄As⁺) or 475nm (for But₄N⁺). Iron concentrations higher than 0.9×10^–6molL^–1 (50µgL^–1) can be detected in the first case, with a relative standard deviation of 1.9% (n=12), a linear application rangeof between 1.34 and 54.0×10^–6molL^–1 (75–3015µgL^–1), and a sampling frequency of 30h^–1. For the ion pair with But₄N⁺, the detection limit is 0.52×10^–6molL^–1 (29µgL^–1), with a relative standard deviation of 1.6% and a linear application range between 0.73 and 54.0×10^–6molL^–1. Under the proposed working conditions, only Pd(IV), Cu(II) and Bi(III) interfere. With the application of the merging zones technique, considerable amounts of organic reagent can be saved. The TBA method was applied to the analysis of iron(III) in tap and industrial waste waters.     

Determination of BTEX Compounds by Dispersive Liquid–Liquid Microextraction with GC–FID

Rapid, inexpensive, and efficient sample-preparation by dispersive liquid–liquid microextraction (DLLME) then gas chromatography with flame ionization detection (GC–FID) have been used for extraction and analysis of BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) in water samples. In this extraction method, a mixture of 25.0 μL carbon disulfide (extraction solvent) and 1.00 mL acetonitrile (disperser solvent) is rapidly injected, by means of a syringe, into a 5.00-mL water sample in a conical test tube. A cloudy solution is formed by dispersion of fine droplets of carbon disulfide in the sample solution. During subsequent centrifugation (5,000 rpm for 2.0 min) the fine droplets of carbon disulfide settle at the bottom of the tube. The effect of several conditions (type and volume of disperser solvent, type of extraction solvent, extraction time, etc.) on the performance of the sample-preparation step was carefully evaluated. Under the optimum conditions the enrichment factors and extraction recoveries were high, and ranged from 122–311 to 24.5–66.7%, respectively. A good linear range (0.2–100 μg L⁻¹, i.e., three orders of magnitude; r ² = 0.9991–0.9999) and good limits of detection (0.1–0.2 μg L⁻¹) were obtained for most of the analytes. Relative standard deviations (RSD, %) for analysis of 5.0 μg L⁻¹ BTEX compounds in water were in the range 0.9–6.4% (n = 5). Relative recovery from well and wastewater at spiked levels of 5.0 μg L⁻¹ was 89–101% and 76–98%, respectively. Finally, the method was successfully used for preconcentration and analysis of BTEX compounds in different real water samples.

     

Determination of Alkylphenols in Water by Dispersive Liquid–Liquid Microextraction Based on Solid Formation without a Disperser

Dispersive liquid–liquid microextraction based on solid formation without a disperser combined with high-performance liquid chromatography has been developed for the determination of 4-tert-butylphenol, 4-n-nonylphenol, and 4-tert-octylphenol. This method is rapid, easy, and uses only 10 µL of a low toxicity organic solvent (1-hexadecanethiol) for the extraction solvent and no disperser solvent. The extraction time and centrifugation time require less than 10 min. The linear range was 1–500 ng mL^?1 for 4-tert-butylphenol, 2–1000 ng mL^?1 for 4-tert-octylphenol, and 5–500 ng mL^?1 for 4-n-nonylphenol with r² ≥ 0.9986. The detection limits were between 0.2 and 1.5 ng mL^?1. The recoveries of lake and river water samples were in the range of 79% to 108%, and the relative standard deviations were 5% to 10%.     

Determination of Aristocularine Enantiomers by Dispersive Liquid–Liquid Microextraction and Chiral High-performance Liquid Chromatography

Here is reported a novel analytical approach for the extractive separation and determination of enantiomeric ratios of aristocularine in bovine serum albumin. The results demonstrate suitable analytical performances. The separation was performed by chiral high-performance liquid chromatography with a 5-µm column using a mobile phase of 1:1 n-hexane:ethanol at a flow rate of 0.7?mL?min^?1 with ultraviolet–visible absorption, circular dichroism, and polarimetric detection. The enantiomers were eluted at 13.2 and 15.6?min for (+) and (?)-aristocularine, with a resolution of 1.58 and a separation factor of 1.27. The analytical parameters for the dispersive liquid–liquid microextraction were optimized; under these conditions, the extraction recoveries were from 88.6% to 93.9% for a two-step extraction. The precision, reported as the percent relative standard deviation, had values from 2.9% to 3.2% for 0.5?µg?mL^?1 of analyte for five replicate measurements using ultraviolet–visible absorption and circular dichroism detection. The limits of detection were between 0.05 and 0.08?µg?mL^?1 with enrichment ratios up to a value of 12.     

LC Determination of Chloramphenicol in Honey Using Dispersive Liquid–Liquid Microextraction

A novel method, dispersive liquid–liquid microextraction coupled with liquid chromatography-variable wavelength detector (LC-VWD), has been developed for the determination of chloramphenicol (CAP) in honey. A mixture of extraction solvent (30 μL 1,1,2,2-tetrachloroethane) and dispersive solvent (1.00 mL acetonitrile) were rapidly injected by syringe into a 5.0 mL real sample for the formation of cloudy solution, the analyte in the sample was extracted into the fine droplets of C₂H₂Cl₄. After extraction, phase separation was performed by centrifugation and the enriched analyte in the sedimented phase was determined by LC-VWD. Some important parameters, such as the kind and volume of extraction solvent and dispersive solvent, extraction time, sample solution pH, sample volume and salt effect were investigated and optimized. Under the optimum extraction condition, the method yields a linear calibration curve in the concentration range from 3 to 2,000 μg kg^?1 for target analyte. The enrichment factor for CAP was 68.2, and the limit of detection (S/N = 3) were 0.6 μg kg^?1. The relative standard deviation (RSD) for the extraction of 10 μg kg^?1 of CAP was 4.3% (n = 6). The main advantages of method are high speed, high enrichment factor, high recovery, good repeatability and extraction solvent volume at μL level. Honey samples were successfully analyzed using the proposed method.     

Dynamic and Rheological Properties of Span 80 at Liquid–Liquid Interfaces

The adsorption characteristics of Span 80 at liquid/liquid interfaces were investigated. The equilibrium interfacial tension values were successfully fitted with a Langmuir isotherm resulting in the determination of a mean molecular area from 25 to 35 Å²/mol. The measured interfacial tension values and deduced adsorption parameters depend on the experimental technique used to obtain them, either Du Noüy ring or profile analysis tensiometry. Two possible explanations to such phenomenon are provided. Adsorption kinetics of Span 80 at liquid/liquid interfaces were studied, and it was concluded that the diffusion of Span 80 molecules from the bulk is the rate determining step of the adsorption. Finally the interfacial rheology properties were investigated and compared to the Lucassen–van den Tempel model. A good match was obtained when the isotherm parameters determined by profile analysis tensiometry were used.      

Dispersive Liquid–Liquid Microextraction for the Preconcentration of Multiple Classes of Pesticides in Water

A simple, rapid, efficient, and environmentally friendly method was developed for the preconcentration of atrazine, simazine, diuron, bentazone, tebuconazole, and fipronil from water. Dispersive liquid–liquid microextraction was employed with determination by liquid chromatography–tandem mass spectrometry. The volumes of extraction and disperser solvents, the concentration of sodium chloride, and the pH were optimized by response surface methodology. The optimum conditions involved the use of 150 µL of 1:1 (v/v) monochlorobenzene:dichlorobenzene as the extraction solvent, 2 mL acetonitrile as the disperser solvent, and 10 mL of sample at pH 3.0. The accuracy was evaluated in terms of recovery values that were from 54 to 112%. The relative standard deviations ranged from 4 to 27%. The limits of quantification were between 0.005 and 0.05 µg L^?1. The optimized method had low matrix effects for the analytes and the results demonstrated application for the determination of pesticides in water.     

Liquid–Liquid–Liquid Micro Extraction Combined with CE for the Determination of Rare Earth Elements in Water Samples

A simple method for determination of rare earth elements (REEs) by liquid–liquid–liquid microextraction (LLLME) coupled with capillary electrophoresis and ultraviolet technique was developed. In the LLLME system, 40 mmol L^?1 4-benzoyl-3-methy-1-phenyl-5-pyrazolinone (PMBP) acted as extractant and 4% (v/v) formic acid was used as back-extraction solution. The parameters influencing the LLLME, including the type of the organic solvent, sample pH, formic acid concentration, PMBP concentration, extraction time, volume of organic solvent, stirring rate and phase volume ratio, were investigated. Under the optimized conditions, the detection limits (S/N = 3) of REEs were in the range of 0.19–0.70 ng mL^?1. The developed method was successfully applied to the determination of trace amounts of REEs in water samples.     

Chemistry of the Positive and Negative Electrical Discharges Formed in Liquid Water and Above a Gas–Liquid Surface

The physical processes and chemical reactions that take place inside different temperature plasma zones in water are only partially understood. The present study uses the emission spectroscopy and hydrogen peroxide measurements as indicators of the processes that take place on the gas–liquid boundary and inside plasma. Based on the hydrogen peroxide measurements with negative and positive high-voltage polarities as a function of solution conductivity, it was concluded that the main difference between positive polarity plasma and negative polarity plasma lies in the active radical concentration inside plasma. Data suggested that in the positive polarity electrical discharge the hydrogen peroxide concentration depends on the solution pH, whereas in the negative polarity discharge, it depends on the solution conductivity. Also, only in the negative polarity discharge do some of the electrons that are emitted from the high voltage electrode diffuse into the bulk where they react with the solutes.