Dispersive liquid–liquid microextraction using a surfactant as disperser agent

A novel and efficient surfactant-assisted dispersive liquid–liquid microextraction combined with high-performance liquid chromatography–photodiode array detection was developed for the determination of phenylurea herbicides in water samples. Based on this procedure, which is a dispersive-solvent-free technique, the extractant is dispersed in the aqueous sample using methyltrialkylammonium chloride. Compared with the conventional dispersive liquid–liquid microextraction, the new extraction method has many advantages such as higher extraction efficiency, low cost, reduced environmental hazards, and consumption of less extracting solvent. A few microliters of chloroform containing an appropriate amount of methyltrialkylammonium chloride (mixture of C8–C10) was used to extract the analytes from water samples. The main parameters relevant to the extraction process (namely, type of surfactant, selection of extractant solvent, extractant volume, surfactant concentration, ionic strength, and extraction time) were investigated. The performed analytical procedure showed limits of detection ranging from 2.3 to 18 ng/L, and precision ranges from 0.6% to 2.0% (as intra-day relative standard deviation, RSD) and from 1.3% to 8.3% (as inter-day RSD) depending on the analyte. The method showed good linearity between 0.04 and 40 μg/L with squared correlation coefficients better than 0.9920. This newly established approach was successfully applied to spiked real water samples.     

Porous electrospun ultrafine fibers via a liquid–liquid phase separation method

This communication reported a new simple method to produce porous ultrafine fibers during electrospinning process. It was named liquid–liquid phase separation method. This method required introducing suitable amount of water into electrospun solution to form similar stable three-phase membrane-forming systems previously. Scanning electron microscopy was employed to evaluate the morphology of porous ultrafine fibers. According to the investigation result, porous ultrafine fibers were prepared by this method. Liquid–liquid phase separation caused by the change of the property of three-phase systems of charged jet that was led by the rapid solvent evaporation during electrospinning process was considered as the key role for the formation of porous structure. This method is very simple and it not only lowers the requirement on the solvents but also does not introduce any new component into the fibers. Therefore, this method provides a new simple way to form porous structure during electrospinning process.     

Voltammetric determination of perchlorate ion at a liquid–liquid microscopic interface

A method is proposed for the electrochemical determination of perchlorate ion by voltammetry at the interface between two immiscible phases (water–o-nitrophenyl octyl ether). A demountable original-design amperometric ion-selective electrode based on a laser-microperforated polymeric membrane was fabricated for voltammetric measurements. The conditions of analytical signal recording in the determination of ClO₄ ^? were determined. The effect of interfering ions was assessed and amperometric selectivity coefficients were calculated. The accuracy of the procedure was verified by the added–found method. The developed electrode was applied to the determination of perchlorate in natural and drinking waters.     

Solid or liquid? Solidification of a nanoconfined liquid under nonequilibrium conditions

There has been a long-standing debate about the physical state and possible phase transformations of confined liquids. In this report, we show that a model-confined liquid can behave both as a Newtonian liquid with very little change in its dynamics and as a pseudosolid, depending solely on the rate of approach of the confining surfaces. Thus, the confined liquid does not exhibit any confinement-induced solidification in thermodynamic equilibrium. Instead, solidification is induced kinetically when the two confining surfaces are approached with a minimum critical rate. This critical rate is surprisingly slow (on the order of 6 A/s), explaining the frequent observation of confinement-induced solidification.     

Electrosynthesis of polyphenylpyrrole coated silver particles at a liquid–liquid interface

This communication reports the production of polyphenylpyrrole coated silver nanoparticles at the liquid/liquid interface by an EC-type mechanism. In the electrochemical step of the reaction N-phenylpyrrole facilitates the transfer of the silver ion from an aqueous to an organic phase. This step is followed by a slow homogeneous electron transfer reaction from the N-phenylpyrrole to the silver ion followed by polymerization and metal cluster growth.     

Automated on-line dispersive liquid–liquid microextraction based on a sequential injection system

A novel approach for sequential injection-dispersive liquid–liquid microextraction (SI-DLLME) has been suggested. The method is based on the aspiration and mixing of a sample and all required aqueous reagents in the holding coil of an SIA system, delivering it into a conical tube and adding in a mixture of extraction solvent, auxiliary solvent and disperser solvent at high flow rate, resulting in the formation of a cloudy state and the extraction of an analyte. The mixture of extraction and auxiliary solvent is immiscible with water and has a density significantly higher than that of water; consequently, the resulting fine droplets in the mixture, which contain the extracted analyte, are self-sedimented in a short time at the bottom of conical tube. Thus, no centrifugation and no use of a microcolumn are required for separation of the extraction phase. Afterwards, the extracted analyte is aspirated and transferred to a micro-volume Z-flow cell, and the absorbance is measured.The performance of the suggested approach is demonstrated by the SI-DLLME of thiocyanate ions in the form of ion associate with dimethylindocarbocyanine reagent, followed by spectrophotometric detection. A mixture of amyl acetate (as extraction solvent), tetrachloromethane (as auxiliary solvent) and acetonitrile (as disperser solvent) was selected for the DLLME procedure. The appropriate experimental conditions for conventional DLLME and automated SI-DLLME were investigated. The analytical performance of both these procedures was compared. The absorbance of the colored extracts at wavelength 555 nm obeys Beer's law in the range of 3.13–28.2 for conventional DLLME and 0.29–5.81 mg L^? 1 of SCN for SI-DLLME, and the limit of detection, calculated from a blank test based on 3 s, is 0.110 for conventional DLLME and 0.017 mg L^? 1 for SI-DLLME.     

Lipophilicity of ions electrogenerated at a Pt coated micropipette supported liquid–liquid interface

A novel method to readily determine the lipophilicity of electrogenerated charged species is reported. This is achieved by local electrolysis at a Pt coated micropipette and, subsequently, driving the electrogenerated species to transfer across the liquid–liquid interface supported at the tip of the micropipette under potential control. The formal potential of ion transfer can then be used to give a measure of its relative lipophilicity. The method proposed is facile and enables the study of potentially unstable charged products of electron transfer reactions.     

Membrane activity of ionisable drugs – a task for liquid–liquid electrochemistry?

Structurally related aminoacridine therapeutics (aminacrine, tacrine, velnacrine and proflavine) were studied electrochemically at the interface between two immiscible electrolyte solutions. To assess both their neutral and ionic partition coefficients, cyclic voltammetry was used to obtain the transfer potential as a function of the aqueous phase pH. Subsequently, in order to create a simple model of a biological membrane, a lipid monolayer was deposited at the liquid–liquid interface using the Langmuir–Blodgett technique. Electrochemical impedance spectroscopy was employed to study the interactions between the drug ions and the biomimetic phospholipid monolayer. Using a model incorporating a charge transfer mechanism coupled with potential dependent adsorption steps, the membrane activity of the studied drug ions could be estimated. The results indicate liquid–liquid electrochemistry to be a versatile tool for detecting differences in membrane activity even between structurally very similar drugs.     

Detection of haemoglobin using an adsorption approach at a liquid–liquid microinterface array

The behaviour of haemoglobin (Hb) at the interface between two immiscible electrolyte solutions (ITIES) has been examined for analytical purposes. When Hb is fully protonated under acidic conditions (pH <pI) in the aqueous phase, it undergoes a potential-dependent adsorption and complexation, at the interface, with the anions of the organic phase electrolyte. When utilised as a simple and fast preconcentration step, consisting of adsorbing the protein at the interface, in conjunction with voltammetric desorption, this opens up the ITIES to the adsorptive stripping voltammetry approach. Utilising a 60 s adsorption step and linear sweep voltammetry, a linear response to Hb concentration in aqueous solution over the range 0.01–0.5 μM was achieved. The equation of the best-fit straight line was I _p ? =?7.46 C???0.109, R?=?0.996, where I _p is the peak current (in nanoampere) and C is haemoglobin concentration (in micromolar). The calculated detection limit (3σ) was 48 nM for a 60 s preconcentration period, while the relative standard deviation was 13.3 % for six successive measurements at 0.1 μM Hb. These results illustrate the prospects for simple, portable and rapid label-free detection of biomacromolecules offered by electrochemistry at arrays of liquid–liquid microinterfaces.     

Determination of carbohydrates in tobacco by pressurized liquid extraction combined with a novel ultrasound-assisted dispersive liquid–liquid microextraction method

A novel derivatization-ultrasonic assisted-dispersive liquid–liquid microextraction (UA-DLLME) method for the simultaneous determination of 11 main carbohydrates in tobacco has been developed. The combined method involves pressurized liquid extraction (PLE), derivatization, and UA-DLLME, followed by the analysis of the main carbohydrates with a gas chromatography-flame ionization detector (GC-FID). First, the PLE conditions were optimized using a univariate approach. Then, the derivatization methods were properly compared and optimized. The aldononitrile acetate method combined with the O-methoxyoxime-trimethylsilyl method was used for derivatization. Finally, the critical variables affecting the UA-DLLME extraction efficiency were searched using fractional factorial design (FFD) and further optimized using Doehlert design (DD) of the response surface methodology. The optimum conditions were found to be 44 μL for CHCl₃, 2.3 mL for H₂O, 11% w/v for NaCl, 5 min for the extraction time and 5 min for the centrifugation time. Under the optimized experimental conditions, the detection limit of the method (LODs) and linear correlation coefficient were found to be in the range of 0.06–0.90 μg mL⁻¹ and 0.9987–0.9999. The proposed method was successfully employed to analyze three flue-cured tobacco cultivars, among which the main carbohydrate concentrations were found to be very different.     

On-column liquid–liquid–liquid microextraction coupled with base stacking as a dual preconcentration method for capillary zone electrophoresis

A simple and efficient dual preconcentration method of on-column liquid–liquid–liquid microextraction (LLLME) coupled with base stacking was developed for capillary zone electrophoresis (CZE) in this paper. Four N-methyl carbamates were used as target compounds to evaluate the enrichment means. The carbamates in sample solutions (donor phase) were extracted into a dodecanol phase immobilized on a porous hollow fiber, hydrolyzed and back extracted into 0.20 μL running buffer (acceptor phase) of 30 mmol/L methylamine hydrochloride (pH 11.6) containing 0.5 mmol/L tetradecyltrimethylammonium bromide inside the hollow fiber, stacked further with 0.5 mol/L NaOH injected at −10 kV for 60 s, and separated by CZE. Analytical parameters affecting the LLLME, base stacking and CZE were investigated, including sample solution volume, pH and temperature, extraction time, stirring rate, buffer component, buffer pH, NaOH concentration, stacking time, etc. The enrichment factors of the carbamates were higher than 1100. The relative standard deviation (RSD) of peak height and limits of detection (LODs) were 4.5–5.5% (n = 6) and 2–4 ng/mL (S/N = 3) for standard solutions, respectively. The proposed method was applied to the analysis of vegetable and fruit samples with the RSD less than 6.0% (n = 3) and LODs of 6–10 ng/g (S/N = 3). The calibration solutions were prepared by diluting the stock solutions with blank sample solutions, and the calibration concentrations ranged from 0.012 to 1.0 μg/mL (r > 0.9951). The analytical results demonstrated that the LLLME coupled with base stacking was a simple, convenient and reliable on-column sample pretreatment method for the analysis of anionic analytes in CZE.     

Molecular structure of mixed adsorption layers surfactant—polymer at a liquid—liquid interface

Isotherms of the binding of dodecyl sulfate anions (DDS^–)and Na⁺ counterions during their coadsorption with a nonionic polymer. proxanol (PR). at the interface of dodecane-water emulsions have been measured by conductometry and Na-selective potentiometry. The adsorption of DDS^– and PR is concurrent. The affinity constant of PR to the interface determined by the Langmuir model decreases as the concentration of PR increases, and the surface concentration of DDS^– tends to a nonzero limiting value at high concentrations of PR. The surface (_o) and electrokinetic () potentials at the interface have been determined at various polymer concentrations by the spin probe and electrophoresis methods. The average dielectric permeability and density of polymer segments in We adsorption layer have been determined by ESR. The lower boundary of the hydrodynamic thickness of the polymer adsorption layer at the interface has been estimated from the dependences of _o and on the ionic strength.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1653–1661, July, 1996.     

What can we learn by squeezing a liquid?

Relaxation times tau(T,upsilon) for different temperatures, T, and specific volumes, upsilon, collapse to a master curve vs Tupsilon(gamma), with gamma a material constant. The isochoric fragility, mV, is also a material constant, inversely correlated with gamma. From these experimental facts, we obtain a three-parameter function that accurately fits tau(T,upsilon) data for several glass-formers over the supercooled regime, without any divergence of tau below Tg. Although the values of the three parameters depend on the material, only gamma significantly varies; thus, by normalizing material-specific quantities related to gamma, a universal power law for the dynamics is obtained.     

Lab in a syringe: fully automated dispersive liquid–liquid microextraction with integrated spectrophotometric detection

A new approach for the integration of various analytical steps inside a syringe (Lab in a Syringe) is presented. Fully automated dispersive liquid-liquid microextraction with integrated spectrophotometric detection is carried out in-syringe using a very simple instrumental setup. The lighter-than-water organic droplets released in the extraction step accumulate at the head of the syringe, where two optical fibers are placed on both sides of the syringe, facing each other and enabling the in situ quantification of the extracted compounds. By this, monitoring of the progressively accumulating droplet in the head of the syringe was further possible. In this first report, the developed instrumental setup has been applied to the determination of the dye rhodamine B in water samples and soft drinks. The main parameters influencing the extraction such as the selection of the extractant and disperser solvents, extractant/disperser and organic/water phase ratios, pH of the aqueous phase, extraction flow rates, and extraction time were investigated. Under the selected conditions, rhodamine B was quantified in a working range of 0.023-2 mg L(-1) with a limit of detection of 0.007 mg L(-1). Good repeatability values of up to 3.2% (RSD) were obtained for ten consecutive extractions. The enrichment factor for a 1 mg L(-1) rhodamine B standard was 23, and up to 51 extractions were accomplished in 1 h.     

Development of a dispersive liquid–liquid microextraction method for the determination of fluoroquinolones in chicken liver by high performance liquid chromatography

A simple and cost effective sample pre-treatment method, dispersive liquid–liquid microextraction (DLLME), has been developed for the extraction of six fluoroquinolones (FQs) from chicken liver samples. Clean DLLME extracts were analyzed for fluoroquinolones using liquid chromatography with diode array detection (LC-DAD). Parameters such as type and volume of disperser solvent, type and volume of extraction solvent, concentration and composition of phosphoric acid in the disperser solvent and pH were optimized. Linearity in the concentration range of 30–500 μg kg⁻¹ was obtained with regression coefficients ranging from 0.9945 to 0.9974. Intra-day repeatability expressed as % RSD was between 4 and 7%. The recoveries determined in spiked blank chicken livers at three concentration levels (i.e. 50, 100 and 300 μg kg⁻¹) ranged from 83 to 102%. LODs were between 5 and 19 μg kg⁻¹ while LOQs ranged between 23 and 62 μg kg⁻¹. All of the eight chicken liver samples obtained from the local supermarkets were found to contain at least one type of fluoroquinolone with enrofloxacin being the most commonly detected. Only one sample had four fluoroquinolone antibiotics (ciprofloxacin, difloxacin, enrofloxacin, norfloxacin). Norfloxacin which is unlicensed for use in South Africa was also detected in three of the eight chicken liver samples analyzed. The concentration levels of all FQs antibiotics in eight samples ranged from 8.8 to 35.3 μg kg⁻¹, values which are lower than the South African stipulated maximum residue limits (MRL).     

Development of a new dispersive liquid–liquid microextraction method in a narrow-bore tube for preconcentration of triazole pesticides from aqueous samples

In the present work a new, simple, rapid and environmentally friendly dispersive liquid–liquid microextraction (DLLME) method has been developed for extraction/preconcentration of some triazole pesticides in aqueous samples and in grape juice. The extract was analyzed with gas chromatography–flame ionization detection (GC–FID) or gas chromatography–mass spectrometry (GC–MS). The DLLME method was performed in a narrow-bore tube containing aqueous sample. Acetonitrile and a mixture of n-hexanol and n-hexane (75:25, v/v) were used as disperser and extraction solvents, respectively. The effect of several factors that influence performance of the method, including the chemical nature and volume of the disperser and extraction solvents, number of extraction, pH and salt addition, were investigated and optimized. Figures of merit such as linearity (r² > 0.995), enrichment factors (EFs) (263–380), limits of detection (0.3–5 μg L^?1) and quantification (0.9–16.7 μg L^?1), and relative standard deviations (3.2–5%) of the proposed method were satisfactory for determination of the model analytes. The method was successfully applied for determination of target pesticides in grape juice and good recoveries (74–99%) were achieved for spiked samples. As compared with the conventional DLLME, the proposed DLLME method showed higher EFs and less environmental hazards with no need for centrifuging.     

Support-free pulsed liquid–liquid chromatography

A simple technique of support-free liquid–liquid chromatography is suggested that operates without incorporation of a centrifuge. The pulsed chromatography apparatus consists of a stationary coiled tube and a pulsation device to produce reciprocating motion of liquid phases within each individual coil segment. This reciprocating motion generates a centrifugal force field varying in intensity and direction that leads to an improved mixing of the two liquid phases and retains the stationary phase in the coiled tubing. The intensity of the back and forth motion of liquid phases within each coil unit can be varied by varying the frequency and/or the amplitude of the pulsations generated by the pulsation device. As the magnitude of the stationary phase retention is of paramount importance for success of the technique, the retention of the stationary phase in the pulsed coil column was experimentally studied. A few experiments were conducted to test the chromatographic behavior of valeric (n-pentanoic) and caproic (n-hexanoic) acids. The results obtained demonstrate the potential of the new separation method for preparative purposes.     

A dispersive liquid–liquid microextraction based on a task-specific ionic liquid for enrichment of trace quantity of cadmium in water and food samples

In this work, a hydrophilic task-specific ionic liquid (TSIL) of 1-chloroethyl-3-methylimidazolium chloride functionalized with 8-hydroxyquinoline was used in a dispersive liquid–liquid microextraction method followed by flame atomic absorption spectrometry for the enrichment and determination of trace amounts of cadmium (Cd²⁺) ions. The simultaneous chelation and extraction of Cd²⁺ ions was carried out by the TSIL. Fine droplets of the water-immiscible TSIL containing target analyte were generated in situ by addition of an anion exchanger potassium hexafluorophosphate (KPF₆) salt to the sample tube. After phase separation by centrifugation for 4 min, the sedimented TSIL was diluted with acidified ethanol for measurement of Cd²⁺ content. Some significant parameters influence the preconcentration of Cd²⁺ ions such as sample pH, TSIL volume, amount of KPF₆, non-ionic surfactant and salt concentration were investigated. Under the optimal conditions, calibration curve was linear in the range of 5–250 µg L^?1 Cd²⁺ with correlation coefficient of 0.9975 and a detection limit of 0.55 µg L^?1. The relative standard deviation for six replicate measurements of 50 µg L^?1 Cd²⁺ was 1.5%. The method was successfully applied for the extraction and determination of Cd²⁺ ions in water and food samples.     

Circulation microchannel for liquid–liquid microextraction

A new method has been developed for liquid–liquid microextraction utilizing a circulation microchannel. A glass microchemical chip having a circular shallow microchannel in contact with a surrounding deeper microchannel was fabricated by a two-step photolithographic wet-etching technique. Surface modification reagent was selectively introduced to the shallow channel by utilizing capillary force, and the surface of the shallow channel was selectively made hydrophobic. With the aid of the hydrophobic/hydrophilic surface patterning, it was possible to keep organic solvent in the circular channel while the aqueous sample solution was continuously flowing in the deep channel. As a result, concentration extraction from sample solution to stationary extractant with a nanoliter scale volume became possible. Concentration extraction has been difficult in a multiphase continuous flow. Function of the newly developed microextraction system was verified with methyl red as a test sample, and concentration extraction to reach equilibrium was successfully carried out. A novel surface modification method utilizing frozen liquid as a masking material was also developed as a reverse process to make the shallow channel hydrophilic and the deep channel hydrophobic. Visualization of circulation motion inside the circular shallow channel induced by flow in the deep channel was observed with a particle tracing method.     

Wagner liquid–vapour pressure equation constants from a simple methodology

A methodology to determine the A, B, C, and D constants from the Wagner equation is presented. The constants for 274 pure substances were determined by minimization in the sum of the squares of the relative deviation in liquid vapour pressure. For 69 chemical compounds, vapour pressures exist over the range from 1 kPa to the critical pressure and an average absolute deviation in vapour pressure of 0.039% was calculated. Using Antoine equation coefficients and initial guesses for a correlation in terms of the acentric factor, Wagner constants were estimated for substances with limited data within the range from (1 to 200) kPa. To validate the proposed methodology, vapour pressure predictions from 1 kPa to the critical pressure were made for 52 substances using Wagner parameters estimated from limited data. A value of 0.27% in average absolute deviation results for those substances. Finally the Waring criterion was applied to check the constants presented in this paper.