Gas injection membrane extraction for fast on-line analysis using GC detection

Membrane extraction has been interfaced with gas chromatography and mass spectroscopy for the analysis of volatile organics in water. The vacuum in a mass spectrometer provides fast mass transport. The time required to complete permeation in a GC interface can be fairly long, because the positive pressure of the carrier gas on the permeate side slows down the analyte permeation. The aqueous boundary layer formed on the membrane is considered to be the biggest contributor to the resistance to mass transfer. Another issue is the dispersion of analyte in the aqueous stream, which broadens the input pulse to the membrane. The overall effect of these two factors is to increase the analysis time. Gas injection of aqueous samples is presented in this paper to address these issues. Gas injection reduces the formation of boundary layer, and increases the overall diffusion coefficient seven times. Axial mixing of the sample with a gaseous eluent is minimal, and this eliminates the tailing in permeation profiles. The overall membrane extraction is found to be significantly faster when a gas is used to inject an aqueous sample. This method is also simpler in terms of instrumentation and operational procedures.     

Three-dimensional wall-bounded laminar boundary layer with span-wise cross free stream and moving boundary

In the current work, the 3D boundary layers of wall-bounded flow configurations were extended to the situations with span-wise cross moving boundary and free stream. The unsteady boundary layer is also addressed for the Falkner–Skan wedge flow with a span-wise oscillating wall or oscillating free stream. The span-wise secondary boundary layer equation is obtained using similarity transformation technique and solved analytically in terms of the primary stream-wise boundary layer flow solutions. Different fluid motion behaviors are found for these new solutions. It is found that for the span-wise secondary boundary layer flow there is no flow separation for any wall cross moving velocity, which is different from the primary stream-wise boundary layers with a reverse flow. For the unsteady boundary layer with an oscillating wall or free stream, it is seen that the solution is different from the Stokes oscillating plate or free stream problem. The unsteady wall drag increases with the increase in the oscillating frequency and decreases with increasing the primary span-wise free stream magnitude. The velocity overshooting near the wall is also seen for an oscillating free stream for a large oscillating frequency or a lower primary stream-wise free stream magnitude.     

Continuous-flow extraction of colloidal components in aqueous samples

In this study we report the development and performance of a system for continuous-flow extraction of dissolved and colloidal analytes in an aqueous matrix. Initial studies, using a classical segmented-flow extraction procedure, showed poor extraction efficiency for the hydrophobic colloidally dispersed analytes. Insufficient contact between the extractant and the colloidal constituents seems to be the primary reason for poor extraction. Improved performance is obtained when mechanical energy is added to the system, to effect a forced contact between the sample and the solvent. This was accomplished by injecting the extractant, with a high velocity, into the continuous flow of analyte through a narrow-bore nozzle. In this way, the solvent stream is dispersed into fine droplets with high kinetic energies. A region of intense turbulence is created, which was studied by high-speed photography using pulsed laser fluorescence. Comparison with classical flow extraction, using a model sample of colloidal wood resin compounds in water, showed that the dissolved components extracted well with both systems, while an extraction enhancement of up to 9 times was experienced with colloidal triglycerides.     

A nanoinjector for microanalysis

We describe a simple miniature injection device that can be used for introduction of nanoliter sample volumes in microfluidic systems. The hybrid microstructure consists of two hydraulically connected parts, a pulse micropump, and a multilevel cross-flow injector. Sample injection is accomplished by creating a transient pressure pulse in the sample line by means of the solenoid-based micropump. The sample line is aligned at right angles to the main carrier flow line. The two flow channels are located in two different parallel planes. The cross section of the two channels is defined by a self-sealing aperture in an elastomer. During the pressure pulse, the sample is introduced through this aperture directly into the main flow stream. Fast impulse-based injection causes rapid mixing of the injected sample with the main flow stream. This permits simple single-line manifold micro flow injection (MFI) systems. The deformation/relaxation of the elastomer is fast and repeatable; as such, rapid serial actuations essentially result in a larger injected sample volume without significantly affecting the peak shape. In the present form, 2-40-nL samples are easily injected by single injection, and the injected volume can be chosen by system parameters. The injection repeatability as observed by a photometric detector is better than 1.2% (n = 100).     

Kinetic model of membrane extraction with a sorbent interface

Membrane extraction with a sorbent interface (MESI) is an unique sample preparation alternative for trace organic analysis. The main features of MESI include its solvent-free nature, the rugged and simple design with no moving parts for long-term reliable performance, the fact that it is a single-step process which ensures good precision, its easy automation, and its feasibility for on-site operation. Among the available membrane extraction modules designed for the MESI system, the headspace configuration has continued to show its superior durability and versatility in membrane applications. The headspace membrane extraction configuration effectively eliminates the need for a sampling pump and flow metering and hence prevents the extraction system from plugging and greatly simplifies the extraction process. The module can be used for extraction of VOCs from gaseous, aqueous, or solid samples. A mathematical model has been developed for headspace membrane extraction of an aqueous sample, based on the assumption that the aqueous phase is perfectly stirred. The model is in good agreement with the experimental benzene extraction results obtained with an efficient agitation method such as high-speed magnetic stirring or sonication. The model has also been used to study the effects of various extraction parameters with respect to the sensitivity and response time of the MESI system. Sample agitation facilities analyte mass transport and hence improves both the system sensitivity and the response time. The sensitivity of the extraction method also increases with an increase of the extraction temperature.     

A mathematical model for kinetic study of analyte permeation from both liquid and gas phases through hollow fiber membranes into vacuum

A mathematical model and a Matlab-5 computer code have been developed to study the dynamic response of the hollow fiber membrane probe. The depletion layer formation at the sample/membrane interface is taken into consideration by the mathematical model for the liquid mobile phase. The code produces concentration profiles within a sample feed stream and in the membrane. Flux values at the vacuum side of the membrane can also be calculated as a function of time. The method can be applied both for gas and liquid feed streams. Concentration profiles in a mobile phase and the flux of analytes through the hollow fiber membrane inlet have been studied with this simulation technique as a function of the liquid-phase flow rate. The influence of the formation of a layer of the analyte depletion during the dynamic response has been considered. The shape of the depleted layer and selectivity of permeation from a liquid mobile phase through the membrane into the vacuum are shown to be dependent on the mobile-phase flow rate. In addition, for studied conditions, formation of a depletion layer is demonstrated to be fast compared with membrane diffusion. Thus, if a homogeneous aqueous sample is coming through the inlet cross-section of a hollow fiber membrane containing pure water, the response time mostly depends on analyte diffusivity in the membrane. However, if the aqueous sample is coming through the inlet cross-section of a hollow fiber membrane containing clean air, response time also depends on equilibrium analyte concentration in the depletion layer.     

Nanoliter solvent extraction combined with microspot MALDI TOF mass spectrometry for the analysis of hydrophobic biomolecules

A nanoliter solvent extraction technique combined with microspot matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is presented. This method involves the use of a nanoliter droplet containing organic solvents at the tip of a small capillary for extraction. The droplet is formed inside a microliter aqueous sample containing the analyte of interest. After extraction, the droplet is deposited onto a MALDI target precoated with a thin matrix layer. Since the nanoliter droplet never touches the sample container wall, any possible extraction of contaminants adsorbed on the plastic or glassware is avoided. In addition, there is no need to concentrate the organic phase after the extraction, thus avoiding any possible loss during the concentration step. The nanoliter volume can be readily deposited onto a MALDI target, producing a high analyte concentration within a microspot. Combined with microspot MALDI, this technique allows for very sensitive analysis of the extracted analyte. The performance of this technique is illustrated in several applications involving the detection of hydrophobic peptides or phospholipids. It is shown that very hydrophobic analytes can be extracted from small-volume samples containing a large amount of salts and/or more hydrophilic analytes, which tend to give dominant signals in conventional MALDI experiments. Nanoliter extraction of analyte from samples containing less than 100 nM hydrophobic analyte and over 1 microM easily ionized hydrophilic species is demonstrated. Finally, using the analysis of the ionophore valinomycin as an example, it is demonstrated that the technique is a more reliable tool for probing metal-peptide complexes than regular MALDI sample preparations.     

Simultaneous extraction and concentration by on-line hollow fiber membrane extraction

In the determination of trace-level pollutants in water, a concentration step is usually needed between extraction and analysis. In this paper, simultaneous extraction and concentration during on-line analysis using hollow fiber membranes is reported. Solvent loss across the membrane resulted in simultaneous concentration during extraction and had pronounced influence on enrichment factor and extraction efficiency. This phenomenon is an important consideration for analytical methods employing solvent extraction across a membrane, because it is possible to eliminate the concentration step. Continuous on-line monitoring of semivolatile compounds was achieved using this approach, and the effects of process variables on enrichment and extraction efficiency were studied. It was found that enrichment increased with solvent loss, even when the extraction efficiency decreased.     

Continuous ultrasound-assisted extraction coupled to flow injection-pervaporation, derivatization, and spectrophotometric detection for the determination of ammonia in cigarettes

A dynamic system for the continuous removal of ammonia from cigarettes with ultrasound assistance and iterative change of the flow direction of the extractant through the sample cell has been developed. A 0.1-g sample of cigarette was subjected to 7 min of ultrasound-assisted extraction (application and duration of pulse 0.7 s, output amplitude 85% of the converter nominal amplitude), and 1 M NaOH solution was used both as extractant and as carrier in the dynamic system. The ultrasound-assisted extractor was coupled to a pervaporation unit through a flow injection interface in order to develop a fully automated method. In arriving at the pervaporator, the ammonia is transferred from the donor-carrier stream to an acceptor stream, where the classical Berthelot reaction takes place--thus favoring pervaporation. The blue complex formed is spectrophotometrically monitored at 655 nm. The method was applied to the determination of ammonia in a selection of 10 European cigarette brands and Kentucky Reference 2R4F cigarettes.     

SFE-GC with quantitative transfer of the extraction effluent to a megabore capillary column

Supercritical fluid extraction was directly coupled with gas chromatography such that the entire extraction effluent was directed onto the GC column. A sealed connection between the restrictor and the column ensured that no loss of analyte could occur. The elevated pressure generated in the column itself resulted in a reduced flow rate through the column, making efficient trapping possible. Metal-jacketed capillary columns were used as they can tolerate the elevated pressures required. In addition to maximizing sensitivity, very practical extraction parameters were possible. For a 25-min extraction, with a 2.2 mL/min flow rate, and the column at 40 degrees C, analytes as volatile as C-14 were trapped and separated. Trapping efficiency improved further with shorter extraction times, mild cooling of the column, or lower flow rates.     

Immunologic trapping in supported liquid membrane extraction

To obtain a high degree of selectivity in sample preparation, supported liquid membrane (SLM) extraction was combined with immunologic recognition. The SLM employs a hydrophobic polymer for supporting the immobilization of an organic solvent, thus forming a nonporous membrane. Said membrane separates the aqueous sample on one side (donor) from a receiving aqueous phase on the other (acceptor). The extraction involves the partitioning of neutral compounds between the sample solution, continuously pumped alongside the membrane, and the membrane. From the membrane, reextraction takes place into a second aqueous phase containing antibodies specific for the target compound(s). Hence, there is a formation of an antibody-antigen complex at the heart of the sample preparation (ImmunoSLM). When the immunocomplex forms, the antigen can no longer redissolve in the organic membrane, thus being trapped in the acceptor. Consequently, the concentration gradient of free antigen over the membrane is ideally unaffected, this being the driving force for the process. With a surplus of antibody, the concentration of analyte in the receiving phase will easily exceed the initial sample concentration. In this work, the so formed immunocomplex was quantified on-line, using a fluorescein flow immunoassay in a sequential injection analysis (SIA) setup. The outlined ImmunoSLM-SIA scheme was successfully applied for the extraction of 4-nitrophenol from spiked water solutions as well as from a spiked wastewater sample, indicating that the immunoextraction can be suitable when dealing with difficult matrixes.     

The role of membrane-like stresses in determining the stability and sensitivity of the Antarctic ice sheets: back pressure and grounding line motion

Membrane stresses act along thin bodies which are relatively well lubricated on both surfaces. They operate in ice sheets because the bottom is either sliding, or is much less viscous than the top owing to stress and heat softening of the basal ice. Ice streams flow over very well lubricated beds, and are restrained at their sides. The ideal of the perfectly slippery bed is considered in this paper, and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces. Propagation distances depend sensitively on the rheological index, and can be very large for ice-type rheologies.A new analytical solution for ice-shelf profiles and grounded tractionless stream profiles is presented, which show blow up of the profile in a finite distance upstream at locations where the flux is non-zero. This is a feature of an earlier analytical solution for a floating shelf.The length scale of decay of membrane stresses from the grounding line is investigated through scale analysis. In ice sheets, such effects decay over distances of several tens of kilometres, creating a vertical boundary layer between sheet flow and shelf flow, where membrane stresses adjust. Bounded, physically reasonable steady surface profiles only exist conditionally in this boundary layer. Where bounded steady profiles exist, adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur (neutral equilibrium). If no solution in the boundary layer can exist, the ice-sheet profile must change.The conditions for existence can be written in terms of whether the basal rate factor (sliding or internal deformation) is too large to permit a steady solution. The critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line. High ice velocity and high stress both favour the existence of solutions and stability. Changes in these parameters can cause the steady solution existence criterion to be traversed, and the ice-sheet dynamics to change.A finite difference model which represents both neutral equilibrium and the dynamical transition is presented, and preliminary investigations into its numerical sensitivity are carried out. Evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem, which will hamper predictability.     

Chemicofunctional membrane for integrated chemical processes on a microchip

Here we report a design and synthesis of a chemically functional polymer membrane by an interfacial polycondensation reaction and multilayer flow inside a microchannel. Single and parallel dual-membrane structures are successfully prepared by using organic/aqueous two-layer flow and organic/aqueous/organic three-layer flow inside the microchannel followed by an interfacial polycondensation reaction. By using the inner-channel membrane, permeation of ammonia species through the inner-channel membrane is successfully achieved. Furthermore, horseradish peroxidase is immobilized on one side of the membrane surface to integrate the chemical transform function onto the inner-channel membrane. Here substrate permeation through the membrane and subsequent chemical transformation at the membrane surface are realized. The polymer membrane prepared inside the microchannel has an important role in ensuring stable contact of different phases such as gas/liquid or liquid/ liquid and the permeation of chemical species through the membrane. Furthermore, membrane surface modification chemistry allows chemical transformation of permeated chemical species. These methods are expected to lead to development of complicated and sophisticated chemical systems involving membrane permeation and chemical reactions.     

Kinetics and the on-site application of standards in a solid-phase microextraction fiber

The kinetics of the desorption of analytes from a SPME fiber into an agitated sample matrix was studied, and a theoretical model was proposed to describe the dynamic desorption process, based on the steady-state diffusion of analytes in the extraction phase and in the boundary layer. It was found that the desorption of analytes from a SPME fiber into an agitated sampling matrix is isotropic to the absorption of the analytes onto the SPME fiber from the sample matrix under the same agitation conditions, and this allows for the calibration of absorption using desorption. The calibration was accomplished by exposing a SPME fiber, preloaded with a standard, to an agitated sample matrix, during which desorption of the standard and absorption of analytes occurred simultaneously. When the standard was the isotopically labeled analogue of the target analyte, the information from the desorption process, i.e., time constant a, could be directly used for estimating the concentration of the target analyte. When the standard varied from the target analyte, the mass-transfer coefficient of the analyte could be extrapolated from that of the standard. These predictions agree well with experimental results. This approach facilitates the full integration of sampling, sample preparation, and sample introduction, especially for on-site or in vivo investigations, where the addition of standards to the sample matrix, or control of the velocity of the sample matrix, is very difficult.     

The thermal state of a porous wall under conditions of transpiration cooling

Results of numerical experiment are used for analysis of fields of temperature in a laminar boundary layer, in a porous wall, and in a cooling gas delivery chamber, as well as for analysis of heat transfer and of distribution of the temperature difference between the cooling gas and the porous wall frame and cooling efficiency. It is demonstrated that heat transfer between a porous wall of finite thickness and a high-temperature gas flow differs significantly from heat transfer with preassignment of the same intensity of injection and of the homogeneous thermal boundary condition directly on the surface subjected to flow. One of the reasons for this is the formation of wall temperature variable along the boundary layer.     

Direct plasma sample injection in multiple-component LC-MS-MS assays for high-throughput pharmacokinetic screening

The simultaneous dosing of numerous compounds followed by multiple-component analysis using LC-MS-MS (the N-in-1 approach) has significantly improved the throughput of the drug-screening process. However, plasma samples still need to be extracted before LC-MS-MS analysis, which frequently limits the throughput of the assay. In this work, a high-throughput on-line extraction technique has been developed for multiple-component LC-MS-MS assays using a high-flow column-switching technique. In N-in-1 LC-MS-MS assays, high sensitivity is required since the dose level is generally reduced to minimize drug-drug interactions. In addition, good chromatographic separation is essential to minimize interference and suppression effects. The direct plasma sample injection method developed in this work has successfully met the two requirements for multiple-component LC-MS-MS assays in high-throughput pharmacokinetic screening. Plasma samples containing a large number of potential drug candidates were directly injected onto an extraction column operated under a flow rate sufficiently high to exhibit a turbulent-flow profile. The extracted analytes were then eluted onto an analytical column via column switching for LC-MS-MS analysis. The use of turbulent flow resulted in a faster and more rugged extraction with reduced carryover compared with results obtained under laminar-flow conditions. Meanwhile, the use of a column-switching method maintained the chromatographic resolving power and high sensitivity of the LC-MS-MS assay. Separation efficiency, dynamic range, accuracy, and precision comparable with those of solid-phase extraction have been achieved with the turbulent-flow column-switching technique. As a result, this technique has been successfully and routinely used for high-throughput pharmacokinetic screening.     

聚丙烯中空纤维膜/二(2-乙基已基)膦酸体系萃取水溶液中铜离子的研究

研究了聚丙烯中空纤维膜接触萃取器中二(2-乙基已基)磷酸萃取金属铜离子的工艺条件以及溶剂夹带,分析了原料的pH值、两相流速、有机相初始铜离子浓度以及萃取膜面积对萃取效率的影响,结果表明,两相流速、萃取膜面积对萃取率基本无影响,而水溶液的pH值和有机相初始铜离子浓度的改变使萃取率在40%～99%之间变化.这主要是由于整个萃取过程的传质阻力主要来源于D2EHPA和Cu2 的界面配位络合反应阻力,当铜浓度比较高时,传质阻力或传质系数与铜浓度无关,其值基本不变;而当铜浓度降低时,传质阻力随着铜浓度的降低而增大,传质系数则随着铜浓度的降低而减小.     

Ligand-assisted extraction for separation and preconcentration of gold nanoparticles from waters

A new two-step extraction procedure is proposed for separation and preconcentration of gold nanoparticles (Au-NPs) from aqueous samples. First, Au-NPs are loaded onto a reversed phase C-18 (RP-C18) column, and then ligand-assisted extraction into chloroform is performed. 1-Dodecanethiol (1-DDT, 5 mM) was used as selective ligand for quantitative extraction under ultrasonic condition. Parameters of the extraction procedure, such as sample volume, organic solvent, concentration and nature of the ligand, ultrasonication time, pH of the sample, and different coating as well as sizes of Au-NPs were investigated in regard to the extraction efficiency of Au-NPs. The optimized procedure allows separation and preconcentration of the Au-NPs with an enrichment factor of up to 250 assuring no changes in size and/or shape of the NPs. This was proved by investigation of the particles by UV-vis spectrometry and transmission electron microscopy (TEM). Furthermore, the presence of potentially interfering other metal nanoparticles (M-NPs) and dissolved organic matter (DOM) was studied. Observed minor recoveries of Au-NPs in DOM model solutions were overcome by hydrogen peroxide pretreatment up to a DOM concentration of about 4 mg/L. Feasibility of the proposed method was proved by application of the optimized procedure to 5 real water samples. Recoveries of Au-NPs in the real waters spiked in a concentration range from 0.15 to 5100 μg/L obtained by this method varied from 68.4% to 99.4%. Consequently, the proposed approach has great potential for the analysis of M-NPs in environmental waters.     

Transient response behavior of an axisymmetric stagnation flow on a circular cylinder due to time dependent free stream velocity

An analysis is presented to investigate the unsteady response behavior of an axisymmetric stagnation flow on a circular cylinder due to transient free stream velocity. The governing boundary layer equations are integrated by the steepest descent method. Numerical results have been presented for the unsteady wall stress by assuming explicit time dependent forms for the free stream velocity.     

汽油脱硫用复合膜的制备及性能

以聚乙二醇(PEG)为活性分离层、聚偏氟乙烯(PVDF)多孔膜为支撑层制备复合膜。PEG涂膜液的固含量提高到16%时,可以减少孔渗现象,提高渗透通量。考察进料温度、流量和膜下游侧压力对复合膜性能的影响。硫富集因子随温度和流量升高先增加后减小,最大值出现在100℃和100mL/min。渗透通量随温度升高而增大;当进料流量大于100mL/min时,渗透通量随流量增加而减小。两个参数均随膜下游侧压力增加而减小。对典型的催化裂化汽油,膜的渗透通量可达2.7kg/(m2.h),硫富集因子为3.6。