Membrane-controlled reagent-delivery systems – a new approach for the continuous production of reagent and standard solutions

A new simple and robust system for the production of standard solutions, based on the mass-transfer of analytes through membranes, is described. The device consists of a cone-shaped reservoir vessel, filled with a concentrated solution of the analyte and separated from a liquid acceptor stream by a membrane. Mass-flow from donor to acceptor solution is controlled by the mass-transfer-affecting properties of the active membrane area, which is determined by the hole in a template (diameter 0.8 mm) placed between the membrane and the acceptor-channel. Using nitrate as model analyte and a track-etched membrane filter (pore size 0.1 μm) dilution factors up to 2,400,000 with long-term reproducible accuracy of < 2% have been achieved. Adjustment of a requested concentration is possible by varying either the flow rate of the acceptor stream or the concentration of the reservoir solution. Received: 22 November 2000 / Revised: 5 March 2001 / Accepted: 7 March 2001     

Experimental evaluation of commercially available semi-permeable membranes for use with parallel-plate dialysers in flow injection systems

Mass transfer in on-line analytical parallel-plate dialysers was studied in the continuous and flow injection (FI) modes in order to characterise the nature of the dialysis process and its dependence on experimental variables. A number of different semi-permeable membrane surfaces were evaluated in the laminar-flow and plug-flow configurations. The fraction of analyte transferred from the donor to the acceptor stream depends on parameters such as type of membrane used, membrane surface, membrane line-length, membrane porosity, concentration of analyte in the donor stream, the use of concurrent and countercurrent flow between the donor and acceptor streams, and flow-rates of the donor and acceptor streams. The dialysis of calcium and chloride ions, in the absence of protein, was studied with different membrane types and dialysers with different dimensions in continuous flow and FI, and the influence of the above mentioned parameters on the dynamic dialysis process of these ions is described.     

Supported liquid membranes for the determination of vanillin in food samples with amperometric detection

A supported liquid membrane system has been developed for the extraction of vanillin from food samples. A porous PTFE membrane is impregnated with an organic solvent, which forms a barrier between two aqueous phases. The analyte is extracted from a donor phase into the hydrophobic membrane and then back extracted into a second aqueous solution, the acceptor. The determination (100–1400 μg ml⁻¹ vanillin) was performed using a PVC-graphite composite electrode versus Ag/AgCl/3MKCl at +0.850 V placed in a wall-jet flow cell as amperometric detector. The solid sample is directly placed in the membrane unit without any treatment, and the analyte was extracted from the sample, passes through the membrane and conduced to the flow cell by the acceptor stream. The limit of detection (3σ) was 44 μg ml⁻¹. The method was applied to the determination of vanillin (9–606 μg g⁻¹) in food samples.     

Residual aqueous ozone determination by gas diffusion reverse flow injection analysis

A novel system based on reverse flow injection analysis with a gaseous diffusion step (GD-r-FIA) has been developed for the analysis of ozone. It includes an automatic microburet injection system. The ozone diffuses through a microporous membrane of polyvinylidene difluoride (PVDF) from the donor stream to the acceptor stream containing nitrite ions. The nitrite concentration in the acceptor solution decreases due to the ozone reduction reaction. In this way, a simple indirect measurement of the ozone concentration can be performed using the Griess–Ilosvay reaction for the nitrite ion. This correlates with the decrease in absorbance of the azoic dye formed with the ozone concentration in the donor stream. The system has been optimised by investigating the effect of the nitrite concentration in the acceptor stream on the diffusion flow. The optimum nitrite concentration was set at 0.250 ppm with a flow rate of 1.5 ml/min. The efficiency of the ozone diffusion through the membrane was only 4.4%. This affects the average sensitivity, which is low (0.0092±0.0012 AU/ppm), although the detection limit is similar to that obtained with other reported methods (0.03 ppm). The main advantage of the system reported here is that it has a linear range that is an order of magnitude broader than those observed for other GD-FIA systems. This is especially useful for continuous monitoring systems, since the residual ozone concentration is normally between 0.05 and 5.0 ppm. Additionally, using the reverse flow injection analysis (FIA) technique minimises chemical consumption and residue generation. Finally, the stability of the ozone solution and the repeatability and reproducibility of the method have been studied.     

Carrier-mediated extraction of bipyridilium herbicides across the hydrophobic liquid membrane

Supported liquid membrane (SLM) method for preconcentration and enrichment of the two bipyridilium herbicides, namely diquat and paraquat, from environmental water samples has been developed. The permanently charged cationic herbicides were extracted from a flowing aqueous solution to a stagnant acidic acceptor solution across a liquid membrane containing 40% (v/v) di-(2-ethylhexyl) phosphoric acid dissolved in di-n-hexyl ether. The mass transfer of analytes is driven by the counter-coupled transport of hydrogen ions from the acceptor to the donor phase. The efficiency of the extraction process depends on the donor solution pH, the amount of the mobile carrier added to the liquid membrane and the concentration of the counter ion in the acceptor solution. The applicability of the method for extraction of these quaternary ammonium herbicides from environmental waters was also investigated by spiking analyte sample solutions in river water. With 24 h sample enrichment concentrations of diquat and paraquat down to ca. 10 ng/L could be detected in environmental waters.     

Flow-injection determination of lead by hydride generation and conductometric detection

Plumbane produced from the lead analyte in a flow-injection manifold by reaction with sodium borohydride is passed through a porous poly(tetrafluoroethylene) membrane in a gas-diffusion cell. The hydride reacts with bromine in the acceptor stream resulting in ionization which is detected by conductivity measurement. Direct mixing of the carrier with a reagent stream yields a limit of detection of approximately 1 mg/L. An improved detection limit of about 200 microg/L can be achieved by the incorporation of an auxiliary stream containing persulphate as oxidizing agent. The application of the method to the determination of lead in road dust and soil samples is demonstrated.     

Investigation of chemical effects on the performance of flow-through dialysis applied to the determination of ionic species

In this paper, the influence of chemical variables on the mass transfer kinetics of ionic species under dynamic conditions in flow-through sandwich-type dialysers is thoroughly investigated. Although the driving force of the mass transport is the existence of a concentration gradient between the two phases separated by a semi-permeable membrane, it has been demonstrated that the chemical composition of both donor and acceptor solutions in terms of concentration and kind of ionic compounds has a significant influence on the mass transfer efficiency. The Donnan effect on passive dialysis and the fast migration of ions concomitantly present with the target species improved the transfer of the analyte ion in the membrane separation process. Thus, for the determination of low molecular weight anions, the addition of cationic species with high transport index, such as oxonium ion, to the donor stream, or multicharged ions (e.g. Al³⁺) to the recipient stream, enhanced the dialysis yields more than 62% with respect to the use of water as acceptor and sample medium.As a consequence of the dependence of the dialysis rate on the composition of the sample matrix, different diffusate concentrations were encountered for the same input concentration of analyte when prepared in different electrolytic media. In order to balance the chemical potential on the donor side, the ionic strength for both standards and sample solutions should be carefully adjusted via incorporation of a modifier stream in the flow manifold (e.g. 1.0 mol l⁻¹ KNO₃ or 0.5 mol l⁻¹ H₂SO₄) as demonstrated in the bulk of the text. Appropriate buffering of the recipient solution was equally effective. Furthermore, these strategies were found suitable to overcome the lack of linearity observed by several researchers in in-line dialytic processes at low concentrations of ionic species caused by polar interactions with the membrane surface.Chloride was selected as a model of target species for assessing the effect of chemical variables on the mass transfer rate in flow-through parallel-plate dialyser units. The spectrophotometric detection scheme for chloride, implemented in a secondary flow configuration, is based on the displacement reaction of thiocyanate from the corresponding mercury salt in the presence of iron(III).     

Use of supported liquid membranes incorporated in a flow system for the direct determination of eugenol in spice samples

A method for the determination of eugenol in spice samples based on the use of supported liquid membranes coupled to a flow system was developed. The solid sample is placed directly in the membrane unit without any treatment and the analyte is extracted from the sample, passes through the membrane and is conducted to the flow cell by the acceptor stream. This stream flows through the detector, allowing the measurement of the analyte by using a PVC-graphite composite electrode, versus Ag/AgCl/3 M KCl at +0.3 V, placed in a well-jet flow cell as amperometric detector. The method allowed the determination of eugenol in the range 0.5-30 micrograms ml-1 with a relative standard deviation of 5%. Results provided by the proposed procedure when applied to clove and seasoning samples agreed well with those obtained by a reference method.     

Indirect determination of tetrahydroborate (BH(-)(4)) by gas-diffusion flow injection analysis with amperometric detection

A rapid, indirect gas-diffusion flow injection analysis (FIA) method with amperometric detection has been developed for the selective and sensitive determination of tetrahydroborate (BH(-)(4)). The injected analyte reduces arsenic(III) to arsine. The arsine formed diffuses through the PTFE (polytetrafluoroethylene) membrane and is quantified amperometrically at a platinum working electrode. The precision of the technique was better than a relative standard deviation of 2.1% at 60 muM levels and better than 0.5% at 0.1 mM, with a throughput of 60 samples/hr. The detection limit of the method was found to be 1 muM (1.5 ng BH(-)(4)) with a linear range up to 1 mM. The dynamic range extends over five orders of magnitude in BH(-)(4) concentration. The effects of working potential, concentration of As(III) and HCl in the reagent stream, type and flow rate of the acceptor solution, temperature and interferents on the FIA signals were studied.     

Flow-injection determination of lead by hydride generation and conductometric detection

Plumbane produced from the lead analyte in a flow-injection manifold by reaction with sodium borohydride is passed through a porous poly(tetrafluoroethylene) membrane in a gas-diffusion cell. The hydride reacts with bromine in the acceptor stream resulting in ionization which is detected by conductivity measurement. Direct mixing of the carrier with a reagent stream yields a limit of detection of approximately 1 mg/L. An improved detection limit of about 200 g/L can be achieved by the incorporation of an auxiliary stream containing persulphate as oxidizing agent. The application of the method to the determination of lead in road dust and soil samples is demonstrated.     

Sampling small volumes of ambient ammonia using a miniaturized gas sampler

The development of a gas sampler for a miniaturized ambient ammonia detector is described. A micromachined channel system is realized in glass and silicon using powder blasting and anodic bonding. The analyte gas is directly mixed with purified water, dissolving the ammonia that will dissociate into ammonium ions. Carrier gas bubbles are subsequently removed from the liquid stream through a venting hole sealed with a microporous water repellent PTFE membrane. A flow restrictor is placed at the outlet of the sampler to create a small overpressure underneath the membrane, enabling the gas to leave through the membrane. Experiments with a gas flow of 1 ml min(-1), containing ammonia concentrations ranging from 9.4 ppm to 0.6 ppm in a nitrogen carrier flow have been carried out, at a water flow of 20 microl min(-1). The ammonium concentration in the sample solution is measured with an electrolyte conductivity detector. The measured values correspond with the concentration calculated from the initial ammonia concentration in the analyte gas, the fifty times concentration enhancement due to the gas-liquid volume difference and the theoretical dissociation equilibrium as a function of the resulting pH.     

微流控芯片流动注射气体扩散分离光度测定系统的研究

提出了纳升级进样量的微流控芯片流动注射气体扩散分离光度检测系统. 制作三层结构微流控芯片, 在玻璃片上加工微反应通道, 用聚二甲基硅氧烷[Poly(dimethylsiloxane), PDMS]加工气体渗透膜和具有接收气体微通道的底片, 实现了生成气体的化学反应、气-液分离和检测在同一微芯片上的集成化. 采用缝管阵列纳升流动注射进样系统连续进样, 用吸光度法测定NH+4以验证系统性能. 结果表明， 该系统对NH+4的检出限为140 μmol/L(3σ), 峰高精度为3.7%(n=9). 在进样时间12 s、注入载流48 s和每次进样消耗200 nL试样条件下, 系统分析通量可达60样/h. 若加大样品量到800 nL, 使接收溶液停流1 min, 该系统对NH+4的检出限可达到35 μmol/L(3σ), 但分析通量降低到20样/h.     

Hollow fibre liquid-phase microextraction of parabens from environmental waters

Parabens (alkyl-p-hydroxybenzoates) are antimicrobial preservatives widely used in cosmetics, toiletries, pharmaceuticals, and food. Nowadays, they are considered emerging pollutants and their determination is becoming increasingly important since they are continuously released into the environment. In this work, a hollow fibre liquid-phase microextraction method has been developed for the extraction of parabens from environmental waters. The parameters affecting the extraction of parabens (organic solvent used as liquid membrane; pH of both sample and acceptor solution; salting-out effect; extraction time and stirring speed) were carefully optimized in order to reach high recoveries for all tested analytes. Under optimum conditions, parabens were extracted from river, reservoir and sea water samples with recoveries ranging from 16.7 to 68.6% depending upon the analyte and the sample analyzed, leading to detection limits lower than 0.2?ng?mL^?1 using a simple HPLC-UV instrument.     

Continuous cytometric bead processing within a microfluidic device for bead based sensing platforms

A microfluidic device for continuous biosensing based on analyte binding with cytometric beads is introduced. The operating principle of the continuous biosensing is based on a novel concept named the "particle cross over" mechanism in microfluidic channels. By carefully designing the microfluidic network the beads are able to "cross-over" from a carrier fluid stream into a recipient fluid stream without mixing of the two streams and analyte dilution. After crossing over into the recipient stream, bead processing such as analyte-bead binding may occur. The microfluidic device is composed of a bead solution inlet, an analyte solution inlet, two washing solution inlets, and a fluorescence detection window. To achieve continuous particle cross over in microfluidic channels, each microfluidic channel is precisely designed to allow the particle cross over to occur by conducting a series of studies including an analogous electrical circuit study to find optimal fluidic resistances, an analytical determination of device dimensions, and a numerical simulation to verify microflow structures within the microfluidic channels. The functionality of the device was experimentally demonstrated using a commercially available fluorescent biotinylated fluorescein isothiocyanate (FITC) dye and streptavidin coated 8 microm-diameter beads. After, demonstrating particle cross over and biotin-streptavidin binding, the fluorescence intensity of the 8 microm-diameter beads was measured at the detection window and linearly depends on the concentration of the analyte (biotinylated FITC) at the inlet. The detection limit of the device was a concentration of 50 ng ml(-1) of biotinylated FITC.     

Preconcentration of quinolones by dialysis on-line coupled to high-performance liquid chromatography.

The parameters influencing dialytic separation of ciprofloxacin (CF) fluoroquinolone were investigated. Dialysis with a porous cellulose acetate membrane was on-line coupled with HPLC and the analysis of dialysate was made by isocratic ion-pairing liquid chromatography using a reversed-phase analytical column and fluorescence detection. Optimisation of the experimental conditions for selective dialytic enrichment are described and explanations of some phenomena affecting dialysis efficiency discussed. By the use of a neutral donor (pH approximately 7) and acidic acceptor solution (pH < 4) a substantial enrichment of quinolones was achieved. Accumulation of CF in the acidic acceptor phase is based on the protonation of the analyte in the acceptor compartment. Continuous-flow of donor solution and a stagnant acceptor solution gave high dialysis efficiency in 5-15 min. Effects of interfering substances present in real samples on the variation of dialysis efficiency can be minimised by successive dialysis runs of the original and spiked samples.     

Simulation of flux during electro-membrane extraction based on the Nernst-Planck equation

The present work has for the first time described and verified a theoretical model of the analytical extraction process electro-membrane extraction (EME), where target analytes are extracted from an aqueous sample, through a thin layer of 2-nitrophenyl octylether immobilized as a supported liquid membrane (SLM) in the pores in the wall of a porous hollow fibre, and into an acceptor solution present inside the lumen of the hollow fibre by the application of an electrical potential difference. The mathematical model was based on the Nernst-Planck equation, and described the flux over the SLM. The model demonstrated that the magnitude of the electrical potential difference, the ion balance of the system, and the absolute temperature influenced the flux of analyte across the SLM. These conclusions were verified by experimental data with five basic drugs. The flux was strongly dependent of the potential difference over the SLM, and increased potential difference resulted in an increase in the flux. The ion balance, defined as the sum of ions in the donor solution divided by the sum of ions in the acceptor solution, was shown to influence the flux, and high ionic concentration in the acceptor solution relative to the sample solution was advantageous for high flux. Different temperatures also led to changes in the flux in the EME system.     

Membrane Introduction Mass Spectrometry (MIMS): A Versatile Tool for Direct,Real‐Time Chemical Measurements

Membrane introduction mass spectrometry (MIMS) is a direct, continuous, on‐line measurement technique. It utilizes a membrane to semi‐selectively transfer analyte mixtures from a sample to a mass spectrometer, rejecting the bulk of the sample matrix, which can be a gas, liquid or solid/slurry. Analyte selectivity and sensitivity are affected by optimizations at the membrane, ionization and the mass spectrometer levels. MIMS can be roughly classified by the acceptor phase that entrains analyte(s) to the mass spectrometer after membrane transport, either a gaseous acceptor phase (GP‐MIMS) or condensed acceptor phase (CP‐MIMS). The aim of this article is to provide an introduction to MIMS as a technique and to explore current variants, recent developments and modern applications, emphasizing examples from our group, the Applied Environmental Research Laboratories as well as selected work from others in this emerging area. Also provided is a synopsis of current and future directions for this versatile analytical technique. Copyright © 2014 John Wiley & Sons, Ltd.     

Flow-injection analysis based on a membrane separation module and a bulk acoustic wave impedance sensor – determination of the volatile acidity of fermentation products

A novel flow-injection analysis (FIA) system has been developed for the rapid determination of the volatile acidity of some fermentation products like vinegars and juices. The proposed method is based on the diffusion of volatile acids, mainly acetic acid, across a PTFE gas-permeable membrane from an acid stream into an alkaline stream, and the acids trapped in the acceptor solution are determined online by a bulk acoustic wave impedance sensor based on changes in the conductivity of the solution. It exhibited a linear frequency response up to 10 mmol · L^–1 acetic acid with a detection limit of 50 μmol · L^–1, and the precision was better than 1% (RSD) at a through-put of 72 h^–1. The effects of operating voltage for the detector, cell constant of the electrode, composition of acceptor stream, flow rates and temperature on the FIA performance were also investigated.     

Liquid-phase microextraction in a microfluidic-chip – High enrichment and sample clean-up from small sample volumes based on three-phase extraction

In this work, a microfluidic-chip based system for liquid-phase microextraction (LPME-chip) was developed. Sample solutions were pumped into the LPME-chip with a micro-syringe pump at a flow rate of 3–4 μL min⁻¹. Inside the LPME chip, the sample was in direct contact with a supported liquid membrane (SLM) composed of 0.2 μL dodecyl acetate immobilized in the pores of a flat membrane of polypropylene (25 μm thickness). On the other side of the SLM, the acceptor phase was present. The acceptor phase was either pumped at 1 μL min⁻¹ during extraction or kept stagnant (stop-flow). Amitriptyline, methadone, haloperidol, loperamide, and pethidine were selected as model analytes, and they were extracted from alkaline sample solution, through the SLM, and into 10 mM HCl or 100 mM HCOOH functioning as acceptor phase. Subsequently, the acceptor phase was either analyzed off-line by capillary electrophoresis for exact quantification, or on-line by UV detection or electrospray ionization mass spectrometry for time profiling of concentrations. The LPME-chip was found to be highly effective, and extraction efficiencies were in the range of 52–91%. When the flow of acceptor phase was turned off during extraction (stop-flow), analyte enrichment increased linearly with the extraction time. After 10 min as an example, amitriptyline was enriched by a factor of 42 from only 30 μL sample solution, and after 120 min amitriptyline was enriched by a factor of 500 from 320 μL sample solution. This suggested that the LPME-chip has great potentials for very efficient analyte enrichments from limited sample volumes in the future.     

Determination of titratable acidity and ascorbic acid in fruit juices in continuous-flow systems

Summary Two continuous-flow systems for the determination of titratable acidity and ascrobic acid in fruit juice samples are described. The assemblies permit on-line dialysis of analytes prior to the reaction step, thus improving selectivity and performing sample dilution. Flow systems are built with a channel carrying the donor phase (sample in both determinations) and another channel carrying an acceptor phase, both of them entering the dialyser. The outcoming stream transporting the dialysed sample fills the valve loop, permitting its injection into a carrier stream which continuously passes through the spectrophotometric detector. For the titratable acidity, acceptor phase and carrier are distilled water, the reagent merged with the carrier channel being a buffered solution of bromothymol blue (pH 7). The analytical signal obtained is then monitored at 616 nm. For ascorbic acid, the acceptor phase was a Fe(III) solution, which reacts with the dialysed analyte to form Fe(II). A buffered solution of o-phenanthroline (pH 4.5) is used as carrier, reacting with Fe(II) to give the analytical signal, which is monitored at 510 nm. Chemical and physical parameters are optimized for both systems. The analytical features of the determination are established. Finally, the proposed procedures are compared with the official volumetric AOAC methods for both parameters. The FIA methods turn out to be suitable for a rapid and accurate control of fruit juice samples, compared with the reference methods; additionally they compete advantageously with the volumetric methods in the case of turbid and highly coloured samples.