An annular dual-membrane continuous cation exchanger packed with ion exchange resin

Design, construction details and performance characteristics are described for an annular perfluorosulfonate (Nafion) dual tubular membrane assembly where inside of the inner membrane tube and outside of the outer membrane tube are packed with ion exchange resin beads. During operation, the solution to be ion exchanged flows in the annular space between the two membrane tubes and a regenerant solution flows countercurrent to the principal flow, through the inner tube and outside the outer tube. With a principal channel hold up volume of ∼ 125 μl and capability of operation at head pressures up to 200 psi, the device can continuously exchange 200-250 μeq of Na⁺ per minute for H⁺. While this particular device was intended for use as a “suppressor” in anion chromatography, the design concept should be of utility in other membrane-based operations.     

An osmotic micro-pump integrated on a microfluidic chip for perfusion cell culture

A novel microfluidic chip integrating an osmosis-based micro-pump was developed and used for perfusion cell culture. The micro-pump includes two sealed chambers, i.e., the inner osmotic reagent chamber and the outer water chamber, sandwiching a semi-permeable membrane. The water in the outer chamber was forced to flow through the membrane into the inner chamber via osmosis, facilitating continuous flow of fluidic zone in the channel. An average flow rate of 0.33 μL min⁻¹ was obtained within 50 h along with a precision of 4.3% RSD (n = 51) by using a 100 mg mL⁻¹ polyvinylpyrrolidone (PVP) solution as the osmotic driving reagent and a flow passage area of 0.98 cm² of the semi-permeable membrane. The power-free micro-pump has been demonstrated to be pulse-free offering stable flow rates during long-term operation. The present microfluidic chip has been successfully applied for the perfusion culture of human colorectal carcinoma cell by continuously refreshing the culture medium with the osmotic micro-pump. In addition, in situ cell immunostaining was also performed on the microchip by driving all the reagent zones with the integrated micro-pump.     

Automated sampling with capillaries

A system is described for the analysis of serum contained in capillaries. The capillaries, filled with samples, are placed directly into a moving stream of diluent which flushes the capillaries, carrying the samples into a continuous flow or discrete system of analysis. The capillaries are inserted into holes in a plastic block which is pushed forward sequentially by a drive mechanism. As each capillary comes into line with an entrance tube and exit tube, reagent is pumped through these tubes and through the capillary. As an alternative, a dispenser is attached to the inlet tube, and as each capillary comes into position, a measured amount of liquid is dispensed through the capillary and into a container. The system is applied to continuous flow analysis of phosphate, alkaline phosphatase, uric acid, and creatinine. The construction of an efficient and reliable peristaltic pump is described for the continuous flow system.     

Repetitive determination of calcium ion and regeneration of a chromogenic reagent using chlorophosphonazo III and an ion exchanger in a circulatory flow injection system.

The spectrophotometric determination of Ca2+ with chlorophosphonazo III (CPN) has been carried out by a circulatory flow injection (FI) method. A cation-exchange mini-column for the on-line regeneration of the main reagent was incorporated in this FT system, allowing a repetitive determination of Ca2+. A solution of 4.0 x 10(-5) M CPN in a 0.05 M acetate buffer (pH 5.0) in a single reservoir (50 ml) was continuously circulated at a constant flow rate of 1.5 ml min(-1). Into the stream, an aliquot (20 microl) of a sample containing Ca2+ was quickly injected by means of a 6-way valve. The complex formed was monitored spectrophotometrically at 670 nm in the flow system. Then, the stream passed through a cation-exchange column, which was introduced after the flow-through cell. A successful ligand-exchange reaction of Ca2+ between the CPN reagent and a cation exchanger, as well as a simultaneous regeneration of the free reagent took place. The stream then returned to the reservoir. The regeneration and recycling of the CPN reagent allowed as many as 300 repetitive determinations of 2.5 mg l(-1) Ca2+ solutions with the same 50 ml circulating solution.     

Development of an on-chip injector for microchip-based flow analyses using laminar flow

A new on-chip injector for microchip-based flow analyses has been designed and characterized. The microchip design utilizes separate laminar flow streams of buffer and sample that are brought into parallel contact for a distance of 300 microm. The buffer flow stream is first routed through a conventional 6-port injection valve fitted with a 5 microm i.d. sample loop. When the 6-port valve is actuated from load to inject for a given time, the on-chip buffer flow stream is constricted and the sample flow stream is pressurized into the buffer flow channel. Once the valve returns to the load state the separate laminar flow streams resume. Fluorescence detection was used to characterize the injector and it was found that 50 injections of a 100 microM fluorescein sample led to an average peak height of 174.32 +/- 2.05 AFU (RSD 1.18%) and average peak skew of 1.37 +/- 0.06. The injector was also interfaced with amperometric detection. Injections of catechol solutions ranging in concentration from 500 nM to 100 microM resulted in a linear response (sensitivity = 2.49 pA microM(-1), r(2) = 0.998) and a limit of detection of 155 nM (S/N = 3). Compared to an off-chip injection scheme, plug dilution, band broadening, and peak asymmetry are much reduced. Finally, the injection and subsequent lysis of an erythrocyte sample was demonstrated, with an injected plug of erythrocytes being lysed 5.72 +/- 0.15 s after injection into a flow stream containing sodium dodecyl sulfate (n = 10). The new injection scheme does not require complex valving mechanisms or high pressures and enables reproducible injections from a continuous sample flow stream in a manner where changes in analyte concentration can be monitored with high temporal resolution.     

Flow-through sampling for electrophoresis-based microfluidic chips using hydrodynamic pumping

This work presents a novel electrophoretic microchip design which is capable of directly coupling with flow-through analyzers for uninterrupted sampling. In this device, a 3 mm wide sampling channel (SC) was etched on quartz substrate to create the sample inlet and outlet and the 75 microm wide electrophoretic channels were also fabricated on the same substrate. Pressure was used to drive the sample flow through the external tube into the SC and the flow was then split into outlet and electrophoretic channels. A gating voltage was applied to the electrophoretic channel to control the sample loading for subsequent separations and inhibit the sample leakage. The minimum gating voltage required to inhibit the sample leakage depended on the solution buffer and increased with the hydrodynamic flow-rate. A fluorescent dye mixture containing Rhodamine B and Cy3 was introduced into the sample stream at either a continuous or discrete mode via an on-line injection valve and then separated and detected on the microchip using laser-induced fluorescence. For both modes, the relative standard deviation of migration time and peak intensity for consecutive injections was determined to be below 0.6 and 8%, respectively. Because the SC was kept floating, the external sampling equipment requires no electric connection. Therefore, such an electrophoresis-based microchip can be directly coupled with any pressure-driven flow analyzers without hardware modifications. To our best knowledge, this is something currently impossible for reported electrophoretic microchip designs.     

Repetitive determination of chloride using the circulation of the reagent solution in closed flow-through system

The precipitation reaction of silver chloride (AgCl) is carried out in a large amount of ammonia (NH₃). This makes possible to adopt a closed-loop flow injection (FI) system and to determine chloride repetitively. A solution of 30 mmol l⁻¹ silver nitrate and 80 mmol l⁻¹ NH₃ in a single reservoir (250 ml) is continuously circulated through the flow cell at a flow rate of 2.0 ml ml⁻¹. The chloride containing sample (100 μl) was introduced into this reagent solution by means of six-way valve. AgCl precipitates formed in the sample zone are monitored spectrophotometrically (at 500 nm) in the flow system. After passing through the flow cell, the excess NH₃ in the circulating reagent solution dissolves AgCl precipitates and the stream then returns to the reservoir. Various variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 3.0 to 30 mg l⁻¹ chloride. Two hundred repetitive injections of 5.0 mg l⁻¹ chloride into the circulating reagent solution have shown unchanged base-line and good reproducibility. The method was successfully applied to the determination of chloride in tap, natural and the reference waters.     

Stopped-in-loop flow analysis of trace vanadium in water.

The new concept of stopped-in-loop flow analysis (SIL-FA) is proposed, and an SIL-FA method for the catalytic determination of vanadium is demonstrated. In an SIL format, a sample solution merges with reagent(s), and the well-mixed solution is loaded into a loop. The solution in the loop is separated by a six-way switching valve from the main stream. While the reaction proceeds in the stationary loop, the SIL-FA system does not need to establish a baseline continuously. This leads to a reduction in reagent consumption and waste generation compared with traditional flow injection analysis.     

Use of intermittent jets to enhance flux in crossflow filtration

This paper deals with the influence of a new flow unsteadiness on the permeate fluxes in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow, causing the formation of large vortices moving downstream along the tubular membrane. The main results of the numerical calculation of such flows are given. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Time evolutions of flux were achieved in steady and unsteady operating conditions. Results concerning the influence and limits of the nozzle to tube diameter ratio and the jet velocities are discussed. The applicability of such an unsteady flow is examined with a view to effects on energy consumption and possible viscosity effects.     

A microporous membrane-based continuous generation system for trace-level standard mixtures of atmospheric gases.

A reliable and convenient system to generate accurate and stable standard gas mixtures of various atmospheric compounds at parts-per-billion levels has been developed. The system is of simple design; the generator is a coil consisting of an inner tube of microporous polytetrafuluoroethylene (PTFE) membrane tubing and an outer tube of silicone tubing. An aqueous solution of the given compound continuously flows through the inner microporous tube and the purge gas flows through the annulus between the inner and outer tubes. In addition to the generation of gas mixtures based on Henry's law, the proposed flow-type system offers generation based on chemical reactions, leading to a distinct advantage of the availability of continuous sources of various compounds. The generation system was tested for preparing standard gas mixtures of HCHO and H2O2 on the basis of Henry's law, and those of HNO2, NO, and SO2 on the basis of chemical reactions. A stable generation of the desired low concentrations of various kinds of gas mixtures can be readily achieved by adjusting the concentration of the solution without the use of high-dilution flow.     

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.     

Enzymatic substrate determination by flow injection analysis in stopped flow modus using a single 5-port-valve for sample and reagent injection

Summary The determination of ethanol with alcohol dehydrogenase is described as an example of enzymatic determination with the flow injection analysis system (FIAS). Both, sample and reagent, are successively injected into the carrier stream by using only one valve. Compared with other techniques, the principle described is more economical with regard to reagent consumption and analysis time. Basic experiments about this kind of reagent addition (dispersion, reproducibility, possibility for gradient dilution) were made by simulation with dye solution. The determination of ethanol is carried out using the stopped flow technique. The peristaltic pumps are stopped when the reaction zone is located in the flow cell, and the change of absorbance with time is monitored. Thus background signals and other matrix influences can be minimized. The method is tested under real conditions for the determination of alcohol in several beverages.     

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.     

Determination of urea in serum by using naturally immobilized urease in a flow injection conductimetric system

A flow injection method was developed, aimed at the determination of urea in human serum. The system makes use of the naturally immobilized urease present in Canavalia ensiformis DC (jack bean). A column is filled with small pieces of this bean, and the sample (50 microliters) containing urea passes through it carried by a 1% NaCl solution. On leaving the column the stream is merged with an alkaline reagent (0.5 mol dm-3 NaOH; 0.5% disodium dihydrogen ethylenediaminetetraacetate). The ammonium ions, arising from the enzymatic reaction that occurs inside the column, are changed into the molecular form, which permeates a polytetrafluoroethylene membrane and is received in a de-ionized water acceptor stream. The ammonia ionizes causing an increase in the conductance, which is proportional to the urea content of the sample. About 40 samples can be processed in 1 h with negligible carry-over and with a relative standard deviation of 1% or less. The results are in agreement with those obtained by a standard spectrophotometric method.     

Gas diffusion with preconcentration for the determination of fluoride in water samples by flow injection

The preconcentration of fluoride is achieved on-line by converting it to trimethylsilane which then diffuses through a gas permeable membrane to be absorbed in a stationary sodium hydroxide acceptor stream. This stream is enclosed in the sample loop of an injection valve and after preconcentration, the fluoride sample is flushed into a flow injection manifold for spectrophotometric analysis by the zirconium/alizarin S procedure at 520 nm. The method is suitable for fluoride analysis in the range 0.1-10 mg/l at a sampling rate of 17/hr. Phosphate does not interfere and aluminium and iron can be tolerated at 200 and 500 times the fluoride concentration, respectively. The LOD was calculated to be 0.055 mg/l and LOQ was found to be 0.18 mg/l.     

Flow-injection extraction with a microvolume module based on integrated conduits

A self-contained module for liquid—liquid extractions is described. The module contains engraved conduits for mixing of sample and ragent, an engraved segmentor, a detachable extraction coil, a membrane separator, and a rinsing system for the flow cell. The membrane in the separator is supported by a teflon-coated steel grid and can be replaced rapidly. The segmented stream enters at the centre of the circular membrane and travels through an engraved, coiled channel in contact with the membrane before leaving the membrane area at the periphery. The volume of the receptor chamber for the organic phase is 10 μl. For a detector flow-cell volume of 8 μl, an aqueous flow rate of 2.0 ml min^?1 and an organic flow rate of 1.2 ml min^?1, the “loss factors” caused by analyte dispersion are 3.2 and 1.5 for caffeine sample volumes of 40 μl and 100 μl, respectively, compared with batch extraction. The system is also tested for extraction of anionic surfactants as their ion-pairs with methylene blue.     

Microfluidic Wheatstone bridge for rapid sample analysis

We developed a microfluidic analogue of the classic Wheatstone bridge circuit for automated, real-time sampling of solutions in a flow-through device format. We demonstrate precise control of flow rate and flow direction in the "bridge" microchannel using an on-chip membrane valve, which functions as an integrated "variable resistor". We implement an automated feedback control mechanism in order to dynamically adjust valve opening, thereby manipulating the pressure drop across the bridge and precisely controlling fluid flow in the bridge channel. At a critical valve opening, the flow in the bridge channel can be completely stopped by balancing the flow resistances in the Wheatstone bridge device, which facilitates rapid, on-demand fluid sampling in the bridge channel. In this article, we present the underlying mechanism for device operation and report key design parameters that determine device performance. Overall, the microfluidic Wheatstone bridge represents a new and versatile method for on-chip flow control and sample manipulation.     

Determination of total nitrogen in food by flow injection analysis (FIA) with a potentiometric differential detection system

A flow injection set-up based on potentiometric detection and gas diffusion device for the determination of total nitrogen in food is described. The detection system consisted of two ammonium-sensitive electrodes placed sequentially and each alternately operating as reference electrode. Tubular electrodes without an inner reference solution were prepared with a PVC membrane composed of nonactin in Tris (2-ethylhexyl) phosphate and potassium tetrakis (4-chlorophenyl) borate to reduce the membrane resistance. The food sample digests were inserted into the system, and the ammonium present was converted into ammonia gas. The gas diffused through a gas-permeable membrane to a buffer acceptor stream with a pH that ensured transformation to the ammonium cation, which was potentiometrically detected. Good agreement between FIA results and those provided by the reference procedure was obtained, with relative deviation errors below 5%. Using the proposed system, low reagent consumption is possible, a sampling rate of about 30 samples/h was achieved, as well as a good reproducibility for consecutive injections of the same sample (variation coefficient < 2%).     

Chemiluminescence detection of hydrazine vapor

An efficient, real-time chemiluminescence detector for hydrazine vapor, N(2)H(4)(g), is described, capable of monitoring sub part-per-billion levels of hydrazine in air. The catalytic oxidation of hydrazine by colloidal platinum forms an intermediate, oxidizing agent (e.g. OH or OOH) which subsequently oxidizes luminol, generating a chemiluminescence signal that is proportional to the hydrazine concentration. Major components of the instrument include a photomultiplier tube (PMT), a short length of glass tubing coiled directly in front of the PMT cathode surface, a vacuum pump for sampling the air, and a peristaltic pump for circulating the liquid reagent. The liquid reagent, a basic solution (pH 13) of luminol and colloidal platinum, is continuously recycled. The detection sequence is initiated by pumping the hydrazine vapor through a short length of teflon tubing that is concurrently transporting the liquid reagent. The liquid is separated from the gas stream in an impinger and quickly pumped to the PMT. We have evaluated the effect of solution pH, luminol and platinum concentrations, and air and liquid flow rates on the analytical characteristics of this system. A linear, dynamic detection range for hydrazine has been obtained from 1 to 2000 ppb in air, with an instrument response that is fully reversible and achieves plateau response in less than 2 min.     

Continuous flow analysis of lead (II) and mercury (II) with substituted diazacrown ionophore membrane electrodes

A novel flow cell for use with ion-selective membrane electrodes is reported in which the carrier stream is drawn through a tube that suppresses the pump noise. PVC membrane electrodes based on 7,16-dithenoyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTODC), and 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC) for lead (II), and 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC) and 7,16-di-(2-methylquinolyl)-1,4,10,13-tertraoxa-7,16-diazacyclootadecane (DQDC), for mercury (II) were prepared and evaluated. The linear ranges were pPb: 5.5-3.0 (DTODC) and 6.0-2.0 (DTAODC); pHg: 5.5-3.0 (DTDC) and 4.5-2.5 (DQDC). With flow rate of 3 ml min(-1) the repeatability of measurements was less than 5% RSD (n = 3). The system was applied to the determination of lead (II) and mercury (II) in spiked natural water samples.