Gas-diffusion Separation Research Articles

Microfluidic paper-based analytical device for the speciation of inorganic nitrogen species

A microfluidic paper-based analytical device (μPAD) utilizing gas-diffusion separation and solid-phase reduction was developed for the first time for the determination of both ammonium and nitrate, which are the dominant inorganic nitrogen species in environmental waters. The device consists of 3 filter paper layers accommodating the sample, reagent and detection zones. The reagent zone is separated from the detection zone by a semipermeable hydrophobic membrane and acts as a solid-phase reactor where nitrate is reduced to ammonia by Devarda's alloy microparticles, integrated into a μPAD for the first time. The detection zone incorporates the acid-base indicators bromothymol blue (BTB) or nitrazine yellow (NY) and changes colour in two steps. Initially the colour change is caused by ammonia generated by the reaction of ammonium and sodium hydroxide in the sample zone. This colour change is followed by a subsequent colour change as a result of the ammonia produced by the reduction of nitrate by the Devarda's alloy microparticles. The corresponding reflectance value changes are used for the quantification of the two inorganic nitrogen species in the ranges 6.5–100.0 or 2.1–15.0 mg N L−1 for ammonium and 18.2–100.0 or 4.2–15.0 mg N L−1 for nitrate when BTB or NY are used, respectively. Under optimal conditions the limits of quantification of ammonium and nitrate in the case of BTB were determined as 6.5 and 18.2 mg N L−1, respectively, while the corresponding values in the case of NY were found to be 2.1 and 4.2 mg N L−1. The newly developed μPAD was stable for 62 days when stored in a freezer and 1 day at ambient temperature. It was validated with a certified reference material and successfully applied to the determination of ammonium and nitrate in spiked environmental water samples and soil extracts.

Design of Gas Diffusion Cascade for Production of Carbon Isotope Using Ethylene as Processing Gas

Carbon-13 isotope has a wide range of applications in medical diagnosis, scientific research, food safety and other fields, and the market demand for carbon-13 isotope is also showing an upward trend. In order to explore the separation and production methods of carbon-13 isotope, the gas diffusion separation experiment with ethylene as the processing gas was carried out, and the cascade was designed on this basis. Through the single-stage separation experiment, it can be obtained that the overall separation factor of the gas diffusion method for separating ethylene under the existing experimental conditions is 1.014. The gas diffusion cascade enrichment of carbon-13 isotope involves two steps. The first step is to separate ethylene using a squared-off cascade to obtain the heavy fraction with the carbon-13 isotope abundance reaching 49%. The second step is to convert ethylene into carbon dioxide, and then use a squared-off cascade to separate the carbon dioxide to obtain the heavy fraction with the carbon-13 isotope abundance reaching 95%.

Scientific search under the pressure of high responsibility: development of the technology of electromagnetic isotope separation in the framework of the USSR Atomic project

One of the ways to obtain nuclear explosives for the manufacture of the first atomic bombs was the separation of uranium isotopes by electromagnetic (mass spectrographic) method. The theoretical justification and experimental separation of uranium isotopes by this method was carried out by L. A. Artsimovich in the Laboratory No. 2 of the USSR Academy of Sciences. By the middle of 1945, L. A. Artsimovich had obtained results at the experimental facility indicating the possibility of stable enrichment of uranium with the U-235 isotope using this method. Having the results of the experiments, the Design Bureau at the “Electrosila” plant (D. V. Efremov), with the participation of the Research Vacuum Institute (S. A. Vekshinsky) and the “A” Institute (M. Ardenne), developed a project of an industrial multi-chamber installation for electromagnetic separation of uranium isotopes SU-20. Such an installation was manufactured and put into operation at the newly created plant in the Urals, which allowed in 1951 to obtain the necessary amount of uranium-235 for the RDS-3 atomic bomb. After 1952, the technology of electromagnetic separation was no more used for the separation of uranium isotopes due to the improvement of more productive equipment for gas-diffusion separation of isotopes. The equipment of the electromagnetic isotope separation plant built in the Urals was used to produce lithium-6, needed to equip the first thermonuclear charge, which was successfully tested in 1953. In the next years, the electromagnetic isotope separation plant was involved in the manufacture of isotope products for scientific, technical and medical applications at wide range of utilization in Russia and abroad.

Effect of two-step calcination on the formation of nickel oxide hollow nanofibers

1D (one-dimensional) hollow nanofibers are of great technological interest. Their formation can be controlled by tuning the calcination process. However, the formation mechanism through the control of the burning stage is still debated in the literature. Herein, the influence of calcination in two steps on the morphology of hollow nanofibers of nickel oxide (NiO), prepared by solution blow spinning (SBS) technique, was investigated. A systematic study using a variety of characterization techniques suggests that a prolonged residence time in the initial step of calcination produces nanofibers with a better definition of the hollow region with better control of tubular wall thickness. Statistically, the calcination methodology studied here does not affect the outer diameter of nanofibers, as shown by Analysis of Variance (ANOVA) and Tukey’s method. According to our findings, the formation mechanism is influenced by gas diffusion and phase separation during fiber spinning and heat treatment, which suggests that neither the Kirkendall effect nor the Ostwald ripening process can be solely responsible for the formation hollow fibers. These findings contribute to a better understanding of the formation of metal-oxide hollow nanofibers induced by heat treatment.

A novel flow injection chemiluminescence method for automated and miniaturized determination of phenols in smoked food samples

An easily performed fully automated and miniaturized flow injection chemiluminescence (CL) method for determination of phenols in smoked food samples has been proposed. This method includes the ultrasound assisted solid-liquid extraction coupled with gas-diffusion separation of phenols from smoked food sample and analytes absorption into a NaOH solution in a specially designed gas-diffusion cell. The flow system was designed to focus on automation and miniaturization with minimal sample and reagent consumption by inexpensive instrumentation. The luminol – N-bromosuccinimide system in an alkaline medium was used for the CL determination of phenols. The limit of detection of the proposed procedure was 3·10−8·molL−1 (0.01mgkg−1) in terms of phenol. The presented method demonstrated to be a good tool for easy, rapid and cost-effective point-of-need screening phenols in smoked food samples.

Development of a gas-diffusion microfluidic paper-based analytical device (μPAD) for the determination of ammonia in wastewater samples.

An inexpensive, disposable and highly selective microfluidic paper-based analytical device (μPAD) is described for the determination of ammonia (molecular ammonia and ammonium cation) in wastewaters which implements for the first time a gas-diffusion separation step on a paper-based platform. Its hydrophilic reagent zones were defined by printing filter paper with a hydrophobic paper sizing agent using a conventional inkjet printer. The sample was introduced into the sodium hydroxide impregnated sample zone of the μPAD. This allowed the quantitative conversion of the ammonium ion to molecular ammonia which diffused across the hydrophobic microporous Teflon membrane of the device into an adjacent hydrophilic reagent zone containing the acid-base indicator 3-nitrophenol or bromothymol blue. The change in indicator color was measured using a desktop scanner for ammonia quantification. Under optimal conditions, the μPAD is characterized by a limit of detection of 0.8 and 1.8 mg N L(-1) and repeatability of 3.1 and 3.7% (n ≥ 10, 20 mg N L(-1)), expressed as relative standard deviation, in the case of 3-nitrophenol or bromothymol blue, respectively. This μPAD was used successfully for the determination of ammonia in sewage and soil water samples. The small dimensions, minimal reagent consumption, low cost, simplicity of operation, and possibility of using a portable scanner make the proposed μPAD suitable for on-site ammonia monitoring in contaminated environmental waters and domestic, agricultural and industrial wastewaters. The successful implementation of the gas-diffusion approach on a paper-based platform is expected to result in the development of other μPADs for volatile analytes.

An integrated platform for gas-diffusion separation and electrochemical determination of ethanol on fermentation broths

An integrated platform was developed for point-of-use determination of ethanol in sugar cane fermentation broths. Such analysis is important because ethanol reduces its fuel production efficiency by altering the alcoholic fermentation step when in excess. The custom-designed platform integrates gas diffusion separation with voltammetric detection in a single analysis module. The detector relied on a Ni(OH)2-modified electrode. It was stabilized by uniformly depositing cobalt and cadmium hydroxides as shown by XPS measurements. Such tests were in accordance with the hypothesis related to stabilization of the Ni(OH)2 structure by insertion of Co2+ and Cd2+ ions in this structure. The separation step, in turn, was based on a hydrophobic PTFE membrane, which separates the sample from receptor solution (electrolyte) where the electrodes were placed. Parameters of limit of detection and analytical sensitivity were estimated to be 0.2% v/v and 2.90μA % (v/v)−1, respectively. Samples of fermentation broth were analyzed by both standard addition method and direct interpolation in saline medium based-analytical curve. In this case, the saline solution exhibited ionic strength similar to those of the samples intended to surpass the tonometry colligative effect of the samples over analyte concentration data by attributing the reduction in quantity of diffused ethanol vapor majorly to the electrolyte. The approach of analytical curve provided rapid, simple and accurate analysis, thus contributing for deployment of point-of-use technologies. All of the results were accurate with respect to those obtained by FTIR method at 95% confidence level.

Microemulsification-Based Method: Coupling with Separation Technique

The outcomes described herein outline the potentiality of the microemulsification-based method (MEC) for development of rapid testing (point-of-use) technologies. MEC was recently proposed by these authors for analytical determinations wherein the detection is conducted in solution with naked eyes. It relies on effect of analyte over the colloid thermodynamics by changing the minimum volume fraction of amphiphile needed to generate microemulsions (MEs) (ΦME), which represents the analytical response of the method. We report in this paper the successfully coupling of MEC-based detection with gas diffusion separation. Such result extends the field of application of MEC in analytical sciences by improving its selectivity. One custom-designed module was constructed on PTFE for the separation measurements. It was utilized in combination with MEC for determining water in ethanol fuels using water/ n-propanol/oleic acid MEs and water-rich compositions. In this situation, accurate direct determinations by MEC are not possible. In addition, further studies on analytical performance and robustness of MEC by using n-propanol amphiphile are described. The method was robust as regards to deviations in dispersion preparing and changes in temperature. Concerning the analytical performance, the analytical curves presented wide linear range with limits of linearity of up to 70.00% v/v ethanol to water (ΦE). The limits of detection (S/N=3) were of 1.03%, 7.21%, and 0.68% v/v ΦE for compositions with water- (region A) and oil-rich (region C) domains as well as equal volumes of water and oil phases (region B), respectively. With respect to the regions A and B, the analytical performance stressed herein exhibited best linearity and comparable sensitivities when compared to these levels reached with ethanol amphiphile (our first publication on MEC) rather than n-propanol.

A microfabricated electroosmotic pump coupled to a gas-diffusion microchip for flow injection analysis of ammonia

We have microfabricated two functional components toward developing a microchip flow injection analysis (FIA) system, i.e., an open-channel electroosmotic pump and a gas-diffusion chip, consisting of two microfabricated glass wafers and a porous polytetrafluoroethylene membrane. This is the first application of gas-diffusion separation in a microchip FIA system. To demonstrate the feasibility of using these two components for performing gas-diffusion FIA, we have incorporated them together with a regular FIA injection valve and a capillary electrophoresis absorbance detector in a flow injection system for determination of ammonia in environmental water samples. This system has a limit of detection of 0.10 mg L−1 NH3, with a good repeatability (relative standard deviation of less than 5 % for 4.0 mg L−1 NH3). Parameters affecting its performance are also discussed.

Gas diffusion as a new fluidic unit operation for centrifugal microfluidic platforms

A centrifugal microfluidic platform prototype with an integrated membrane for gas diffusion is presented for the first time. The centrifugal platform allows multiple and parallel analysis on a single disk and integrates at least ten independent microfluidic subunits, which allow both calibration and sample determination. It is constructed with a polymeric substrate material and it is designed to perform colorimetric determinations by the use of a simple miniaturized optical detection system. The determination of three different analytes, sulfur dioxide, nitrite and carbon dioxide, is carried out as a proof of concept of a versatile microfluidic system for the determination of analytes which involve a gas diffusion separation step during the analytical procedure.

Caprolactone Ring-Opening Molecular Layer Deposition of Organic-Aluminum Oxide Polymer Films

New hybrid organic-inorganic thin films are finding use in electronics, biomedical, chemical protection, and energy storage systems applications. In a unique vapor-source molecular layer deposition film formation reaction, we find that cyclic ɛ-caprolactone reacts with surface-adsorbed methyl-aluminum species to build a metal-organic hybrid coordination polymer thin film at substrate temperatures between 60 and 120°C. Infrared and X-ray photoelectron spectroscopy analysis confirms (–Al–O–(C8H16)–O–)n bonding, expected from the a Lewis acid catalyzed surface ring-opening reaction. In-situ IR analysis confirms the surface reaction sequence. The materials show good stability and maintain their physical structure in ambient. This surface acid-catalyzed vapor process could extend to other metal organics and vapor/surface ring opening reactions to yield new organic-inorganic film materials for use in gas diffusion barriers, separation membranes and chemical resistance coatings.

Development of a Gas Diffusion Multicommuted Flow Injection System for the Determination of Sulfur Dioxide in Wines, Comparing Malachite Green and Pararosaniline Chemistries

A flow system based on the multicommutation concept was developed for the determination of free and total sulfur dioxide in table wines, exploiting gas diffusion separation and spectrophotometric detection. The system allowed the comparison of malachite green and pararosaniline chemistries, using the same manifold configuration. Free and total SO(2) were determined within the ranges 1.00-40.0 and 25.0-250 mg L(-1), at determination throughputs of 25 and 23 h(-1), respectively. Employing the malachite green reaction, detection limits of 0.3 and 0.8 mg L(-1) were attained for free and total SO(2), respectively. Pararosaniline chemistry provided detection limits of 0.6 mg L(-1) for free SO(2) and 0.8 mg L(-1) for total SO(2). Relative standard deviations better than 1.8 and 1.4% were obtained by the malachite green and pararosaniline reactions, respectively. With regard to the two tested chemistries, 18 wines were analyzed and the results achieved by the pararosaniline reaction compared better with those furnished by the recommended procedure.

Highly sensitive gas-diffusion sequential injection analysis based on flow manipulation

The present paper describes approaches utilizing the powerful flow manipulation capabilities of sequential injection analysis (SIA) to substantially improve the efficiency of gas-diffusion separation compared to its traditional implementation in flow injection analysis (FIA). Ammonia, ethylamine, diethylamine and triethylamine were used as model analytes in this study. Eleven flow manipulation approaches involving continuous flow, stop-flow, oscillating flow, and the introduction of air bubbles to separate the sample zone from the donor solution were tested. Improvement in sensitivity compared to traditional gas-diffusion FIA exceeding one order of magnitude was achieved. It was observed that this improvement increased with the molecular size of the analyte.

Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat

A simple flow injection (FI) conductometric system with gas diffusion separation was developed for the determination of Kjeldahl nitrogen (or proteins) in milk and chicken meat. The sample was digested according to the Kjeldahl standard method and the digest was diluted and directly injected into the donor stream consisting of 4 M NaOH. In alkaline medium, ammonium was converted to ammonia, which diffused through the PTFE membrane to dissolve in an acceptor stream (water). Dissociation of ammonia caused a change in conductance of the acceptor solution, which was linearly proportional to the concentration of ammonium originally present in the injected solution. A conductometric flow through cell and an amplifier circuit was fabricated, which helped improve sensitivity of the conductometric detection system. With using a plumbing Teflon tape as a gas diffusion membrane and without thermostating control of the system, a linear calibration graph in range of 10–100 mg L −1 N-NH 4 was obtained, with detection limit of 1 mg L −1 and good precision (relative standard deviation of 0.3% for 11 replicate injections of 50 mg L −1 N-NH 4). The developed method was validated by the standard Kjeldahl distillation/titration method for the analysis of milk and chicken meat samples. The proposed system had sample throughput of 35 h −1 and consumed much smaller amounts of chemical than the standard method (275 mg vs 17.5 g of NaOH per analysis, respectively).

Sensitivity enhancement in membrane separation flow injection analysis by ultrasound

The effect of ultrasound on gas-diffusion and pervaporation flow injection separation was investigated. Ammonia and three aliphatic amines (propylamine, tri-ethylamine and di- n-butylamine) with different volatility and surface activity were used as model analytes. Under the experimental conditions used, sonication did not enhance gas-diffusion separation efficiency and resulted in up to 62% improvement in pervaporation mass transfer. Based on these findings and taking into account the surface activity of the analytes studied which decreased with their molecular mass it was postulated that ultrasound-induced surface rippling was primarily responsible for the enhanced evaporation in the donor chamber of the pervaporation cell. The results reported in this paper suggest that ultrasonic pervaporation separation could extend the applicability of this on-line flow injection separation technique to the direct determination of higher molecular mass volatile and semi-volatile analytes in ‘dirty’ samples.

Exploiting Sulphide Generation and Gas Diffusion Separation in a Flow System for Indirect Sulphite Determination in Wines and Fruit Juices

A flow system for sulphite determination in wines and fruit juice samples has been developed exploiting H2S generation and gas diffusion separation. The composed sandwich gas diffusion cell was coupled in the flow manifold to permit sulphite reduction to H2S and H2S diffusion through a microporous PTFE membrane to produce methylene blue resulting from the reaction with N,N-dimethylphenylene diamine (DMPD) and Fe3+. The flow parameters were optimized, and the best results were obtained by using NaOH solution for H2S collection. Interferences of H2S, CO2 and ascorbic acid were evaluated. Limits of detection (3σ) for the free and total sulfite were calculated at 0.12 and 0.25 mg L−1, respectively. The results obtained by the proposed process were in agreement with the reference methods for a 95% confidence level.

Interfacing in-line gas-diffusion separation with optrode sorptive preconcentration exploiting multisyringe flow injection analysis

An automatic multisyringe flow injection analysis (MSFIA) system coupling a flow-through optical fiber diffuse reflectance sensor with in-line gas-diffusion (GD) separation is proposed for the isolation, preconcentration and determination of traces of volatile and gas-evolving compounds in samples containing suspended solids, with no need for any preliminary batch sample treatment. The flowing methodology overcomes the lost of sensitivity of the in-line separation technique, when performed in a uni-directional continuous-flow mode, through the implementation of disk-based solid-phase extraction schemes. The high selectivity and sensitivity, the low reagent consumption and the miniaturization of the whole assembly are the outstanding features of the automated set-up. The proposed combination of techniques for separation, flow analysis, preconcentration and detection was applied satisfactorily to sulfide determination in environmental complex matrixes. The method based on multicommutation flow analysis involves the stripping of the analyte as hydrogen sulfide from the donor channel of the GD-module into an alkaline receiver segment, whereupon the enriched plug merges with well-defined zones of the chormogenic reagents (viz., N, N-dimethyl- p-phenylenediamine (DMPD) and Fe(III)). The in-line generated methylene blue dye is subsequently delivered downstream to the dedicated optrode cell furnished with a C 18 disk, while recording continuously the diffuse reflectance spectrum of the pre-concentrated compound. This procedure provides a linear working range of 20–500 μg l −1 sulfide with a relative standard deviation of 2.2% ( n = 10) at the 200 μg l −1 level, and a detection limit of 1.3 μg l −1.

A contactless conductivity detection cell for flow injection analysis: Determination of total inorganic carbon

New, very simple cells for contactless conductivity detection have been designed and tested in flow injection analysis. The cells consist of two insulated wire electrodes placed inside a PTFE tubing at a close mutual distance and oriented either across the tube or along the tube axis; the detection takes place within the space limited by the two electrodes. The electrodes are insulated by a 5 μm thick film of a polyimide (the dielectric). The electrodes with thin film of dielectric represent capacitors with higher capacitance which allows a higher ac current to flow between the electrodes, as compared to contactless conductivity cells used, e.g., in capillary electrophoresis, and the measuring sensitivity is thus enhanced. The cells were tested on the determination of total inorganic carbon in model water samples by the flow injection analysis combined with a gas diffusion separation. A concentration range from 0.01 to 1 mmol l −1 NaHCO 3 was tested with a linear dynamic range of up to 0.1 mmol l −1. Detection limits of 0.6 and 6 μmol l −1 with RSD values of 4.1 and 4% were obtained for detectors with the electrodes placed axially and across the tube, respectively. A throughput of 15 samples per hour was attained under an experimental procedure used. The analytical parameters were compared with those obtained using a contactless conductivity cell with tubular electrodes placed outside the PTFE tubing.

Enzymatic Determination of Urea in Milk by Sequential Injection with Spectrophotometric and Conductometric Detection

In this work, an analytical system based on the coupling of gas diffusion separation and sequential injection analysis for urea determination in milk is presented. A versatile manifold that could simultaneously be used for either spectrophotometric or conductometric detection was constructed. The sample and urease solution are sequentially aspirated into the holding coil and sent to a thermoreactor, where urea is enzymatically hydrolyzed by urease and converted into ammonium. This stream merges an alkaline solution at a confluence point where ammonia is formed. Ammonia diffuses through a hydrophobic membrane and modifies the bromothymol blue indicator color, when spectrophotometric detection is used, or changes the conductance of a boric acid solution acceptor stream, when conductometric detection is used. This methodology was applied to the determination of urea in 18 milk samples and the results were statistically comparable with those furnished by the enzymatic recommended procedure. The detection limits were 2.6 x 10(-4) and 2.8 x 10(-5) mol L(-1) for conductometric and spectrophotometric detection, respectively. Repeatability (relative standard deviation, RSD) was better than 3.7% and 2.6% for conductometric and spectrophotometric detection, respectively.

In-line membrane separation method for sulfide monitoring in wastewaters exploiting multisyringe flow injection analysis

A fully automated multisyringe flow injection analysis (MSFIA) system coupled to gas-diffusion (GD) separation is proposed for the spectrophotometric determination of sulfide in urban wastewaters containing suspended solids. No preliminary batch sample treatment is required. The method carried out in a multicommutated system is based on analyte release as hydrogen sulfide from the donor channel of the GD module into a stagnant recipient solution composed of N, N-dimethyl- p-phenylene diamine (DMPD) and Fe(III). The methylene blue dye generated in-line via oxidative coupling is subsequently delivered downstream to a miniaturized flow-through light-emitting diode (LED)-based optical fiber plug-in spectrophotometer. The effect of various experimental variables on gas transfer across the membrane and concentration enhancement in the receiver solution via iterative flow reversals are discussed. Under the optimized chemical and hydrodynamic variables, the GD–MSFIA method features coefficients of variation better than 0.8% ( n=10) at the 10 mg l −1 level, a linear working range of 0.5–20 mg l −1 sulfide, a 3 σ blank-detection limit of 0.03 mg l −1 and an injection throughput of 13 h −1 for a sample volume of 700 μl and a forward flow mode. The analytical performance of the proposed GD–MSFIA system is also critically compared with that of in-line dialysis separation schemes using passive semi-permeable hydrophilic membranes. Accuracy was assessed by using the batch APHA–AWWA–WPFC standard method as an external reference.