Flow-injection Configuration Research Articles

Fundamental Study of CH4-Air Combustion Under an Axisymmetric Small-scale Rectangular Combustor Using Computational Modeling

The optimization of the design and operating conditions of industrial combustors depends on the fundamental study of combustion dynamics and flow behaviors. Complete combustion increases the thermal efficiency as well as reduces the emission significantly. A study of this kind also allows exploring alternative fuels that would increase the combustion efficiency thus the life cycle of the systems. To develop a highly-performed combustion system for rocket engines or power plants, fundamental research under an axisymmetric small-scale combustor is considered in this study. The k-Ɛ (2 Eqn.) and species transport model (STM) are used to study the flow turbulence and combustion behavior, respectively. A Parallel flow injection configuration of fuel and air is considered. In this study, combustion behavior is investigated at a wide range of fuel and air flowrate conditions while keeping the air slot dimension (240 mm) and fuel injection slot diameter (10 mm) constant. The fuel velocity (FV) and air velocity (AV) are changed from 2 m/s to 30 m/s so that a better test matrix could be proposed. At each run, turbulence, the flame temperature, reaction heat release rate, mass fraction of CO2, etc are studied. It is seen that the combustion temperature increases with the increase in fuel injection velocity. The static flame temperature reaches its maximum (2177 K-2287 K) and falls within the standard limits of CH4-Air combustion. The mass fraction of CO2 is found to be within the acceptable limit (0.121-0.153). The heat of the reaction is found to be high at variable Reair and ReCH4 conditions. It is observed that the computational models used in this study are capable of predicting the flow and combustion behaviors accurately.

A rebinding-assay for measuring extreme kinetics using label-free biosensors

In vitro kinetic measurements allow mechanistic characterization of binding interactions and are particularly valuable throughout drug discovery, from confirmation of on-target binding in early discovery to fine-tuning of drug-binding properties in pre-clinical development. Early chemical matter often exhibits transient kinetics, which remain challenging to measure in a routine drug discovery setting. For example, characterization of irreversible inhibitors has classically relied on the alkylation rate constant, yet this metric fails to resolve its fundamental constituent rate constants, which drive reversible binding kinetics and affinity complex inactivation. In other cases, extremely rapid association processes, which can approach the diffusion limit, also remain challenging to measure. To address these limitations, a practical kinetic rebinding assay is introduced that may be applied for kinetic screening and characterization of compounds. The new capabilities afforded by this probe-based assay emerge from mixed-phase partitioning in a flow-injection configuration and have been implemented using label-free biosensing. A finite element analysis-based biosensor model, simulating inhibition of rebinding within a crowded hydrogel milieu, provided surrogate test data that enabled development and validation of an algebraic model for estimation of kinetic interaction constants. An experimental proof-of-principle demonstrating estimation of the association rate constant, decoupled from the dissociation process, provided further validation.

Silver nanoparticle impacts on gold electrode surfaces in flow-injection configuration

Flow-injection techniques using nanoparticles are important in several applications. This work reports experimental work on the effect of flow rates on the fate of nanoparticles impacting an electrode surface, in real, commercially available flow systems. Resonance Enhanced Surface Impedance (RESI) and Particle Chronoamperometry were used for monitoring silver nanoparticles (AgNPs) collisions in a FIA format, on gold working electrodes using dodecanethiolate (DDT-AgNP) and (ω-ferrocenylhexanethiolate) silver NPs (FcHT-AgNP). The results are intuitive, showing that factors such as flow rates and ligand electroactivity have significant impact on nanoparticle-electrode collision events. A slow flow rate, (5 μL/min) resulted in the detection of broad spikes, suggestive of large agglomerated particles comprised of 100–150 NPs compared to narrower spikes, comprised of 4–5 NPs detected at higher flow rate (100 μL/min). FcHT-AgNPs, monitored at 5 μL/min, at the electrode held at an oxidising potential for ferrocene, showed spikes suggestive of reduced agglomeration on the surface of the electrodes. The electrode surface recovered following the dwell time of the NP containing solutions on the electrode surface. RESI and CA used in conjunction provide capabilities for isolating capacitive effects from faradaic effects.

Application of a tyrosinase microreactor – detector in a flow injection configuration for the determination of affinity and dynamics of inhibitor binding

An original self-contained enzymatic microreactor − detector in a thin layer flow-through configuration has been developed for the screening of tyrosinase inhibitors. The microreactor- detector consisted of a gold disk for tyrosinase immobilization adjacent to a glassy carbon disk as working electrode (GCE). Using l-tyrosine as substrate, the enzymatic product formed (dopaquinone) was detected amperometrically at the GCE. Experimental parameters most affecting the l-tyrosine response were optimized with a central composite design. Measurements were performed in phosphate buffer pH 6.0 with an applied potential of +50mV vs Ag/AgCl and a flow rate of 100μL/min. The determined Kmapp was 1.9±0.1mM. The setup permitted the determination of the efficiency and the duration of inhibition of twelve compounds known as tyrosinase inhibitors namely kojic acid, gallic acid, azelaic acid, benzoic acid, l-ascorbic acid, hydroquinone, glabridine, resveratrol, quercetin dihydrate, captopril, l-cysteine and l-glutathione reduced. l-Ascorbic acid and kojic acid were found to be the most potent inhibitors for the two studied criteria with IC50 of 32μM and 156μM, respectively.

Liquid Chromatography, Chemical Oxidation, and Online Carbon Isotope Dilution Mass Spectrometry as a Universal Quantification System for Nonvolatile Organic Compounds

A procedure for the universal detection and quantification of polar organic compounds separated by liquid chromatography (LC) based on postcolumn carbon isotope dilution mass spectrometry (IDMS) was developed. The eluent from the LC column is mixed online with a continuous flow of (13)C-enriched sodium bicarbonate, and the sodium persulfate oxidation reaction in acidic media is employed to achieve isotope equilibration. All carbon-containing compounds eluting from the column are oxidized to (12)CO(2) and (13)CO(2), respectively, and the carbon dioxide is separated from the aqueous phase using a gas-permeable membrane. The gaseous carbon dioxide is then carried to the mass spectrometer for isotope ratio measurements. Different water-soluble organic compounds were evaluated using a flow injection configuration to assess the efficiency of the oxidation process. Most water-soluble organic compounds tested showed quantitative oxidation. However, chemical structures involving conjugated C═N double bounds and guanidinium-like structures were found to be resistant to the oxidation and were further studied. For this purpose, (13)C(1)-labeled creatine (with the isotopic label in the guanidinium group) was employed as model compound. Specific conditions for the quantitative oxidation of these compounds required lower flow rates and the addition of metallic catalysts. This novel approach was tested as a universal detection and quantification system for LC. A simple standard mixture of four amino acids was separated under 100% aqueous conditions and quantified without the need for specific standards with good accuracy and precision using potassium hydrogen phthalate as internal standard. The main field of application of the developed method is for the purity assessment of organic standards with direct traceability to the International System of Units (SI).

Modulation of Acoustic Coupling by Photo-Oxidation of Self-Assembled Monolayers

A thickness-shear mode acoustic wave device operated in a flow-injection configuration has been used to study the effects of oxygen on thiol-based self-assembled monolayers at the liquid-sensor interface when exposed to UV light. The films were composed of dodecanethiol or undec-5-yne-1-thiol. Piezoelectric AT-cut quartz sensors with gold electrodes were submerged in thiol solutions with varying amounts of oxygen present in the solution by altering the amount of time that the solutions were exposed to nitrogen. Oxygen was also removed or incorporated into the system by sparging the liquid buffer used in the flow system with helium gas. When exposed to UV radiation, the device with thiol monolayers in place exhibited reversible changes in series renounces frequency and motional resistance only in the presence of oxygen. This result is ascribed to a photochemically induced oxidation of the sulfur moiety at the device-liquid interface. The response of the device under these conditions is governed by acoustic coupling phenomena and not mass effects. Finally, the in situ cross-linking of undec-5-yne-thiol does not result in any additional changes in response other than those indicated for dodecanethiol confirming that thickness viscoelastic changes at the monolayer level are not relevant to device response.

Depolarization of surface-attached hypothalamic mouse neurons studied by acoustic wave (thickness shear mode) detector

Isolation of neurons from animal tissue is an important aspect of understanding basic biochemical processes such as the action of hormones and neurotransmitters. In the present work, the focus is on an effort to evaluate the utility of acoustic wave physics for the study of such cells. Immortalised hypothalamic neuronal cells from mouse embryos were cultured on the surface of the gold electrode of a 9.0 MHz thickness-shear mode acoustic wave sensor. These cells, which are clonal, are imposed on the surface of the device at a confluence in the range of 80-100%. The coated sensor is incorporated into a flow-injection configuration such that electrolytes can be introduced in order to examine their effects through measurement by network analysis. Both series resonance frequency, fs, and motional resistance, R(m), were measured in a number of experiments involving the injection of KCl and NaCl into the sensor-neuron system. The various responses to these electrolytes were interpreted in terms of changes in cellular structure associated with the depolarization process. The sensor-neuron system was found to elicit different responses to the addition of KCl and NaCl. Preliminary findings indicate that the TSM sensor does not purely measure changes in the membrane potential upon KCl addition. Typical changes in fs for 15 mM, 30 mM and 60 mM KCl additions were 54 +/- 15, 80 +/- 26 and 142 +/- 58 Hz (mean +/- standard deviation) respectively. Typical changes in R(m) for these KCl additions were 7 +/- 3, 13 +/- 4 and 23 +/- 6 Omega, respectively. These results were concluded after 17 runs at each concentration. Despite the large relative standard deviations, the dependence of f(s) and R(m) with respect to concentration was apparent. Controls performed by coating the TSM sensor with laminin or a cell attachment matrix showed no significant changes in either f(s) or R(m) for the same solutions tested on the sensor-neuron system.

Passive Control of Pressure Oscillations in Solid Rocket Motors: Cold-Flow Experiments

Cold-gas experiments are used to study the pressure oscillations occurring in solid rocket motors and the performance of different ways of passive control of these oscillations. Previous studies stated that flow-acoustic coupling is mainly observed for nozzles including a cavity. The nozzle geometry has an effect on the pressure oscillations through a coupling between the acoustic fluctuations induced by the cavity volume and the vortices traveling in front of the cavity entrance. An important reduction of pressure oscillations (1 order of magnitude) is obtained, both for the axial and radial flow injection configurations, by removing the cavity located around the nozzle head. However, the nozzle integration cannot be avoided and this solution then cannot be implemented on a real flight. A permeable membrane (with holes to allow the combustion gas to pass through) placed in front of the cavity allows a reduction by a factor 1.5. The Helmholtz resonator shows a small attenuation of the pressure oscillations; however, its design could be optimized to maximize the acoustic damping. The three-dimensional-shaped inhibitors show a good attenuation of the pressure fluctuations, especially when the opening cross section is increased. This increase results in a shift of the Mach number associated with excitation. For a similar cross section, the asymmetric inhibitor (crenel-shaped) provides a reduction of pressure oscillations by a factor of 2 compared with an axisymmetric inhibitor. Therefore, the asymmetry of the inhibitor seems to be the best candidate for reducing the pressure oscillations.

FIA determination of ascorbic acid at low potential using a ruthenium oxide hexacyanoferrate modified carbon electrode

The anodic oxidation of ascorbic acid on a ruthenium oxide hexacyanoferrate modified electrode was characterized by cyclic voltammetry. On this modified surface, the electrocatalytic process allows the determination of ascorbic acid to be performed at 0.0 V and pH 6.9 with a limit of detection of 2.2 μM in a flow injection configuration. Under this experimental condition, no interference from glucose, nitrite and uric acid was noticed. Lower detection limit values were obtained by measuring flow injection analysis (FIA) responses at 0.4 V (0.14 μM), but a concurrent loss of selectivity is expected at this more positive potential. Under optimal FIA operating conditions, the linear response of the method was extended up to 1 mM ascorbic acid. The repeatability of the method for injections of a 1.0 mM ascorbic acid solution was 2.0% ( n = 10). The usefulness of the method was demonstrated by an addition-recovery experiment with urine samples and the recovered values were in the 98–104% range.

Flow injection determination of choline in milk hydrolysates by an immobilized enzyme reactor coupled to a selective hydrogen peroxide amperometric sensor

A choline oxidase (ChO) immobilized enzyme reactor (IMER) prepared by glutaraldehyde coupling of the enzyme on aminopropyl modified controlled pore glass beads is described. The ChO–IMER was coupled, in a flow injection configuration system, to an interference free hydrogen peroxide amperometric sensor based on a Pt surface modified by an overoxidized polypyrrole film. The resulting analytical device responds selectively to choline and displays a sensitivity of 46.9 ± 0.2 μC mM −1 and a limit of detection, calculated at a signal-to-noise ratio equal to 3, of 7 μM. Sensitivity remains constant for about 20 days and then starts to slowly deteriorate and after 2 months a 70% of the initial sensitivity was still retained. The application to choline determination in milk hydrolysates is demonstrated. Short- and long-term drift observed in the analytical response can be corrected by a bracketing technique.

Thin layer distillation for matrix isolation in flow analysis

Analyte transfer from the matrix in a thin layer distillation (TLD) cell and its subsequent measurement were investigated in a flow injection configuration. We designed the cell such that the donor and acceptor streams flowed in parallel channels separated by a thin dividing wall. The matrix transfer process involved room-temperature distillation of the analyte into the headspace of the TLD cell and its subsequent condensation/uptake by a concurrently flowing acceptor stream. There are no membranes; hence there are no membrane-related problems. The TLD system design was optimized with respect to its dimensions and operational parameters. Throughput and sensitivity were compared with a conventional pervaporation flow injection (PFI) system for ammonia and five different amines. For the higher molecular mass amines, the TLD approach provided comparable or superior performance. The TLD technique should be an attractive approach for online analysis of volatile chemical species in ‘dirty’ samples, especially for volatile analytes of higher molecular mass.

Flow injection chemiluminescent determination of amiodarone in pharmaceutical preparations using photogenerated tris(2,2′-bipyridyl)ruthenium(III)

A flow injection configuration was developed and evaluated for the chemiluminescent determination of amiodarone. The method is based on the reaction of the drug with tris(2,2′-bipyridyl)ruthenium(III), which was generated through the on-line photo-oxidation of tris(2,2′-bipyridyl)ruthenium(II) with peroxydisulfate. Under the optimum experimental conditions, a linear calibration graph was obtained over the range 3.0–60.0 μg ml −1 with a detection limit of 0.28 μg ml −1. The proposed method allows 120 injections h −1 with excellent repeatability and precision (R.S.D. less than 0.5% and 2.8%, respectively) and a reagent consumption of only 0.37 μmol (0.27 mg) of Ru(bpy) 3Cl 2·6H 2O per determination. The method was successfully applied to the determination of amiodarone in commercial pharmaceutical formulations.

Chemiluminescent determination of oxalate based on its enhancing effect on the oxidation of methyl red by dichromate

A flow injection configuration for the chemiluminescence determination of oxalate is proposed. The procedure is based on the catalytic effect of oxalate on the oxidation of methyl red by dichromate. The chromium(III) formed catalyzes the luminescent reaction between luminol and hydrogen peroxide. The proposed method is simple, inexpensive, sensitive and suitable for concentrations of oxalate between 0.24 and 8.11 μg ml −1. The applicability of the method was demonstrated by determining oxalate in vegetables.

Evaluation of on-line preconcentration and flow-injection amperometry for phosphate determination in fresh and marine waters

Dissolved reactive phosphorus (DRP) was determined as orthophosphate (PO 4-P) in fresh and saline water samples by flow-injection (FI) amperometry, without and with in-valve column preconcentration. Detection is based on reduction of the product formed from the reaction of DRP with acidic molybdate at a glassy carbon working electrode (GCE) at 220 mV versus the Ag/AgCl reference electrode. A 0.1 M potassium chloride solution was used as both supporting electrolyte and eluent in the preconcentration system. For the FI configuration without preconcentration, a detection limit of 3.4 μg P l −1 and sample throughput of 70 samples h −1 were achieved. The relative standard deviations for 50 and 500 μg P l −1 orthophosphate standards were 5.2 and 5.9%, respectively. By incorporating an ion exchange preconcentration column, a detection limit of 0.18 μg P l −1 was obtained for a 2-min preconcentration time (R.S.D.s for 0.1 and 1 μg P l −1 standards were 22 and 1.0%, respectively). Potential interference from silicate, sulfide, organic phosphates and sodium chloride were investigated. Both the systems were applied to the analysis of certified reference materials and water samples.

Flow injection chemiluminescent determination of N-nitrosodimethylamine using photogenerated tris(2,2′-bipyridyl) ruthenium (III)

A flow injection configuration was developed and evaluated for the chemiluminescent determination of N-nitrosodimethylamine. The method is based on the on-line cleavage of the N NO bond of the nitrosamine by irradiation with ultraviolet light. The dimethylamine generated was subsequently reacted with tris(2,2′-bipyridyl) ruthenium (III), which was generated through the on-line photo-oxidation of tris(2,2′-bipyridyl) ruthenium (II) with peroxydisulfate. After selecting the best operating parameters, the emitted light showed a linear relationship with the concentration of N-nitrosodimethylamine between 1.5 and 148 ng ml −1, with a detection limit of 0.29 ng ml −1. The repeatability was 1.6% expressed as relative standard deviation ( n = 10) and the reproducibility, studied on five consecutive days, was 3.2%. The sample throughput was 50 injections per hour. The method was applied to studying the recoveries of N-nitrosodimethylamine in water and different cured meat products.

Spectrofluorimetric determination of formaldehyde by a flow-injection method based on its catalytic effect on the acridine yellow–bromate reaction

A flow-injection configuration for the determination of formaldehyde is proposed. The method is based on the enhancing effect of formaldehyde on the oxidation of acridine yellow by bromate in acidic medium. The proposed procedure is simple, inexpensive, sensitive and suitable for concentrations of formaldehyde between 1 and 56 microg mL(-1). A sampling-rate of 60 samples h(-1) was achieved. The effect of several organic and inorganic species was studied. The method was applied to the determination of formaldehyde in pharmaceuticals, milk and air in work environments. The accuracy of the method was confirmed by comparing the results with those obtained using the standard acetylacetone method.

Flow Injection Spectrophotometric Determination of Ferbam Based on a Photochemical Reaction

A flow-injection configuration for the spectrophotometric determination of ferbam is proposed. The procedure is based on the “online” decomposition of ferbam and complexation of the released Fe3+ with oxalate. The ferrioxalate formed is decomposed upon irradiation with visible light. The Fe2+ produced in the photochemical reaction was monitored by measuring the absorbance after complexation with ferrozine (λmax=562 nm). The method shows a linear range between 1.0 and 30 µg mL−1 (2.5×10−6−7.2 × 10−5 mol L−1) with a limit of detection of 0.2 µg mL−1. Repeatibility was 0.53% expressed as relative standard deviation (n=10), and reproducibility, studied on five different days, was ±1.23%. The sampling frequency was about 45 samples h−1. The applicability of the method was tested in the determination of ferbam in waters and grains.

Flow injection chemiluminescent method for the successive determination of l-cysteine and l-cystine using photogenerated tris(2,2′-bipyridyl) ruthenium (III)

Flow injection configurations were developed for the individual determination of l-cysteine and l-cystine and for the mixture of both analytes. The method is based on the reaction of l-cysteine with tris(2,2′-bipyridyl) ruthenium (II) and peroxydisulphate under UV irradiation to produce chemiluminescence. Cystine is determined after reduction to cysteine in a copper-coated cadmium reductor mini-column in the flow system. The inclusion of a selection valve in the configuration allows the successive determination of the two analytes. Calibration was linear between 2×10 −6 and 5×10 −4 mol l −1 for cysteine and between 1×10 −6 and 2×10 −4 mol l −1 for cystine. When applied to pharmaceutical formulations, the procedure was free from interferences from common excipients. The results obtained for the assay of commercial formulations compared well with those obtained by an official method and demonstrated good accuracy and precision.

S(IV) induced autoxidation of Cu(II)/tetraglycine complexes in the presence of aldehydes: mechanistic considerations and analytical applications

An indirect amperometric method for the determination of sulfite in a flow injection configuration is described. The procedure is based on the amperometric measurement ( E=0.1 V) of Cu(III) chemically generated by the S(IV) induced oxidation of Cu(II)/tetraglycine by dissolved oxygen. The dynamic analytical range was 5–100 μM with a detection limit of 1 μM. The repeatability for the determination (10 μM) was evaluated to be 1.4% ( n=20). The method was employed for the determination of S(IV) in wine after its extraction from acidified samples by bubbling argon and the results were in agreement with those obtained by the iodometric procedure. The influence of formaldehyde and acetaldehyde on the Cu(III) formation is also discussed on the basis of the mechanism of the induced reaction.

Biological fluid screening and confirmation of bile acids by use of an integrated flow-injection-LC-evaporative light-scattering system

An integrated flow-injection-liquid chromatographic system is presented for the screening of biological fluids for bile acids, and for subsequent confirmation. It is based on the use of a flow-injection configuration with an evaporative light-scattering detector (ELSD), for routine screening for the total concentration of the analytes, followed by the individual determination, by liquid chromatography, of only those samples for which the total concentration is close to or above the normal level in healthy individuals. Bile acids are extracted from human serum and urine withn-hexane under acid conditions. After phase separation the organic layer is evaporated to dryness under a stream of nitrogen and redissolved in weakly alkaline solution (pH 8). The aqueous phase is then passed through a miniaturized Amberlite XAD-7 column for preconcentration and clean up, and eluted with acetonitrile-methanol, 65∶35 (v/v). The effluent is introduced directly into the ELSD for determination of the total bile-acid content of the sample. For confirmatory analysis another aliquot of the sample is processed in the screening module and the effluent from the adsorbent column is then directed to the chromatographic column to obtain the bile acid profile of the sample. Good agreement was obtained in the analysis of urine and serum samples by both procedures.