Fabrication and characterization of high-temperature microreactors with thin film heater and sensor patterns in silicon nitride tubes

In this paper the fabrication and electrical characterization of a silicon microreactor for high-temperature catalytic gas phase reactions, like Rh-catalyzed catalytic partial oxidation of methane into synthesis gas, is presented. The microreactor, realized with micromachining technologies, contains silicon nitride tubes that are suspended in a flow channel. These tubes contain metal thin films that heat the gas mixture in the channel and sense its temperature. The metal patterns are defined by using the channel geometry as a shadow mask. Furthermore, a new method to obtain Pt thin films with good adhesive properties, also at elevated temperatures, without adhesion metal is implemented in the fabrication process. Based on different experiments, it is concluded that the electrical behaviour at high temperatures of Pt thin films without adhesion layer is better than that of Pt/Ta films. Furthermore, it is found that the temperature coefficient of resistance (TCR) and the resistivity of the thin films are stable for up to tens of hours when the temperature-range during operation of the microreactor is below the so-called "burn-in" temperature. Experiments showed that the presented suspended-tube microreactors with heaters and temperature sensors of Pt thin films can be operated safely and in a stable way at temperatures up to 700 degrees C for over 20 h. This type of microreactor solves the electrical breakdown problem that was previously reported by us in flat-membrane microreactors that were operated at temperatures above 600 degrees C.     

Differential Pulse Polarographic Determination of Trace Amounts of Lead in Alloys and Biological Samples after Column Preconcentration Using Nitroso-S and TDBA

A highly selective, sensitive, rapid, and economical differential pulse polarographic method has been developed for the determination of trace amounts of lead in various standard alloys and biological samples after the adsorption of its 2-nitroso-l-naphthol-4-sulfonic acid (nitroso-S)—tetradecyldimethylbenzylammonium (TDBA) chloride on microcrystalline naphthalene in the pH range of 8.0–10.5. After filtration, the solid mass is shaken with 9.0 mL of 1 M hydrochloric acid, and lead is determined by differential pulse polarography (DPP). Lead can alternatively be quantitatively adsorbed on 2-nitroso-l-naphthol-4-sulfonic acid-tetradecyldimethylbenzylammonium-naphthalene adsorbent packed in a column and determined similarly. In this case, 1.0 g of lead can be concentrated in a column from 500 mL of an aqueous sample in which its concentration is as low as 2 ng/mL. Characterization of the electroactive process included an examination of the degree of reversibility. Various parameters, such as the pH effect, volume of aqueous phase, HCl concentration, reagent concentration, naphthalene concentration, shaking time, and the interference of a number of metal ions on the determination of lead were studied in detail to optimize the conditions for determination in standard alloys and standard biological samples.     

GC-FTIR-MS analysis of volatile radiolysis products of nitrophenol solutions in carbon tetrachloride

Capillary gas chromatography — Fourier transform infrared spectroscopy combined with capillary gas chromatography — mass spectrometry is employed for the analysis of major volatile products in the -radiolysis of isomeric nitrophenol solutions in carbon tetrachloride. Isomeric di- and trichlorophenols, chloronitrophenols and dichloroisocyanatobenzenes in addition to those formed with m- and p-isomers, are among the important products formed in the solution of o-nitrophenol in carbon tetrachloride. Formation of dichloroisocyanatobenzene is explained by the interaction of dichlorocarbene with the nitro group followed by the ipso-substitution of OH and H atom by chlorine atom.This paper constitutes part of the Ph.D. thesis of M.K. Sahoo and was presented at the 10th conference on Chromatographic method and its significance for human health with international participation, Stará Lesná, High Tatras, Slovakia, Nov. 4–6, 1992.     

Feasibility of Tubular Microreactors for Emulsion Polymerization

The viability of a tubular microreactor for the production of polymer dispersions by emulsion polymerization was investigated. The reactor setup and operational conditions necessary to reach a very stable reactor operation were determined. It was found that stability of pre‐emulsion plays a very important role in achieving steady‐state performance. Further, the flow pattern of the microreactor was characterized, observing that the dispersion model can be used because of the effect of the radial molecular diffusion. Finally, a comparative study of the styrene emulsion polymerization in microreactor and batch laboratory reactor was performed. Similar kinetics and properties were achieved, indicating enough mixing in the microreactor.

     

A new cataluminescence sensor for carbon tetrachloride using its catalytic reduction by hydrogen on palladium/carbon surface

A new cataluminescence (CTL) sensor was developed based on the chemiluminescence (CL) emission from the catalytic hydrodechlorination of carbon tetrachloride on the surface of palladium/carbon catalyst. The factors influencing the CTL signal, such as the catalyst, carrier gas, gas flow rate, temperature and the CL wavelength, were investigated in detail. Under the optimal conditions, the linear range of the CTL intensity versus concentration of carbon tetrachloride was 4.7–235 μg/mL (R = 0.9944, n = 7), with a limit of detection of 0.7 μg/mL (σ = 3). GC/MS results suggest that the possible CTL mechanism of the reduction is the formation of CCl₃ radicals. The CCl₃ radicals combine with H free atoms or capture hydrogen atoms from H₂ molecules to form excited CHCl₃ intermediates, which decay from the excited-state to the ground giving CTL emission for the detection. It is also found that some benzene derivatives with α-H of branched-chain, such as toluene, ethylbenzene and xylenecan, can play a role of catalyst in the reaction.     

Effect of the microchannel plate design on the capacity of methanol steam reformers

Methanol steam reforming in microreactors is considered, and the effects of the microreactor geometry (cylindrical and rectangular) and microchannel plate (MCP) design on the hydrogen capacity of the microreactor is analyzed. The MCPs were made from aluminum foil, stainless steel, and foamed nickel by laser engraving, electrochemical etching, and pressing. The amount of catalyst powder (CuO/ZnO = 40: 60 mol/mol) fixed on one MCP was 0.04–2.5 g. The specific hydrogen capacity (U _w) of the cylindrical microreactor is more than 3 times as high as the U _w of the rectangular microreactor and is 6 times as high as the U _w of a conventional fixed-bed catalytic reactor. This gain in hydrogen capacity is due to the more efficient use of the catalyst in the microreactors. The MCP design, which determines the residence time of the reactants in the microreactor, also has a significant effect on the capacity of the microreactor.     

Modelling a biosensor based on the heterogeneous microreactor

Modelling of the amperometric biosensors based on carbon paste electrodes encrusted with a single heterogeneous microreactor is analyzed. The microreactor was constructed from CPCsilica carrier and was loaded with glucose oxidase. The model is based on nonstationary diffusion–reaction equations containing a nonlinear term related to the enzymatic reaction. A homogenization process having an effective algorithm for the digital modelling of the operation of the microreactor is proposed. The influence of the size, geometrical form, and the position of a microreactor on the operation of biosensors are investigated.     

Second-Derivative Spectrophotometric Determination of Trace Copper after Preconcentration with the Ion Pair of 2-Nitroso-1-Naphthol-4-Sulfonic Acid and Tetradecyldimethylbenzylammonium Chloride on Microcrystalline Naphthalene or a Column

Copper is quantitatively retained by 2-nitroso-1-naphthol-4-sulfonic acid and tetradecyldimethylbenzylammonium chloride on microcrystalline naphthalene in the pH range 7.1–10.7 from large volumes of aqueous solutions of various samples. After filtration, the solid mass consisting of a copper complex and naphthalene is dissolved with 5 mL of dimethylformamide, and the metal is determined by second-derivative spectrophotometry. The copper complex can alternatively be quantitatively adsorbed on tetradecyldimethylbenzylammonium–naphthalene adsorbent packed in a column and determined similarly. About 1.5 g of copper can be concentrated in a column from 300 mL of aqueous sample, where its concentration is as low as 5 ng/mL. The effects of pH, the volume of the aqueous phase, and interferences from a number of metal ions on the determination of copper have been studied in detail to optimize the conditions for its determination in standard alloys and biological samples.     

A low perfusion rate microreactor for continuous monitoring of enzyme characteristics: application to glucose oxidase

This report describes a versatile and robust microreactor for bioactive proteins physically immobilized on a polyether sulfone filter. The potential of the reactor is illustrated with glucose oxidase immobilized on a filter with a cut-off value of 30 kDa. A flow-injection system was used to deliver the reactants and the device was linked on-line to an electrochemical detector. The microreactor was used for on-line preparation of apoglucose oxidase in strong acid and its subsequent reactivation with flavin adenine dinucleotide. In addition we describe a miniaturized version of the microreactor used to assess several characteristics of femtomole to attomole amounts of glucose oxidase. A low negative potential over the electrodes was used when ferrocene was the mediator in combination with horseradish peroxidase, ensuring the absence of oxidation of electro-active compounds in biological fluids. A low backpressure at very low flow rates is an advantage, which increases the sensitivity. A variety of further applications of the microreactor are suggested. Figure Preparation of apoGOx and restoration of enzyme activity using a soluton of FAD     

A portable microreactor with minimal accessories for polymerase chain reaction: application to the determination of foodborne pathogens

The authors describe a microreactor for performing a multiplexed polymerase chain reaction (PCR) which is operated with minimal accessories such as a single heater for gene amplification and a hand-held syringe for sample actuation. It was fabricated by wrapping a polytetrafluoroethylene (PTFE) tube around a rectangular poly(dimethylsiloxane) (PDMS) block having a predetermined thickness. The resulting portable microreactor was applied to PCR using a single heater because the rectangular PDMS block spatially segregated the upper and lower arrays of the PTFE tube. This warrants the adjustment of distinct temperatures inside the upper and lower tubes. A hand-held plastic syringe was connected to the inlet of the PTFE tube and used as a portable pump to achieve a homogeneous transport of a sample liquid inside the microreactor. The use of gas-impermeable PTFE prevents the formation of bubbles during thermal cycling. The microreactor was successfully applied to the amplification of typical DNA fragments of three foodborne pathogens in less than 30 min. In our perception, this method paves the way to the construction of a truly portable PCR chip that is applicable to rapid clinical diagnosis and the detection of foodborne pathogens.

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Indirect micromanipulation of single molecules in water-in-oil emulsion

Based on real-time observation and micromanipulation, analytical methods for single DNA molecules have been under development for some time. Precise manipulation, however, is still difficult because single molecules are too small for conventional techniques. We have developed a chemical reaction system that uses water droplets in oil as containers of materials. The water droplets can be manipulated by optical force. The manipulation of the water droplets permits the fusion of two selected droplets. This process corresponds to mixing of different samples. We designate this system as "w/o (water-in-oil emulsion) microreactor system", and each droplet can be thought of as a "microreactor". In this system, single molecules can be manipulated readily, as a molecule can be contained in a microm-sized microreactor. The microreactor utilizes extremely small quantities of samples, therefore, reactions are rapid, as diffusion times in the microreactor are very short. The manipulation technique of the microreactors based on optical force has been applied to induce fusion between microreactors loaded with DNA and YOYO, a fluorescent dye that binds to DNA. This fusion induced a rapid binding of YOYO.     

A gas chromatographic assay for quantitative analysis of volatiles in solid materials by discontinuous gas extraction

Summary A method has been developed for the quantitative analysis of volatile compounds in solid samples. The method is based on a stepwise gas extraction of the volatiles with subsequent analysis of the extracted material and is termed discontinuous gas extraction. Any quantitative analysis requires an exhaustive extraction, which, however, is often too time-consuming for routine analysis. It is shown how the total amount of each volatile compound can be calculated from only a few extractions. Such a calculation is possible because for analytical purposes it is the information of the extraction process and not the extracted material that is needed. This method is useful for samples which are insoluble, such as certain polymers or residual solvents in printed foils, and which cannot be analyzed quantitatively by headspace gas chromatography, since no calibration solution can be prepared. It is further shown how discontinuous gas extraction can also be used to calibrate headspace analysis. Thus, both methods combine well together in that discontinuous gas extraction provides the accuracy while the headspace analysis gives convenience and speed of sample throughput, particularly if carried out with an automated headspace analyzer.     

Investigation of stereoisomers of 4-alkenyl-trans-decahydroquinol-4-ols in mixtures by the method of reaction chromato-mass spectrometry

The mass spectra of the appropriate 4-ethyl derivatives, which provide the determination of the configuration of the 4 position in the initial alcohols, are registered in the chromato-mass spectrometric investigation of mixtures of the stereoisomeric 4-vinyl-trans-decahydroquinol-4-ols using a microreactor for hydrogenation; the microreactor is placed before or after the chromatographic column.Communication 13 of the series Reaction chromato-mass spectrometry. For Communication 12, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1215–1217, September, 1986     

Coupling of Methanol and Isobutanol to Ethers over γ-Alumina Catalyst. Kinetic Studies

The kinetic of methanol (MeOH) and isobutanol (iBuOH) coupling to ethers has been studied. Catalytic tests were carried out in the gas phase in a fixed-bed flow microreactor, in the temperature range of 463-533 K, under atmospheric pressure, using -Al₂O₃ as the catalyst. The quasi-homogeneous model for the process was assumed. The reaction rates were described by simple kinetics.     

A Novel Method for Separating the Anodic Voltammetric Peaks of Dopamine and Ascorbic Acid

This paper presents a simple and reliable method for separating the anodic peaks of dopamine (DA) and ascorbic acid (AA). Good electrochemical responses of DA and AA can be obtained at an electrochemically pretreated glassy carbon electrode (PGCE). The electrode reaction of DA is a quasi-reversible process, but the oxidation of AA is entirely irreversible. Consequently, the PGCE can be used to separate the anodic peaks of DA and AA. The separation can be higher than 290mV, and the detection limit (S/N=3) of DA is 6.0×10^–8M. This PGCE is very stable and can be used for as long as four weeks.     

Alpha-activation analysis of nitrogen in boron-doped silicon single crystal

The rapid dissolution of silicon and the substoichiometric separation of¹⁸F were studied in order to determine nitrogen in a boron-doped silicon single crystal by activation analysis via¹⁵N(,n)¹⁸F. The silicon dissolution method was developed by using a nitric acid-based solution containing a known amount of ammonium fluoride, instead of hydrofluoric acid, as the¹⁸F carrier. The silicon could be dissolved to a depth of 100 m with a 10 mm Ø in 9 minutes. The conditions for¹⁸F steam distillation and substoichiometric precipitation as lanthanum fluoride were improved. 50% of the fluorine could be separated substoichiometrically even though the carrier amount was increased from 6–12 mmol to more than 16 mmol in order to add it to the dissolving solution. This dissolution method and improved substoichiometric separation were used to determine nitrogen in a boron-doped silicon single crystal. The nitrogen concentration was found to be less than the detection limit (50 ppb).     

Measuring hydrogen by cold-neutron prompt-gamma activation analysis

By irradiating with cold neutrons and avoiding hydrogenous materials of construction, we have developed a PGAA instrument at the Cold Neutron Research Facility at NIST with hydrogen detection limits in the microgram range in many materials. Quantities of 5–10 g H/g are presently measurable in gram-sized samples of silicon or quartz, and of order 0.01 wt % can be quantitatively measured in complex silicate rocks.Contributions of the National Institute of Standards and Technology are not subject to copyright.     

Indirect spectrofluorimetric determination of arsenic at nanotrace levels in alloys, underground water, industrial waters and sewage sludge

An inexpensive and virtually specific nonextractive Spectrofluorimetric method for the determination of As(III) (1 ng/ml-1 g/ml) in 0.18–1.08N H₂SO₄ has been developed. A known excess of Cr(VI) is allowed to quantitatively oxidize As(III) in the essential presence of a mixture of iodide and chloride as catalyst, followed by the addition of excess 2-(-pyridyl) thioquinaldinamide and measurement of the fluorescence intensity (_em(max) = 488 nm _ex(max) = 360 nm) caused by Cr(VI) remaining after As(III) oxidation. The measurement is repeated without As(III) addition and the decrease in fluorescence intensity gives the As(III) concentration with the help of a calibration graph. The method is very precise and accurate (S. D. = ± 0.474, R. S. D. = 0.95%, for the analysis of 50ng/ml As(III), 11 replicates). The detection limit is 0.3ng/ml As(III). Large excesses of over fifty cations, anions and some complexing agents do not interfere. The method has been successfully applied for arsenic determination in various complex matrices: environmental waters, sewage sludges and several certified reference materials (alloys). This method has also been extended to determine As(V), after its reduction to As(III), in water samples, waste discharge water and sewage sludges.Presented at International Conference Arsenic in Ground Water: Cause, Effect and Remedy, Calcutta, India, February, 1995     

Chromatographic Determination of Silicon and Phosphorus as Molybdic Heteropoly Acids with Preconcentration

The chromatographic behavior of molybdic heteropoly acids of silicon and phosphorus as ion pairs with tetrabutylammonium bromide was studied by ion-pair high-performance liquid chromatography on the C₁₈reversed phase (UV detection at 310 nm). Heteropoly acids were preconcentrated as ion pairs with tetrabutylammonium bromide to increase the sensitivity of the determination. Optimal conditions were selected for the chromatographic determination of silicon and phosphorus in water in the presence of each other. Detection limits for silicon and phosphorus are (1.1 × 0.3) × 10^–3and (6.7 × 1.2) × 10^–3g/mL, respectively. Calibration plots are linear in the concentration ranges 0.01–0.1 g/mL (silicon) and 0.02–0.15 g/mL (phosphorus). The procedure was used for the analysis of distilled water.     

SIMS Investigations on the Distribution of Trace Elements in Modified Aluminium–Silicon–Magnesium Alloys

 Strontium modified aluminium–silicon cast alloys are well known for their outstanding mechanical properties as they combine excellent strength with good ductility that is due to a modification of brittle Si in the eutectic with traces of Sr (0.3–0.5 wt%). Although the level of Sr addition is very low, formation of ternary AlSiSr phases with deleterious effects on the ductility can take place. Consequently, there is a certain need of alternative modifying elements. Following the theory of Lu and Hellawell which predicts an optimal atomic size for good modification, Eu, which has an atomic radius close to that optimal size, was additionally used as a modifying element. This study presents SIMS (secondary ion mass spectrometer) investigations of the 3D-distributions of the main alloying and trace elements in Al–Si feedstock alloys for thixocasting modified with Sr and Eu, respectively. Received October 1, 2001; accepted May 21, 2002