On-line determination of lactic acid during kefir fermentation based on a fibre-optic lactic acid biosensor and flow-injection analysis

Lactic acid was monitored on-line for 13 h during a kefir fermentation by means of a fibre-optic lactic acid biosensor in combination with flow-injection analysis. The biosensor, which is based on an oxygen optrode with immobilized lactate oxidase (LOD), is described. The consumption of oxygen was determined via dynamic quenching of the fluorescence of an indicator by molecular oxygen. LOD was adsorbed on a sheet of carbon black and cross-linked with glutaraldehyde. Carbon black was used for optical isolation to protect the optrode from interference from ambient light and sample fluorescence. With the zone sampling technique the linear range (0.02–0.5 mM) for l-lactate was extended up to 60 mM. The maximum sample throughput is 20 h^?1. For five repeated measurements an r.s.d. of 3% at the 60 mM level was observed. It was possible to do continuous l-lactate analyses with this enzyme optrode for at least 2 days.     

Determination of glucose in wine and fruit juice based on a fibre-optic glucose biosensor and flow-injection analysis

A flow-injection system for the determination of glucose in various samples using a glucose biosensor based on an oxygen optrode with immobilized glucose oxidase (GOD) is described. The consumption of oxygen was determined via dynamic quenching of the fluorescence of an indicator by molecular oxygen. GOD was adsorbed on a sheet of carbon black and cross-linked with glutaraldehyde. Carbon black was used as an optical isolation to protect the optrode from interference from ambient light and sample fluorescence. The system is linear for 0.1–500 mM glucose, with an r.s.d. of 2% at 100 mM (5 measurements). The application of the system to glucose in wine and fruit juice is demonstrated. It was possible to analyse up to 60 samples per hour. The enzyme optrode was stable for more than 400 h in continuous use and was easily maintained.     

A novel luminescent lifetime-based optrode for the detection of gaseous and dissolved oxygen utilising a mixed ormosil matrix containing ruthenium (4,7-diphenyl-1,10-phenanthroline)3Cl2 (Ru.dpp)

A novel luminescent lifetime optrode is presented for the detection of gaseous and dissolved oxygen. The optrode utilises ruthenium (4,7-diphenyl-1,10-phenanthroline)₃Cl₂ as the sensing fluorophore immobilised in a hydrophobic ormosil matrix. The ormosil matrix is synthesised at room temperature from octyltriethoxysilane and methyltriethoxysilane precursors. Investigations of different ormosils were conducted and the most effective one was selected for optrode production. Optrodes were tested for responses to gaseous and dissolved oxygen. Their responses were modelled using traditional two-site or two-exponential methods and feed-forward artificial neural networks. Comparison of the two modelling methodologies is presented and further improvements in modelling and ormosil design are suggested.     

A sodium-selective optrode

An ion selective optrode (ISO) for continuous determination of sodium is presented. It is based on the optical measurement of the membrane potential between an aqueous sample solution and a lipid membrane phase with the help of a potential-sensitive dye. The lipid membrane is composed of rhodamine B C-18 ester together with the sodium selective ionophore ETH 157 and either arachidic acid or 1-octadecanol, and is prepared by using the Langmuir-Blodgett (LB) technique. The fluorescence intensity of the dye reversibly responds to the sodium ion concentration. Fluorescence decreases with increasing sodium concentraction. Interferences by potassium or calcium can be compensated for by using a reference optrode. The sodium sensor works in the 1 to 100 mM sodium ion concentration range with a maximal signal change of –4.2% of the full scale signal for 100 mM. The relationship between the relative signal change (I/I) and the negative logarithm of the sodium concentration is linear over two orders of magnitude. The selectivity coefficients over potassium or calcium are better than 1 × 10⁵ when use is made of a reference optrode. The sensors can be stored under air for more than half a year without loss in fluorescence intensity or changes in response function.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

Design of two-dimensional biomimetic uranyl optrode and its application to the analysis of natural waters

A two-dimensional biomimetic optrode for the detection and quantification of uranium in natural waters was fabricated. The sensing element was designed by the inclusion of uranyl ion imprinted polymer particles into polymethyl methacrylate followed by casting a thin film on a glass slide without any plasticizer. The ion imprinted polymer material was prepared via covalent immobilization of the newly synthesised ligand 4-vinyl phenylazo-2-naphthol by thermal polymerization. Operational parameters such as pH, response time and the amount of sensing material were optimized. The response characteristics of the imprinted and the corresponding non-imprinted polymer inclusion optrodes of uranium were compared under optimum conditions. The imprinted polymer inclusion optrode responds linearly to uranium in the concentration range 0–1.0 μg mL⁻¹ with a detection limit of 0.18 μg mL⁻¹, which is much better than the solution studies using 4-vinyl phenylazo-2-naphthol (1.5 μg mL⁻¹). Triplicate determinations of 100 μg of uranium(VI) present in 250 mL of solution gave a mean absorbance of 0.018 with a relative standard deviation of 8.33%. The superior sensitivity of imprinted polymer inclusion optrode is exemplified by lower detection limits and broader dynamic range over non-imprinted polymer inclusion optrode. The developed imprinted polymer inclusion optrode was found to give stable and precise response for 3 months and can be used without any loss in sensitivity. The applicability for analysing ground, lake and tap-water samples collected in the vicinity of uranium deposits was successfully demonstrated.     

Thermal behaviour and S-O bonding state of alkaline-earth metal sulfites

The thermal behaviours of magnesium sulfite, strontium sulfite and barium sulfite were investigated in the atmospheres of argon and air. the thermal behaviours of magnesium sulfite were different from those of the other two sulfites. The oxidation of magnesium sulfite in air does not occur.The bonding state of the SO ₃ ^2– in each sulfite was compared. The SO ₃ ^2– in magnesium sulfite was coordinated through the sulfur, while those in the other sulfites were coordinated through the oxygen.It appears that the difference in thermal behaviour between magnesium sulfite and the other sulfites depend upon the difference in bonding state of the SO ₃ ^2– We wish to thank Mr. K. Takahashi of MAC Science Inc. for TG and DTA measurements, mass spectra in Ar and his valuable discussions.     

Observations on the determination of oxygen in silver halides using inductively coupled plasma-atomic emission spectrometry

This paper reports on a novel method for the determination of oxygen in silver halides using inductively coupled plasma-atomic emission spectrometry (ICP-AES). A heating system was designed and set up to heat the sample and to release oxygen which was then sent into the plasma by the argon carrier gas. A demountable extended ICP torch was assembled to prevent air from entering the analytical region of the ICP. The nonresonance near infrared atomic oxygen line, O(I) 777.19 nm, was used for the determination of oxygen. The detection limit of the method was 1.6 μg of oxygen. Pure oxygen was used for calibration. The method had a precision of 4.74% RSD for about 15 μg of oxygen in samples.     

A tensiometric study of diamond (111) and (110) faces

Tensiometric contact angle measurements of diamond surfaces in both the air:water and octane:water systems reveal moderate hydrophobicity, as might be expected of a surface with a minority of oxygen-containing groups. High-temperature reaction with oxygen or hydrogen leads to large changes in the contact angles. Spectroscopic methods show a “one-third” coverage by oxygen. The response to pH and adsorption of ferric ion suggests that the oxygen is present as an hydroxyl group. Steric resistance to higher oxide coverage, because of the small lattice constant of the crystal, may be an explanation for the natural hydrophobicity of the diamond surface.     

Multiresidue method using SPME for the determination of various pesticides with different volatility in confined atmospheres

An analytical method is described for assessing the vapour concentration of 11 pesticides (bioallethrin, chlorpyriphos methyl, folpet, malathion, procymidone, quintozene, chlorothalonil, fonofos, penconazole and trimethacarb) in confined atmospheres (e.g. a greenhouse after pesticide application). This study is a successful extension of a method previously developed by the authors for dichlorvos to much less volatile pesticides. Sampling was performed by using polydimethylsiloxane–solid phase micro-extraction (PDMS–SPME) fibres immersed in a 250-mL sampling flask through which air samples were dynamically pumped from the analysed atmosphere. After a 40-min sampling duration, samples were analysed by GC/MS.Calibration was performed from a vapour-saturated air sample. The linearity of the observed signal versus pesticide concentration in the vapour phase was proved from spiked liquid samples whose headspace concentrations were measured by using the proposed method. This procedure gave calibration curves with regression coefficients (R²) greater than 0.98, and the repeatability of these measurements was found with RSDs of 1.9–7.6%. As a field application test, this analysis procedure was used for the determination of gaseous procymidone concentrations as a function of time in the atmosphere of an experimental 8-m² and 20-m³ greenhouse. The pesticide was sprayed according to real cultivation conditions, and measurements were made for 80 h after application (8 measurements). The observed concentrations found ranged from 200 to 500 µg m^–3, thus indicating the level of contamination of the air breathed by people in such working conditions.Abbreviations GC/MS gas chromatography/mass spectrometry - SIM selective ion monitoring - FC43 perfluorotributylamine - RSD relative standard deviation - LOD limit of detection - LOQ limit of quantification     

Charge Transport in BaCe<Subscript>0.85</Subscript>R<Subscript>0.15</Subscript>O<Subscript>3 − δ</Subscript> (R = Sm,Pr, Tb) in Oxidizing and Reducing Environment

Overall conductance (in moist and dry air at partial water vapor pressures of 2.34 and 40 Pa, below 970° C) and transport numbers for ions and protons in ceramics BaCe_0.85R_0.15O_{3 − δ} doped by cations with constant (R = Sm) and variable (R = Pr, Tb) oxidation degrees and in BaCeO₃ are measured. Partial conductances (by ions, protons, oxygen ions, holes) are determined in moist air at partial oxygen pressures of 10⁵–10⁻¹³ Pa at 900°C. Various models for the hydrogen incorporation in the oxides under study are discussed.__________Translated from Elektrokhimiya, Vol. 41, No. 6, 2005, pp. 748–754.Original Russian Text Copyright © 2005 by Sharova, Gorelov, Balakireva.     

Comparison of Experimental Techniques for Measuring Isosteric Heat of Adsorption

Experimental measurements of heats of adsorption published in the literature are often in disagreement; differences of 10–20% are common. The three most widely used experimental methods are: (1) differentiation of adsorption isotherms at constant loading; (2) measurement of adsorption isosteres; (3) calorimetry. Results from these methods were compared for the systems nitrogen on CaA, oxygen on CaA, and carbon dioxide on NaX. Although the same materials and similar degassing procedures were used for all experiments, calorimetric heats are about 2 kJ/mol higher than the heats from isoteric measurements. Additional experiments are needed to bring these methods into exact agreement.     

Oxygen production using La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) perovskite hollow fibre membrane modules

A phase-inversion/sintering technique has been employed in the production of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−α (LSCF) hollow fibre membranes, a bundle of which has then been placed in a high-temperature furnace for production of high purity oxygen from air at temperatures between 980 °C and 1060 °C. By applying a vacuum in the hollow fibre lumens, a product stream containing oxygen purity of 97.15% has been obtained. The downstream vacuum degree higher than 99 kPa shows negligible effect on the oxygen production rate. Studies on long-term operation suggest that the LSCF hollow fibre membranes are less stable for the oxygen production due to the segregation of the constituent membrane elements and the formation of new phases on the outer membrane surfaces. The effect of the operating cycle on the retrogression of membrane performance is much larger than that of duration used in a single cycle.     

Determination of137Cs and134Cs in air samples collected near Belgrade /Yugoslavia/ following the Chernobyl accident

This paper presents the results of measurements of¹³⁷Cs and¹³⁴Cs content in air sampled during May 1986. Maximum concentrations:¹³⁷Cs 2.94±0.01 Bq m^–3 and¹³⁴Cs 1.38±0.01 Bq m^–3 were registered on May 3. Several other long lived radionuclides having -energies in the region 33 keV to 1365 keV were registered in the same samples two years later. The results of measurements of the total -activity in air for the same period are also presented.     

Can clay emit light? Ru(bpy)3-modified clay colloids and their application in the detection of glucose

In this paper, we describe the electrochemiluminescent (ECL) behavior of Ru(bpy)₃³⁺-incorporated clay colloids. Experimental results based on the electrochemical-quartz-crystal-microbalance (EQCM) techniques showed that Ru(bpy)₃³⁺ could be adsorbed by the clay colloids (montmorillonite K10, denoted K10). The resulting clay particles could emit light (λ_em 610 nm) when they were fabricated as thin films sandwiched by two conductive ITO electrodes with opposite biases. These Ru(bpy)₃³⁺-incorporated clay-modified electrodes could also emit light in aqueous oxalate solutions (pH 10) when potentials more positive than 0.9 V vs. SCE were applied. EDTA was an effective promoter for the Ru(bpy)_{3 (clay)}³⁺-oxalate ECL reaction. The resulting ECL showed a remarkable sensitivity to oxygen. A glucose optrode was thus fabricated based on the Ru(bpy)₃³⁺-incorporated K10 colloids and glucose oxidase (GOx). The ECL signals behaved as a function of [glucose], covering a range from 0.1 to 10 mM at pH 10. The detection limits reached a level of 0.1 mM at this pH.     

Defect concentration in Ti-substituted YIG from TG curves

The article discusses theory and experiment about the measurement of defect concentration in Ti-substituted yttrium iron garnet by means of thermogravimetry techniques. The two possible cases arising from oxygen interchange with atmosphere, oxygen vacancies and interstitial cations, may be analyzed quantitatively from the derived expressions. The possibility of another type of defects being present in the samples, not associated to oxygen evolution, is not excluded. Measurements were carried out in air and CO₂ for different Ti contents. The substitution tends to increase slightly the defect parameter θ in YIG, while a CO₂ reducing atmosphere is much more effective than the Ti substitution for increasing θ. Calculations for a given single-phased sample sintered in air show two possibilities: a deficit of one oxygen atom for every 2.5 unit cells of 96 atoms, or one excess cation for every 3.5 cells. Other samples show very similar results. The accuracy involved in the measurements is about 2–3%.     

Schiff base complex sol–gel method for LaCoO3 perovskite preparation with high-adsorbed oxygen

A novel Schiff base complex sol–gel method has been used to prepare LaCoO₃ producing high ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β). The as-prepared gels, characterized by Fourier transform infrared spectroscopy (FTIR), showed that both lanthanum and cobalt ions were complexed before calcinations. IR spectra revealed that CO₃²⁻ and NO₃⁻ presented on the sample surfaces during heat treatment. High-resolution transmission electron microscopic (HRTEM) images of all samples showed resolved lattice fringes with the inter-planar spacing 0.37–0.39 nm of the (0 1 2) plane in hexagonal perovskite. BET surface areas of LaCoO₃ nano-crystals were 11.7–18.6 m²/g. Ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β) quantified by X-ray photoemission spectroscopy showed that LaCoO₃ prepared by the Schiff base complex method produced higher ratios when bases had higher nitrogen content in molecules. Carbonate and nitrate which were resulting from the oxidation of functional groups in the Schiff base complex, can produce gaseous compounds and leave vacant sites for oxygen in the gas phase to adsorb.     

The oxygen content in supercooled water

Summary The oxygen content in liquid water has been measured in the temperature range between 0°C and –7°C. The measurements have been carried out with an amperometric needle sensor in glass-capillaries with an inner diameter of 1.7 mm. It has been obtained that the oxygen content in water is rapidly increasing as the temperature is lowered below 0°C. At –5°C the concentration of oxygen in water at constant partial pressure of oxygen is by 13% higher than that at +3°C. The increase of oxygen content seems to be related to the unusual temperature dependence of heat capacity, density and isothermal compressibility of supercooled water.Herrn Professor Ulrich Wannagat in alter Verbundenheit zum 70. Geburtstag gewidmet.     

Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation

Although Nernst observed ionic conduction of zirconia–yttria solutions in 1899, the field of oxygen separation research remained dormant. In the last 30 years, research efforts by the scientific community intensified significantly, stemming from the pioneering work of Takahashi and co-workers, with the initial development of mixed ionic–electronic conducting (MIEC) oxides. A large number of MIEC compounds have been synthesized and characterized since then, mainly based on perovskites (ABO_3−δ and A₂BO_4±δ) and fluorites (A_δB_1−δO_2−δ and A_2δB_2−2δO₃), or dual-phases by the introduction of metal or ceramic elements. These compounds form dense ceramic membranes, which exhibit significant oxygen ionic and electronic conductivity at elevated temperatures. In turn, this process allows for the ionic transport of oxygen from air due to the differential partial pressure of oxygen across the membrane, providing the driving force for oxygen ion transport. As a result, defect-free synthesized membranes deliver 100% pure oxygen. Electrons involved in the electrochemical oxidation and reduction of oxygen ions and oxygen molecules respectively are transported in the opposite direction, thus ensuring overall electrical neutrality. Notably, the fundamental application of the defect theory was deduced to a plethora of MIEC materials over the last 30 years, providing the understanding of electronic and ionic transport, in particular when dopants are introduced to the compound of interest. As a consequence, there are many special cases of ionic oxygen transport limitation accompanied by phase changes, depending upon the temperature and oxygen partial pressure operating conditions. This paper aims at reviewing all the significant and relevant contribution of the research community in this area in the last three decades in conjunction with theoretical principles.     

The Effects of Gas Composition on Active Species and Byproducts Formation in Gas–Water Gliding Arc Discharge

The effects of gas composition on hybrid gas–water gliding arc discharge plasma reactor have been studied. The voltage cycles are characterized by a moderate increase in the tension which is represented by a peak followed by an abrupt decrease and a current peak in the half period (10 ms). Emission spectrum measurements revealed that ^•OH hydroxyl radicals are present in the discharge with feeding any gas. The H₂O₂ concentrations reach 38.0, 15.0, 10.0, and 8.0 mg/l after 25 min plasma treatment with oxygen, argon, air, and nitrogen, respectively. O₃ was produced when oxygen and air are used, but not when nitrogen and argon. The O₃ concentration reached the highest value 1.0 mg/l after 25 min plasma treatment with oxygen feeding gas, but gradually decreased to 0.2 mg/l after that. With feeding nitrogenous gas, NO₂ ⁻ and NO₃ ⁻ byproducts were formed by the plasma chemical process.