Oxygen Transport Processes Research Articles

Antioxidative Effect of Glucose Oxidase and Catalase in Mayonnaises of Different Oxidative Susceptibility. II. Mathematical Modelling

A theoretical assessment was made of whether the enzyme system glucose oxidase/catalase (GOX/CAT) has an antioxidative effect in a food model consisting of an O/W emulsion packed in an oxygen-permeable plastic bag. This model represents food products such as mayonnaises and other high-fat dressings. The various oxygen transport and consumption processes were analysed theoretically and their relative significance estimated. This showed that when GOX/CAT is present in the emulsion, lipid oxidation rate, enzymatic oxygen removal rate, and oxygen penetration through the packaging material were significant for the overall reaction rate, whereas all the diffusional resistances in the oil and in the aqueous phase could be neglected in a mathematical modelling of the system. The derived model predicts that the lipid oxidation rate in a packaged, stagnant emulsion is proportional to the oxygen permeability of the packaging material and inversely proportional to a term containing the GOX concentration and characteristic kinetic constants for the enzyme reaction and for the lipid oxidation reaction. Experimentally obtained results, where lipid oxidation rate was varied by adjusting the fatty acid composition of the mayonnaise, were in satisfactory agreement with the theoretical model.

Electrochemical filters for oxygen sensors with improved surface properties

Oxygen sensors based on YSZ (yttria stabilized zirconia) are best known for high temperature measurements. The protection of such oxygen sensors with dense electrochemical filters is an effective way to increase their life time in aggressive industrial atmospheres. This work reports on the performance of sensors protected by composite or single phase mixed conducting filters. The first group includes materials based on GCO (Gd2O3 doped CeO2)+LC (LaCoO3), and on YSZ+LCM (La(Mn,Co)O3). Sensors protected with GCO+LC composites showed the best performance amongst the composite filters already evaluated. Electroded single phase GCO filters were also tested to check on the role of surface reaction kinetics on the overall oxygen transport process across the filter. The fastest sensor responses to a sudden decrease in oxygen partial pressure were obtained with deposited electrodes either on both surfaces of the filter or on the filter cathode side. These preliminary results suggest that the cathodic process is more relevant than the anodic one in determining the filter permeability (and protected sensor performance), which fulfils the logical requirement of electrode free surfaces in contact with the furnace atmosphere.

The effects of thin surface-applied oxide coating films on the selective oxidation of alloys

The effects of thin surface-applied Al 2O 3-Y 2O 3 composite films on the selective oxidation of chromium in Ni-10Cr in air at 1000 °C have been studied. An integral chromia scale was established on specimens coated with an A1 2O 3-Y 2O 3 film during oxidation for 100 h and the weight gains were almost two orders of magnitude lower than those of uncoated specimens. Mechanisms to account for such effects have been explored. It is proposed that two oxygen transport processes influence the selective oxidation of an alloy: short-range diffusion of oxygen at the scale/alloy interface and long-range diffusion of oxygen in the bulk alloy. When the flux of oxygen due to shortrange diffusion at the scale/alloy interface is much greater than that due to long-range diffusion in the bulk alloy, a chromia scale is able to form at the scale/alloy interface, that is, selective oxidation of chromium is promoted. Otherwise, internal oxidation of chromium in this alloy occurs. It has been confirmed that increasing the number of sites for oxide to nucleate, by the application of a surface oxide film, accelerates short-range diffusion and reduces the critical content of chromium in the alloy required for the formation of a chromia scale. Such selective oxidation may be promoted by increasing the thickness of the film or by applying a film in which the transport of oxygen is much reduced. The beneficial effects of an Al 2O 3Y 2O 3 composite film on the selective oxidation of chromium in Ni-10Cr can be explained by this hypothesis.

Empirical modeling for oxygen transport processes and related physiological and bioprocess systems.

Engineering analysis is based upon quantification of systems via mathematical modeling and computer simulation. The most fundamental approach is to begin with the basic principles of physics, chemistry and biology and formulate phenomenological models that are based upon mass, energy and momentum balances. This procedure evolves equations with specific system parameters such as, diffusivity, velocity, rate constants, etc., that can be examined to determine system sensitivity to various inputs. The theoretical modeling approach has many advantages because it yields a primary understanding of system behavior at the process parameter level.

High resolution low energy resonance depth profiling of18O in near surface isotopic tracing studies

The exceptional depth profiling potential of the ≈ 100 eV narrow resonance in the 18O(p, α)15N nuclear reaction at 151 keV is hindered by the low counting rates due to its low cross section. This drawback is in fact associated with Van de Graaff type accelerators, that provide small proton beam currents at these low energies. In this paper we demonstrate experimentally that this disadvantage may be overcome with ion implantation type accelerators, which may deliver high proton currents. The very high depth resolution of this method in the first hundreds Ångströms of solids was put to benefit efficiently and without hindrance in 18O isotopic tracing experiments applied to the systematic study of oxygen transport processes in various oxides. The method is illustrated by the study of various oxidation processes of silicon and of recoil implantation of oxygen into the silicon substrate from very thin thermal oxide layers under As and Sb ion bombardment. Deep 18O profiles could be measured in perpendicular geometry with energy straggling limited resolution. Shallow 18O profiles in SiO2 films with thicknesses below 100 Å could be deduced from excitation curves recorded in grazing incidence geometry.

Surface oxygen exchange properties of bismuth oxide-based solid electrolytes and electrode materials

The surface oxygen exchange coefficient, k s, has been measured for the solid solution (Bi 2O 3) 0.75(Er 2O 3) 0.25 and (Bi 2O 3) 0.6(Tb 2O 3) 0.4 (abbreviated BE25 and BT40), using gas-phase 18O exchange techniques. The activation enth alpy of k s amounts to ΔE=110 kJ/ mol for BT40 and ΔE=130 kJ/ mol for BE25. The magnitude of k s for the purely ionic conducting BE25 is comparable with values obtained from electrode polarization ( I−V) measurements ( ΔE=140 kJ/mol.) The comparatively high k s values show a ( P O 2 ) n dependence on oxygen pressure with values of n close to 0.5, indicating surface control in the oxygen transport process. Bismuth oxide containing solid solutions show a large activity in the oxygen exchange reaction with the gas phase.

Distribution of skeletal muscle blood flow during locomotory exercise.

If submaximal, rhythmic exercise is to be maintained for an extended period of time (60 min for example), the ATP used to fuel muscular contractions must be provided primarily by oxidative metabolism. As a result, oxygen transfer from air to active skeletal muscle tissue must be matched to the metabolic rate of the muscles. In this regard, up to this point in this symposium, we have considered the determinants of blood oxygenation. The transfer of the oxygenated blood to the skeletal muscle capillaries is the next step in the oxygen transport process. After this convective process, oxygen transfer into the myocytes is determined by the perfused capillarity of the tissue and the determinants of diffusion (i.e., O2 concentrations in blood and tissue, diffusion distance, etc.).

Study of oxygen transport processes during plasma anodization of Si between room temperature and 600 °C

Studies of oxygen transport mechanisms during plasma anodization of Si have been carried out using a combination of 18O tracing techniques and nuclear microanalysis. A thin 18O-enriched ZrO2 overlay film has been used as a source of oxygen tracers during subsequent plasma anodization of Si. The enhancement effect of the zirconia layer on the kinetics of Si anodization allows one to study the mechanisms of Si anodization between room temperature and 600 °C. Results of the 18O depth distributions show that the order of oxygen atoms is largely preserved in the SiO2 and that oxygen ions migrate during film growth. For the temperature range (25–600 °C) studied the microscopic mechanism of oxygen transport during plasma anodization of Si involves a short-range oxygen ion migration, in contrast to the case of high-temperature thermal oxidation. The contribution of a pure thermal process via the diffusion of neutral oxygen species is negligible compared to the high-field-assisted oxygen ion migration in these plasma anodization experiments.

Isotope Labeling Studies of the Oxidation of Silicon at 1000° and 1300°C

Oxygen‐18 profiles in thermal oxides grown on silicon at 1000° and 1300°C in sequential oxidation steps in the parabolic oxidation regime indicate a change in the dominant oxygen transport process. At 1000°C, oxygen‐18 pile‐up at the surface and at the oxide/silicon interface is consistent with a large flux of oxygen via molecular oxygen permeation and very slow diffusion of lattice oxygen ions . At 1300°C, the oxygen‐18 concentration is uniformly high across the oxide, which suggests a much larger lattice oxygen diffusivity and a total flux due to lattice oxygen diffusion which contributes significantly to the supply of oxygen to the growth interface.

A mathematical model of transmural transport of oxygen to the retina.

A mathematical model of transmural transport of oxygen to a metabolizing retina is presented based on the equations of fluid dynamics. The equations of oxygen transfer are derived and then solved subject to the condition that the capillaries begin to transport oxygen at an initial time. The resulting transient analysis gives us insight into how diffusive and filtrative processes lead to the oxygen distributions both inside and outside capillaries. On the other hand, the steady state solution allows us to predict the cutoff intraocular pressure above which no oxygen is transferred to retinal tissue. It also gives quantitative relationships which allow us to postulate how intracapillary hypertension counterbalances elevated intraocular pressures and how low pressure glaucoma may arise from ineffective diffusive and filtrative processes of oxygen transport.

Study by Nuclear Microanalysis and O18 Tracer Techniques of the Oxygen Transport Processes and the Growth Laws for Porous Anodic Oxide Layers on Aluminum

The method of nuclear microanalysis of stable oxygen isotopes by the reactions was used to investigate the mechanisms of growth, dissolution, and transport associated with oxygen during the formation of relatively thin (≤7000Aå) porous anodic layers on aluminum. The average charge associated with an oxygen ion fixed in the film was calculated on. the basis of the direct measurement of sulfur incorporation in the films. The current efficiency of the oxidation was determined (and found to be low, ∼55%) by comparing the amount of oxygen fixed on the metal surface deduced from nuclear microanalysis to that deduced from the amount of charge fed into the circuit. The rate of chemical dissolution of the oxide was measured in open circuit and under polarization and was found to be two orders of magnitude lower than that which would explain the low current efficiency. Hence the existence of an electrochemical process of cation transfer must be assumed. It was found, by using oxygen 18 tracer techniques, that the growing oxide layer is formed at the metal/oxide interface by oxygen transport through the base layer. The results show that about 60% of the ionic current is transported by oxygen ions and about 40% by cations during the growth of porous oxide layers on aluminum.

Geometric considerations in modeling transport processes in tissue.

Numerical solution of partial differential equations describing transport processes in capillaries and tissue is used for calculation of oxygen transport to brain tissue. Calculation is based on a three-dimensional network model which covers inhomogeneities in capillary blood flow in a relation of 27:1, This study was performed in order to obtain information about the influence of the main physiological parameters on oxygen tension frequency distribution pattern. Following results were obtained: a)Increase (or decrease) of the oxygen consumption rate in tissue to about twice (or half) of the normal case for cerebral grey matter causes an extreme left (or right) shift of the oxygen tension frequency distribution = histogram with a decrease (or increase) in venous oxygen tension from 35 to 18 (or 61) mmHg. b)A decrease of capillary blood flow at the input of the network in steps of about 15% from the normal value causes a stepwise left shift of the oxygen tension frequency distribution with venous oxygen tensions from 35 to 22 (or 13) mmHg. c)An increase of the critical oxygen tension from 1 to 10 mmHg causes a considerable right shift with an increase of the venous oxygen tension from 35 to 49 mmHg.