Electron spin resonance spectroscopy for evaluation of early oxidative events in semisolid palm oil

Electron spin resonance (ESR) spectroscopy was applied to study early oxidative events in semisolid palm oil in bulk. Oil was stored at mildly accelerated conditions of 50°C for 7 days and the free radical formation was followed with the addition of spin trap N‐tert‐butyl‐α‐phenyl‐nitrone. Dissolution of the oil samples in isooctane prior to ESR measurements stabilized the ESR signal allowing the changes in relative free radical concentrations during oil storage to be monitored. Formation of lipid hydroperoxides as primary lipid oxidation products was found to correlate with the tendency for the formation of free radicals in the oil during the storage and accordingly, ESR spectroscopy may be used to detect the early events in lipid oxidation in palm oil. However, the interference of added rosemary extract (RE) in ESR analyses was seen as an increased ESR signal while the efficiency of RE as antioxidant in palm oil was confirmed by isothermal DSC. Practical applications : ESR spectroscopy may be used to evaluate early events of oxidation in semisolid oils such as palm fat, which is widely used in food industry.     

Electron spin resonance spectroscopy for determination of the oxidative stability of food lipids

Evaluation of the oxidative stability of food lipids based on the tendency of formation of radicals is shown to be possible using electron spin resonance (ESR) spectroscopy and the spin-trapping technique. Induction time can be determined for mildly accelerated conditions (50°C for lipid fraction from mayonnaise enriched with fish oil), and the length of the induction time decreases during storage and γ-tocopherol depletion. The protection by ethylenediaminetetraacetic acid against initiation of lipid oxidation is also detected in the new assay. For more oxidatively stable lipids (butter, rapeseed oil, dairy spread) the mildly accelerated conditions can be used in the assay, provided that difference in signal height for fixed times replaces determination of induction time. ESR spin trapping provides a sensitive method for evaluating the oxidative stability of food lipids. Detection of radicals in the lipid as an early event in oxidation allows mild conditions to be used, and future experiments should also include sensory evaluation in relation to determination of practical shelf life.     

Carbon radicals generated by solid polymers: Electron spin resonance spectroscopy for detection of species in water

Radicals generated in plastic medical devices (solid phase) by γ-rays or electron-beam irradiation during sterilization are known to cause oxidation of protein drugs, resulting in a loss or reduction in drug efficacy. The generation of radical species in water by the radical species in solid polymers has not been proved. Using electron spin resonance (ESR) spectroscopy, we confirm the generation of new radicals in water by γ-ray irradiated cyclic olefin polymers (COP). ESR measurements are obtained using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) as a spin probe and 5-(2,2-dimethyl-1,3-propoxycyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) as a spin trap, in which the irradiated COP was immersed. The ESR signals indicate the TEMPOL radicals decline over time, suggesting the generation of new radicals. Conversely, the characteristic ESR signals of the adduct formed by the reaction between CYPMPO and the hydroxyl radical are observed. Thus, hydroxyl radicals are generated because of the migration of the radicals from COP to water. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48604.     

Electron spin resonance in glassy carbon

The ESR was investigated for a series of samples of glassy carbon heat-treated to various temperatures in the range 1000–3000°C using a Q and X band spectrometers. For the solid material, the width first increases slightly, then goes through a minimum at around 1400°C and increases greatly above HTT 2600°C. For a ground material, a large broadening in the transition range (~1300°C) is observed. The intensity for all samples passes through a minimum at HTT 1200°C and increases to a strong maximum at around HTT 1550°C after which it decays to a value 0·5 × 10¹⁹ spin/g. The influence of neutron irradiation and of subsequent anneal are also reported. The irradiated material shows a large broadening in the transition range (~ 1300°C) similar to the broadening observed for irradiated soft carbons. Temperature dependence measurements of the ESR signal were carried out for nonirradiated and for irradiated samples, in order to determine the contribution of the localized spin and of the conduction carriers to the total intensity.     

Electron spin resonance spectroscopy study on reduction of constrained‐geometry catalyst systems

The reduction of a constrained‐geometry catalyst (CGCTiMe₂) activated with a series of cocatalysts, including modified methylaluminoxane (MMAO), tris(pentafluorophenyl) borane [B(C₆F₅)₃], and combined B(C₆F₅)₃/MMAO, was experimentally investigated using an on‐line electron spin resonance spectroscopy (ESR) technique. The effects of the solvent type, cocatalyst/catalyst ratio, and temperature on the reduction were examined. In the CGCTiMe₂/MMAO system, increasing the Al/Ti molar ratio from 20 to 250 significantly increased the Ti(III) content. Adding trimethylaluminum to the system lowered the Ti(III) content. Three trivalent Ti species, Ti(a) (g = 1.972), Ti(b) [g = 1.992, α(H) = 7.4 G], and Ti(c) (g = 1.995), were observed. In the CGCMe₂/B(C₆F₅)₃ system, only one trivalent Ti species (g = 1.988) was observed. The CGCTiMe₂/B(C₆F₅)₃ system was more stable to reduction than the CGCTiMe₂/MMAO system. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2474–2482, 2003     

Electron spin resonance from impurities in coal-derived pyrites

The first observation of several e.s.r. lines in coal-derived pyrites (FeS₂) is reported. These lines are due to Mn²⁺ impurities in FeS₂ and to free radicals in coal kerogens associated with pyrites as inclusions.     

Interpretation of spin concentration measurements from a bituminous coal using in situ electron spin resonance spectroscopy

The spin population in a UK bituminous coal has been measured by in situ ESR as a function of temperature in a conventional sealed tube and in a novel flow cell. It was found that the reproducibility of the results from the closed-tube cell was generally worse, indicating that the contact of evolved volatiles with the char complicates these measurements. Temperature dependence data reinforce this conclusion in that the dependences obtained from the flow cell are much more linear and therefore consistent with Curie Law behavior, than those from a closed tube. Detailed examination of the data suggests that heat-treated coals possess nonzero and variable Weiss constants, which calls into question the blind application of the Curie Law correction term to high-temperature in situ ESR measurements. These results are independent of dilution with silica and so are uncomplicated by sample conductivity effects, which have been invoked to explain some previous anomolous behaviors.     

Novel method for rapid monitoring of lipid oxidation by FTIR spectroscopy using disposable IR cards

A new FTIR approach was investigated for assessing edible oil oxidative stability with the use of polymer IR (PIR) cards as sample holders. This approach allows oil oxidation to be monitored at moderate temperatures owing to the fairly rapid rate at which unsaturated oils oxidize on the PIR cards. To assess the FTIR/PIR card method, pure TAG—triolein, trilinolein, and trilinolenin—were loaded onto cards and placed in a chamber where warm air (55°C) flowed over them continuously to facilitate oxidation. At periodic intervals, individual cards were removed and their FTIR spectra scanned, after which they were replaced in the aeration chamber. All spectra were normalized to compensate for variations in PIR card path lengths or oil loadings, and for each card the initial spectrum recorded (t=0) was subtracted from all subsequent spectra taken over time to produce differential spectra. With the use of a peak-find algorithm, the absorbance minimum in the cis region (3017–3000 cm⁻¹) and the absorbance maxima in the hydroperoxide (3550–3200 cm⁻¹), isolated trans (977–957 cm⁻¹), and conjugated trans regions (995–983 cm⁻¹) were measured in the differential spectra and plotted as a function of time. For all three TAG, the loss of cis double bonds was linearly related to the development of hydroperoxides and isolated trans bonds for much of the oxidation process, whereas for the polyunsaturated TAG a similar relationship also existed for conjugated trans species. Based on experimentally determined hydroperoxide (ROOH) absorbance slope factor (0.06 mAbs/PV), ROOH absorbance changes were converted to PV, allowing direct PV monitoring as a function of time using the PIR cards. Trilinolenin, trilinolein, and triolein attained a PV of 100 mequiv/kg oil after, 43,98, and 2889 min, respectively, their relative reaction rates being similar to ratios published in the literature. The assessment of the FTIR/PIR card method using TAG indicates that it may be a practical and rapid means of oxidizing lipids and tracking their oxidative state in terms of PV so as to provide a measure of their oxidative stability.     

Electron spin resonance study of free radicals in Athabasca asphaltene

The asphaltene of the Athabasca bitumen exhibits a single, structureless e.s.r. signal at a g-value of 2.0032 with an intensity corresponding to approximately one free spin per ninety molecules. From the temperature dependence of the absorption it is concluded that absorption is due to stable polycycloaromatic doublet- and triplet-state radicals. Singlet-state e.s.r. inactive biradicals are also present in thermal equilibrium with the triplets which lie 300 cm^?1 above the singlet level. The internuclear separation of the radical sites in the triplet biradicals is estimated to be greater than 1.7 nm. Oxygen causes a reversible enhancement of the e.s.r. absorption by inducing the conversion of singlet biradicals to doublets. A quantitative treatment of the enhancing effect based on the Langmuir adsorption isotherm made it possible to derive an activation energy of approximately 17 kJ/mol for the dissociation of the oxygen-asphaltene complex and to evaluate the temperature variation of the number of radical pairs which contribute to the intensity variation. The kinetics of the spontaneous relaxation of the O₂-asphaltene system were also investigated and it was possible to show that the rate-controlling step, which is diffusion of oxygen into the interior of the solid asphaltene, has an activation energy of 25–29 kJ/mol.     

Electron spin resonance studies on silver atoms in imogolite fibers

The formation and stabilization of reduced silver species in imogolite have been studied by electron spin resonance (ESR) spectroscopy. Ag-loaded imogolite samples after degassing and dehydration were γ-irradiated at 77 K and monitored by ESR as the temperature increased. Some samples were exposed to methanol vapour after dehydration. It was found that imogolite shows exceptional ability to stabilize silver atoms. In dehydrated Ag-imogolite silver atoms generated at low temperature remain stable at room temperature. Silver atoms are also formed in imogolite samples exposed to methanol. However, in contrast to silver agglomeration in molecular sieves and smectites exposed to methanol there is no indication of the formation of cationic silver clusters in Ag-imogolite. It is postulated that there are special trapping sites in imogolite structure which effectively stabilize silver atoms.     

Tocotrienols from palm oil: Electron spin resonance spectra of tocotrienoxyl radicals

The major vitamin E components present in palm oil, viz. α-tocopherol, α⁻, ψ-and δ-tocotrienols, have been isolated and their structures verified by the NMR spectra of their acetate and succinate derivatives. Oxidation of γ-and δ-tocotrienols with alkaline K₃Fe(CN)₆ gave isolable dimeric species, which were studied by¹³C NMR. Free radicals generated from the monomeric and dimeric tocotrienols were investigated using ESR spectroscopy. The distinction between antioxidant activity and antioxidant capacity of vitamin E isomers is discussed.     

MIR spectroscopy: A new method for studying the hydrolysis of hexamethylcyclotrisilazane in air

A new method for studying the rate of hydrolysis of hexamethylcyclotrisilazane has been developed. This method, MIR spectroscopy in the infrared region of 4000–625 cm^?1, was employed to characterize the reactions. The change in relative intensity of characteristic bands of the reactant is plotted as a function of time to give the required rate curves. The effects of temperature, pressure, exterior magnetic fields, and ultraviolet–visible radiation on the rate curves are also discussed.     

Electron spin resonance of VO radical-ion in sub- and supercritical water

Electron spin resonance (ESR) spectra of VO²⁺ radical-ions in sub- and supercritical water are observed. Upon increasing the temperature from 20 to 100 °C, the fine structure line widths in the ESR spectra of the vanadyl ion are observed to be reduced that is associated with the effective averaging of the g-factor anisotropy and the hyperfine interaction. With further increasing the temperature, the spectrum components of the hyperfine structure are broadened significantly resulting in the unresolved low-intensity line in supercritical water with ΔH_pp ∼ 300 G. The data obtained allow behavior peculiarities of the paramagnetic VO²⁺ ions in sub- and supercritical water including rotational dynamics and spin exchange between the radicals to be elucidated. The registration of the unresolved low-intensity line in supercritical conditions points to an increase in the local ion concentration in the system that can be an initial stage for the formation of vanadium-based particles in supercritical conditions. The study demonstrates that ESR is the powerful tool to investigate properties of sub- and supercritical water in situ.     

Electron spin resonance study of a cured epoxy resin exposed to high-energy radiation

Tetraglycidyl-4,4′-diaminodiphenyl methane cured with 4,4′-diaminodiphenyl sulphone was irradiated with varying dosages of 0.5 MeV electrons and 1.17 and 1.33 MeV gamma rays and was studied using electron spin resonance. The radical concentration increases with increasing radiation dose and decays, at ambient temperature, in a way consistent with a model that assumes two simultaneous second-order reactions occurring in different zones.     

Electron spin echo spectroscopy: A tool to probe structural properties of coals

Electron spin echo (e.s.e.) spectroscopy has been used in these laboratories to investigate proton hyperfine interactions in whole coal, separated coal macerals, synthetic lignites and model materials comprising perylene radical ions adsorbed on alumina and silica-alumina catalysts. The e.s.e. technique provides information that is complementary to electron nuclear double resonance in studying such interactions in coal. Coupling constants measured so far in evacuated samples of Illinois No. 6 agree well with models for condensed ring aromatic structures. Analysis of the proton matrix interactions are best fitted by r = 0.75 ± 0.05 nm, A_iso = 0 and n = 40 protons. Matrix interactions from ¹³C nuclei also are seen. Coupling constants observed in a vitrain component separated from Illinois No. 6 coal are similar to those seen in the whole coal; the fact that the maceral does not exhibit all of the interactions found in whole coal suggests molecular structures to be specific to individual maceral types. Differences in the g-values of macerals may provide a direct, non-destructive route to the analysis of maceral-specific structure.     

Low temperature processes in a bituminous coal studied by in situ electron spin resonance spectroscopy

The free spin concentrations in a whole and extracted bituminous coal, were measured by electron spin resonance spectroscopy (e.s.r.). In situ high temperature measurements of spin populations were made with a novel flow cell which enables volatiles formed to be swept clear of the e.s.r. active zone. The development of the spin population profile with temperature was measured up to 400 °C, and three regions, delimiting different processes taking place in the coal, can be defined. The interpretation of these profiles was made in conjunction with thermogravimetric and differential thermal analysis. It is suggested that the regions, in order of ascending temperature, correspond with desorption, recombination and thermal bond-breaking processes, respectively. A very simple model of coal pyrolysis suggests that more bonds are broken than are detected by e.s.r., in agreement with the estimated lifetimes of radicals expected to be formed on pyrolysis. These calculations, and the stability of spin concentration measurements over long time periods, cast doubt on efforts to establish a direct relationship between e.s.r. spin counts and the pyrolytic reactivity of the coal.     

Electron spin resonance measurements of Cu++ distribution in cation exchange membranes

Electron spin measurement of the local Cu⁺⁺ concentrations have been performed in different cation exchange membranes by comparison with reference frozen aqueous solutions of Cu⁺⁺ salts. Comparison of the obtained values with the average chemical concentrations has therefore permitted direct evidence of microphase separation in these systems. Local concentrations approximately four times larger than average chemical concentrations have been found. Information about composition of this ion-containing phase has also been obtained from changes in local Cu⁺⁺ concentrations versus water content.