Assessment of the Antioxidant Capacity of Standard Compounds and Fruit Juices by a Newly Developed Electrochemical Method: Comparative Study with Results from Other Analytical Methods

The antioxidant capacity (AOC) of some standard compounds and commercial beverages was determined using an electroanalytical method which measures the OH radical scavenging ability (OH‐RSC). It is based on the degradation of a thin polyphenol film electrodeposited on glassy carbon following the attack by OH radicals produced by hydrogen peroxide photolysis. The degradation is prevented in presence of increasing concentrations of antioxidants. The same samples were also analyzed by DPPH, ABTS, and ORAC assays. The OH‐RSC method provided AOC values highly correlated with those resulting from the application of ORAC assay for both juices (R=0.90) and standard antioxidants (R=0.96).     

Protein Protection by Antioxidants: Development of a Convenient Assay and Structure‐Activity Relationships of Natural Polyphenols

In this work, a convenient test of antioxidant activity was developed, with BChE‐contaminated HSA as the target of AAPH‐induced oxidation and its esterase activity as the marker of protein integrity or degradation. The method is relatively simple, of low cost, and convenient to use. Its application to natural polyphenols showed that quercetin ( 1 ), verbascoside ( 2 ), chlorogenic acid ( 3 ), caffeic acid ( 4 ), 1,3,6,7‐tetrahydroxyxanthone ( 5 ), and mangiferin ( 6 ), are good antioxidants (IC₅₀<9 μM ). 1,5‐Dihydroxy‐3‐methoxyxanthone ( 7 ), flemichin D ( 8 ), and cordigone ( 9 ) showed modest activities (ca. 50 μM <IC₅₀<350 μM ), whereas danthrone ( 10 ) was inactive. Complementary experiments with two of the more active antioxidants, namely quercetin ( 1 ) and chlorogenic acid ( 3 ) showed that both antioxidants were better radical scavengers than chain‐breaking antioxidants. The relative adiabatic oxidation potential (ΔH_ox), the relative H‐bond dissociation energy (ΔH_abs), and the first oxidation potential measured by cyclic voltammetry were found to be related to the radical‐scavenging activity of these antioxidants.     

Evaluation of long‐term performance of antioxidants using prooxidants instead of thermal acceleration

Evaluation of stabilization efficiencies of different antioxidants in polymers at low temperature and relatively short time was performed using incorporation of a prooxidant for catalytic oxidation. Comparisons were made between polypropylene films stabilized with primary antioxidants (Irganox 1076, Irganox 1010, and α‐tocopherol), with or without the prooxidant manganese stearate at different temperatures. A faster degradation was obtained in the presence of a prooxidant than without it. The relative efficiency of the antioxidants at prooxidant acceleration correlated better to low temperature long‐term test than at the thermal acceleration. The results were affected by initial differences in the amounts of the antioxidants present after the processing of the films. These differences were corrected for by a recalculation using microwave‐assisted extraction (MAE) and high performance liquid chromatography analysis from the exponential decrease in the amount of antioxidant in the films. The fastest comparison of the antioxidants efficiency was obtained from oxidation induction times, using total luminescence intensity measurements, but reliable results could also be obtained from the time to apparent failure. © 2005 Wiley Periodicals,Inc. J Polym Sci Part A: Polym Chem 43: 4537–4546, 2005     

Thermo‐ and photo‐oxidative stability and improved processability of polyamide stabilized with a new functional additive

During processing polyamides can undergo thermo‐mechanical degradation due to the mechanical stress and to the high temperature applied to the melt. Degradation can occur also during the polyamides lifetime due to many driving forces, like temperature, ultraviolet radiations, etc. The mechanisms of degradation are not well understood and the stabilization with usual stabilizers, like phenol antioxidants, is not so efficient as for other classes of polymers. The aim of this work is the study of the effects of the addition of a new multifunctional additive on the processing and on the thermo‐ and photo‐stabilization of polyamide‐6. This additive is able to prevent the thermo‐mechanical degradation and also to enlarge the molecular weight distribution improving the processability of this polymer in operations where the elongational flow is involved. Moreover, the presence of this additive extends significantly the lifetime of the polyamide when subjected to thermo‐ and photo‐oxidative stress. Copyright © 2005 John Wiley & Sons, Ltd.     

Combined chromatographic strategy based on macroporous resin,high‐speed counter‐current chromatography and preparative HPLC for systematic separation of seven antioxidants from the fruit of Terminalia billerica

In the present study, combined chromatographic strategy based on macroporous resin, high‐speed counter‐current chromatography and preparative high‐performance liquid chromatography for systematic separation of antioxidants from crude samples guided by high‐performance liquid chromatography with 1,1‐diphenyl‐2‐picrylhydrazyl has been successfully established. Based on this strategy, seven antioxidants including isorugosin A, _β‐1,2,3,6‐tetragalloyl‐D ‐glucose, chebulinic acid, 1,2,3,4,6‐penta‐O‐galloyl‐_β‐D‐glucose, chebulagic acid, ethyl gallate, and gallic acid were obtained from the fruit of Terminalia billerica. First, high‐performance liquid chromatography with 1,1‐diphenyl‐2‐picrylhydrazyl experiment showed the presence of seven main antioxidants in the crude extract of the fruit of Terminalia billerica. Then, a macroporous resin column chromatography method was developed for the enrichment of these seven antioxidants. Finally, an efficient method based on high‐speed counter‐current chromatography and preparative high‐performance liquid chromatography was developed for the separation of these antioxidants. In the selection of solvent systems, it was found that acetic acid could be a good regulator for modifying the partition coefficient values of tannins. The present study provides a reference for systematic separation of antioxidants from crude samples. Considering the general existence of antioxidants in crude samples, this combined chromatographic strategy might lead to broader application prospects.     

An Experimental and Mechanistic Investigation of the Complexities Arising During Inhibition of the Briggs?Rauscher Reaction by Antioxidants

A method to determine the relative antioxidant capacity of radical scavengers based on the inhibition of the oscillations of the Briggs? Rauscher (BR) oscillating reaction was previously reported. A semiquantitative mechanistic interpretation of the inhibitory effects required two steps to obtain simulated inhibition times in very good agreement with the experimental ones. The first step is inhibitory, involving H‐atom transfer from antioxidant to the HOO^. radical; the second step is a first‐order degradation of the antioxidant to unspecified products. Since the degradation may be due to oxidation and/or iodination of the antioxidant, we studied the kinetics of the subsystems IO (H⁺)+antioxidant and I₂(H⁺)+antioxidant. We used 2,5‐ and 2,6‐dihydroxybenzoic acids, caffeic acid (=3‐(3,4‐dihydroxyphenyl)prop‐2‐enoic acid), ferulic acid (=3‐(4‐hydroxy‐3‐methoxyphenyl)prop‐2‐enoic acid), pyrocatechol (=benzene‐1,2‐diol), and hydroquinone (=benzene‐1,4‐diol) as antioxidants. Spectra in the wavelength range 500–250 nm were repeated at given time intervals to follow the peaks of the iodine and oxidation products, which were mainly quinones. For the iodination of the above diphenols (=benzenediol derivatives) the substitution and/or addition reactions with I₂ or HOI were found to be relatively slow compared to oxidation by IO . Approximate rate constants for oxidation were obtained on the basis of a reasonable kinetic model by using a suitable numerical integration program. Although these complexities can arise also in the completely inhibited BR oscillator, we believe that the inhibitory effects are due to the HOO^. scavenging action by diphenols or by quinones since HOO^. radicals are also potential reducing agents. We propose two steps that could maintain a small reservoir of diphenol, while both quinone and diphenol deplete HOO^. radicals. In short, the complexities do not affect the method for monitoring the relative activity of antioxidants based on the BR oscillating reaction. The effects of temperature on the inverse of the oscillatory time in the BR‐uninhibited system, on the inverse of inhibition times, and on the time length of the resumed oscillations for four antioxidants were also investigated. Apparent average activation energies were obtained.     

New oligomeric thioether antioxidants for polymers

Polyfunctional oligomeric antioxidants with a sterically hindered phenolic fragment were prepared. The influence of these antioxidants on oxidation and degradation of SKI-3 rubber was analyzed.     

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low‐energy collision‐induced dissociation (LE‐CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three‐dimensional ion trap (3D‐IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive‐ion electrospray ionization (ESI) and LE‐CID. Various types of molecular ions (e.g. [M]^+?, [M + H]⁺, [M + NH₄]⁺ or [M + Na]⁺) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative‐ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE‐CID behavior: (1) they formed stable radical product ions and (2) C? C bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE‐CID. Comparing the two instruments, the main benefit of applying the LIT‐Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D‐IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT‐Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrospray ionization and atmospheric pressure matrix‐assisted laser desorption/ionization mass spectrometry of antioxidants applied in lubricants

The aim of this study was to investigate the utility of ion trap mass spectrometry (ITMS) in combination with the two desorption/ionization methods, electrospray (ESI) and atmospheric pressure matrix‐assisted laser desorption/ionization (AP‐MALDI), for the detection of antioxidants which are applied in lubricants. These experiments should form the base for future investigations of antioxidants in tribologically formed thin layers on the surface of frictional systems. Seventeen different antioxidants were selected out of the group of hindered phenolic and aromatic aminic compounds. Practically all antioxidants could be characterized by positive ion ESI‐ and AP‐MALDI‐ITMS, forming various types/species of molecular ions (e.g. [M]^{+ .} , [M+H]⁺, [M+Na]⁺ or [M–2H+H]⁺). A few compounds could be analyzed by negative ion ESI‐MS, too, but none by negative ion AP‐MALDI‐MS. The influence of target materials in AP‐MALDI‐MS (gold‐ and titanium nitride (TiN)‐covered stainless steel, micro‐diamond‐covered hard metal, hand‐polished and sand‐blasted stainless steel targets) with respect to the molecular ion intensity and type of molecular ion of two selected antioxidants was evaluated. The surface properties are of particular interest because in friction tests different materials with different surface characteristics are used. However, the MS results indicate that optimal target surfaces have to be found for individual antioxidants in AP‐MALDI‐MS but in general smooth surfaces were superior to rough surfaces. Finally the gold‐covered stainless steel MALDI target provided the best mass spectra and was selected for all the antioxidants investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

Structure‐property relationships: phenolic antioxidants with high efficacy and low color contribution

Antioxidants can be divided into two broad classes, primary and secondary, depending upon their mode of action. The most broadly used primary antioxidants are hindered phenols. Phenolic antioxidants have traditionally been based on 2,6‐di‐t‐butyl‐4‐methylphenol (BHT). This functional moiety has been incorporated into larger molecules affording products with lower volatility or greater polymer compatibility. More recently, molecules have been introduced which vary in steric hindrance about the phenol and also the way in which multiple phenol functional groups are linked to form larger molecules. This has led to structures which have lower color contribution. Surprisingly, in some cases these molecules have shown higher levels of efficacy relative to other antioxidants with similar phenol/molecular weight ratios. This has included enhanced levels of synergism with secondary antioxidants. An attempt is made to correlate structural features of these molecules to the enhanced performance and/or lower color.     

Rapid screening and identification of antioxidants in the leaves of Malus hupehensis using off‐line two‐dimensional HPLC–UV–MS/MS coupled with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay

The leaves of Malus hupehensis have a strong antioxidant activity and are commonly consumed as a healthy tea. However, detailed information about its antioxidants is incomplete. Herein, we developed an effective strategy based on combining off‐line two‐dimensional high‐performance liquid chromatography with ultraviolet and tandem mass spectrometry detection with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay to rapidly screen and identify the antioxidants from the leaves of M. hupehensis. In the orthogonal two‐dimensional liquid chromatography system, a Venusil HILIC column was used for the first dimension, while a Universil XB‐C₁₈ column was installed in the second dimension. As a result, 32 antioxidants, including ten dihydrochalcones, two flavanones, nine flavonols, four flavones, and seven phenolic acids were tentatively identified, out of which 23 compounds, as far as we know, were isolated and characterized from the leaves of M. hupehensis for the first time. To the best of our knowledge, this is the first systematic investigation of the antioxidants from the leaves of M. hupehensis. The results indicated that the proposed method is an efficient technique to rapidly investigate antioxidants, especially for coeluted and minor compounds in a complex system.     

Evaluation of the 3-hydroxy pyridine antioxidant effect on the thermal-oxidative degradation of HTPB

The effect of the mixture of two antioxidants has been evaluated on the thermal-oxidant degradation of the hydroxyl-terminated polybutadiene (HTPB) because of its importance in the coatings and adhesives industries. 2,2-Methylene bis(4-methyl-6-tertiarybutylphenol) or A.O.2246 and 3-hydroxy pyridine have been considered as antioxidants in this study as a common HTPB antioxidant and an active antioxidant, respectively. The thermal-oxidant degradation behavior of the HTPB has been investigated in the presence of a mixture of two antioxidants by TGA and DTG tests, and, subsequently, the results of these tests have been interpreted by two model-free methods, e.g., Kissinger–Akahira–Sunose and Friedman methods. The results revealed that the mixture of two antioxidants affected the activation energy of the thermal-oxidant degradation reaction of the HTPB. The calculated activation energy value obtained from the Kissinger–Akahira–Sunose method was about 199 ± 1 kJ⋅mol⁻¹. In addition, the E_a value at various conversion rates has also been calculated by using the Friedman method. This method showed that the highest E_a value in the thermal-oxidant degradation reaction belonged to the initiation step of the reaction (about 299 kJ⋅mol⁻¹). Moreover, the lowest activation energy value was correlated to the second step of the degradation reaction at a conversion rate of 0.6 (about 184 kJ⋅mol⁻¹).     

An Electrochemical Method for Sensitive Determination of Antioxidant Capacity

A simple and sensitive electrochemical method for the determination of antioxidant capacity has been studied. 4‐hydroxybenzoic acid (4‐HBA) was used as a trapping agent for photogenerated ^.OH radicals, leading to 3,4‐dihydroxybenzoic acid (3,4‐DHBA). Square‐wave voltammetry was used to quantify the amount of 3,4‐DHBA formed from the reaction between 4‐HBA and ^.OH. Addition of antioxidants induced the competition between 4‐HBA and antioxidants toward ^.OH elimination, resulting in a decrease of the measured current. The IC₅₀ for different standard antioxidants was calculated and expressed in Trolox equivalent antioxidant capacity (TEAC). The proposed method was successfully compared to a fluorimetric one.     

Extraction of polypropylene additives and their analysis by HPLC

Summary The polypropylene additives were extracted by dissolution-precipitation and Soxhlet. The Soxhlet method was adapted for the extraction of phosphorous antioxidants. The RP HPLC method with quaternary gradient elution separated five chemical groups of additives: lower molecular mass di-tert-butyl phenol (D.T.B.P.), hindered amine light stabilizers (Tinuvin 326), hindered phenolic antioxidants (Irganox 1010) and phosphorous antioxidants (Irgafos 168 and Ultranox 626) with their degradation products.     

Selection of Pharmaceutical Antioxidants by Hydrodynamic Voltammetry

Antioxidant compounds are routinely included in pharmaceutical formulations in order to minimize the oxidative degradation of the active pharmaceutical ingredient(s) (API). The effectiveness of the antioxidants being considered is often assessed by creating several formulations with the API(s) and single or multiple antioxidants in order to place each of these formulations on stability under accelerated conditions. This trial and error selection process is costly and time consuming, often taking weeks before the oxidation is observed at a detectable level. Using a flow injection coulometric detector based system, a potential tool has been developed to guide initial antioxidant selection for hindering the oxidative degradation for a given drug substance in its formulation environment. The relative potential for the API to be oxidized can be measured by matching the conditions of the coulometric experiment to the formulation matrix. Using this selection criterion, the choice of antioxidants to study for individual formations can be greatly narrowed, thus leading to a significant savings in development time and cost.     

Quantitative characterization of degradation behaviors of antioxidants stabilized in lipid particles

This study describes a flexible approach that allows us to characterize the long-term stability of antioxidants by using a thermodynamically extended Arrhenius equation. We use retinol, Vitamin A, as a model antioxidant and its degradation behaviors are characterized for both stabilized and non-stabilized systems; in this study, by using a fluid bed technique, we immobilize the retinol in lipid particles, thus increasing its thermal stability in complex formulations, such as aqueous polymer gels and emulsions. Our approach demonstrates that the degradation behaviors of the retinol show a functional relationship with temperature and time, which makes it possible to use the Arrhenius approach. This result allows us to precisely characterize the stability of antioxidants in complex formulations for long time.     

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

Hindered amine light stabilisers (HALS) are the most effective antioxidants currently available for polymer systems in post‐production, in‐service applications, yet the mechanism of their action is still not fully understood. Structural characterisation of HALS in polymer matrices, particularly the identification of structural modifications brought about by oxidative conditions, is critical to aid mechanistic understanding of the prophylactic effects of these molecules. In this work, electrospray ionisation tandem mass spectrometry (ESI‐MS/MS) was applied to the analysis of a suite of commercially available 2,2,6,6‐tetramethylpiperidine‐based HALS. Fragmentation mechanisms for the [M + H]⁺ ions are proposed, which provide a rationale for the product ions observed in the MS/MS and MS³ mass spectra of N‐H, N‐CH₃, N‐C(O)CH₃ and N‐OR containing HALS (where R is an alkyl substituent). A common product ion at m/z 123 was identified for the group of antioxidants containing N‐H, N‐CH₃ or N‐C(O)CH₃ functionality, and this product ion was employed in precursor ion scans on a triple quadrupole mass spectrometer to identify the HALS species present in a crude extract from of a polyester‐based coil coating. Using MS/MS, two degradation products were unambiguously identified. This technique provides a simple and selective approach to monitoring HALS structures within complex matrices. Copyright © 2010 John Wiley & Sons, Ltd.     

Effectiveness of antioxidants: Suppression of evolution of gaseous degradation products from low-density polyethylene during thermo-oxidation

The present investigation is part of a project concerning improvements of the working environment for polymer manufacturing processes at high temperatures; for example, welding and extrusion coating.The evolution of gaseous degradation products from unstabilized and antioxidant stabilized low-density polyethylene during thermo-oxidation in air at 210°C has been studied by a simple technique using a modified gas chromatograph. Of the thirteen antioxidants investigated (phenols, phosphites, phosphonites, sulphides and metal dithiocarbamates), only phenols (radical scavengers) effectively suppress the formation of gaseous degradation products from the polymer during an induction period.The gaseous degradation products were analysed with combined gas chromatography-mass spectrometry (GC-MS), using direct trapping on the GC column (Porapak® QS). Apart from water and carbon dioxide, the major products are formaldehyde, acetaldehyde, formic and acetic acids. The addition of effective antioxidants strongly reduces the absolute amounts of the degradation products during the induction period, but does not change their relative quantities.     

The effects of gamma irradiation on the fate of hindered phenol antioxidants in food contact polymers. Analytical and 14C-labelling studies

The extractable levels of two hindered phenol antioxidants (Irganox 1076 and Irganox 1010) present in PVC, polyethylene and polypropylene, have been monitored by HPLC techniques following progressive exposure of the polymers to ⁶⁰Co γ radiation. There is a gradual diminution in the extractable levels of each antioxidant as irradiation progresses as a result of transformation of the antioxidants in the oxidation reactions ensuing during irradiation. Experiments involving the use of a ¹⁴C-labelled sample of Irganox 1076 in polyolefins have provided evidence of covalent binding of antioxidant degradation products to the polymer following gamma irradiation. However, there is also evidence of the formation of extractable degradation products, the identity of which is as yet unknown.     

Effect of the antioxidants on the stability of poly(vinyl butyral) and kinetic analysis

The effect of several kinds of antioxidants on the stability of poly(vinyl butyral) (PVB) under air atmosphere is studied by thermogravimetry–differential scanning calorimetry method and kinetic analysis. After mixed with antioxidants, the thermal oxidative stability of PVB increases significantly, because the antioxidants could inhibit the oxidation of copolymer (stage I). The thermal oxidative stability increases in the following order: PVB < PVB/1010 < PVB/B215 < PVB/1098. However, the thermal oxidative degradation rate of PVB increases markedly after 320 °C, due to the loss of chemical activity for antioxidants gradually. The thermal stability of antioxidants increases in the following order: B215 < 1010 < 1098.