Temperature-programmed Mode Research Articles

Large-scale statistical study of the dependence of retention index on heating rate in temperature-programmed gas chromatography

Retention indices are values that characterize the retention of a compound in gas chromatography. In practice, retention indices are often assumed to depend only on the structure of the molecule and the type of the stationary phase, but this approximation is incorrect. This study is devoted to studying the dependence of retention indices on the column heating rate in the linear temperature programming mode, using a large and diverse data set. In the NIST 20 database, most data records are recorded in this mode. For stationary phases based on poly(5%-diphenyl-95%-dimethyl)siloxane (5%-phenyl-PDMS), there is a high proportion of records with heating rates of 10-15 K/min. In practice, such a high heating rate is rarely used and the use of such data may cause errors. A search was made for groups of records that were taken from the same primary source, recorded for the same compound and the same stationary phase, but differing in a heating rate. For each of these groups, the value D, the angular coefficient (slope) of the dependence of the retention index on the heating rate, was calculated. This value can take both positive and negative values. The highest values and the greatest variation of D values are observed for polar stationary phases, but further consideration was performed for 5%-phenyl-PDMS due to its greater practical significance. For these stationary phases, the highest D values are observed for aromatic and polyaromatic molecules; oxygen-containing compounds, on the contrary, exhibit lower D values. Negative D values are observed for many trimethylsilyl derivatives. A data set of D values for 756 molecules was selected and published online. There is almost no correlation between D and the retention index, lipophilicity factor logP, and molecular weight. Significant correlations with the number of cycles, the number of rotatable bonds, and the number of aromatic atoms were observed. Linear equations quantitatively relating the molecular descriptors to the D value were constructed. A number of cycles and halogen atoms were shown to contribute positively to the D value, while a number of oxygen atoms and bonds subject to internal rotation contributed negatively. The strong influence of the values related to the conformational rigidity of molecules and the weak influence of polarity allow us to suppose that the entropic factor has a key influence on the D value. A simple empirical linear equation for estimating the value of D is derived and presented in this study. Several machine learning methods for predicting D are compared. The best results are shown by gradient boosting and a random forest. However, the random forest does not achieve high accuracy in predicting the retention indices themselves.

Statistical Evaluation of the Standard Deviation of Chromatographic Retention Time in the Capillary Column Temperature Programming Mode

In the qualitative analysis of complex mixtures of unknown composition, a promising direction for increasing the reliability of the identification of substances is the joint use of several simultaneously determined analytical parameters. A combination of chromatographic methods with FTIR spectroscopy, mass spectrometry, or other methods is used. In solving complex analytical problems of industrial nature, it is economically expedient to use only chromatographic methods. In analyzing complex multicomponent mixtures with a high density of peaks in the chromatogram, the precision of the determination of retention time determines the reliability of the identification of substances and of the result of analysis, using identification both directly by the absolute retention time and by relative parameters derived from it (relative retention times, retention indices). The standard deviation of a measurement of the chromatographic retention time is a measure of precision. Experimental chromatographic retention times were obtained for analyzing a mixture by the following scheme: initial isotherm, temperature programming, final isotherm. A sharp increase (by one order of magnitude) in the numerical value of the standard deviation was noted at the step of temperature programming. A regression equation was obtained linking the point estimate of the standard deviation of the retention time and column temperature. The performance characteristic of the regression equation is obtained as a function that enables calculating the total standard uncertainty in the estimate of the standard deviation of the chromatographic retention time. The results can be used to refine chromatographic retention time and indices that ensure the reliable identification of substances.

SILICA GEL MODIFICATION BY NICKEL(II) COMPLEXES WITH NITROGEN- AND OXYGEN-CONTAINING ORGANIC LIGANDS FOR GAS CHROMATOGRAPHY

Gas chromatographic sorbents based on Siloсhrome S-80 adsorptively modified with 8-hydroxyquinolinate, 1-phenylazo-2-naphtholate and 2-nitroso-1-naphtholate of nickel(II) have been obtained. It has been established that the thermal stability (up to 170–350 °C) of chelate-containing sorbents is sufficient for carrying out gas chromatographic separations. The IR and Raman spectra of the obtained materials confirmed the success of the complexation processes in the synthesis of complex nickel(II) compounds and the retention of coordination bonds after the deposition of chelates on silica gel. The characteristics of the surface and the porous structure of the prepared sorbents are investigated. As a result of adsorptive modification of Silochrome S-80 with nickel(II) chelates, it’s specific surface area (84 m2/g) decreases by 12–32 m2/g along with a decrease in the average pore size, which indicates the fixation of modifiers at the edges of large pores. It was shown that modification of Silochrome S-80 with 8-hydroxyquinolinate, 1-phenylazo-2-naphtholate, 2-nitroso-1-naphtholate of nickel(II) has a significant effect on the chromatographic polarity and selectivity of sorbents. The presence of a polar nitroso-groups in the composition of nickel(II) 2-nitroso-1-naphtholate makes the metal center more accessible for donor-acceptor interactions with sorbates (pyridine, carbonyl compounds). Also, the nitroso-group is active in the formation of hydrogen bonds with alcohols, which complicates their elution at column temperatures below 170 °C. The greatest affinity to the homologous of arenes is shown by silochrome modified by a nickel(II) complex with the least bulky 8-hydroxyquinolinate ligands, which leads to an increase in the retention times of aromatic and polyaromatic compounds along with an increase of the sorbent selectivity. The possibility of practical application of a chromatographic column filled with Silochrome S-80 with deposited nickel(II) 8-hydroxyquinolinate for gas chromatographic (FID) separation of mixtures of polyaromatic compounds in the temperature programming mode is shown.

Synthesis and determination of analytical characteristics and differentiation of positional isomers in the series of N-(2-methoxybenzyl)-2-(dimethoxyphenyl)ethanamine using chromatography-mass spectrometry.

N-(2-Methoxybenzyl)-2,5-dimethoxyphenethylamines (NBOMes) are synthetic phenethylamine derivatives emerging on the global drug market and reported to be associated with untoward effects in people who use drugs. Its action involves agonism at serotonin 5-HT2A receptors, affecting cognitive and behavioral processes. However, certain isomers of NBOMes may not show any psychoactive effects. They are not controlled by legislation and can be tested as pharmaceutical drugs. This study deals with the differentiation among positional isomers of 25H-NBOMe differing in the position of the two methoxy groups in the phenylethyl moiety of the molecule, using chromatography-mass spectrometry methods. The gas chromatography analysis showed that the isothermal mode was more efficient than the usually applied temperature-programming mode for the separation of the mentioned isomers. Electron ionization mass spectra of 25H-NBOMe isomers were highly similar, often resulting in a high probability of erroneous identification. However, mass spectra of their trifluoroacetyl or pentafluoropropanoyl derivatives were easily identified as they contained fragments with many significant differences. The proposed analysis using liquid chromatography-tandem mass spectrometry could distinguish the isomers of 25H-NBOMe without the need for any derivatization.

Experience with the application of gas chromatography-mass spectrometry in combination with solid-phase microextraction for the study of the composition of volatile organic compounds in matrices with the high fat content

Current article presents a methodological approach for a comparative study of the composition of volatile organic compounds ( VOCs ) in a natural product and a nature-identical flavoring substance. The identification of VOCs was performed by gas chromatography-mass spectrometry without reference compounds. The identification was made based on coincidence of the experimental and reference mass spectra and linear retention indices. During the first step, the closest match factor substance was selected based on the library search. During the second step, the result was verified by the means of experimental and reference retention indices data proximity. The reference retention indices were selected considering the identity of the experimental conditions (chromatographic stationary phase, temperature programming mode). The impossibility of identification of aroma components in acetonitrile extract of fragrance was shown due to the overlapping of their peaks with peaks of di- and tri- glycerides of fatty acids. The extraction and concentration of VOCs from vapor were carried out using the head space solid-phase microextraction. The fragrance content investigation revealed 39 volatile organic compounds including propylene glycol and triacetin as the markers of synthetic fragrances. In natural cocoa butter, also 39 volatile organic compounds were identified. All of them were common for the fermented cocoa beans. Pyrazines derivatives, characteristic of cocoa aroma, were presented with the following 7 components: 2,3-dimethylpyrazine; 2,5-dimethylpyrazine; ethylpyrazine; 2-methyl-6-ethylpyrazine; 2,3,5-trimethylpyrazine, 2,5-dimethyl-3-ethylpyrazine; tetramethylpyrazine. In quantitative terms, the isomeric 2,3-butandiols predominate in the head space of natural cocoa butter accounting for 37 percent of total VOC content. The analytical technique used allows investigation of multicomponent objects including the heterogeneous ones. Key words: gas chromatography-mass spectrometry, head space solid-phase microextraction, mass spectra, retention indices, fragrance DOI: http://dx.doi.org/10.15826/analitika.2020.24.1.003 E. Savelieva E. Kessenikh and L. Gustyleva Research Institute of Hygiene, Occupational Pathology and Human Ecology, g/p Kiz’molovskii, Vsevolozhskii raion, Leningradskaia oblast’, 188663, Russian Federation

Oxygen Mobility in the Materials for Solid Oxide Fuel Cells and Catalytic Membranes (Review)

Oxygen transport (including oxygen mobility and surface reactivity) is one of the important factors governing electrochemical activity of solid oxide fuel cells electrodes as well as oxygen and hydrogen separation membranes based on materials with mixed oxide-ionic and electronic conductivity. In this work, oxygen mobility data obtained for a series of materials destined for such devices using modern techniques of oxygen isotope heteroexchange are summarized. Series of solid oxide fuel cells’ and membranes’ materials were studied by isotope exchange of their oxygen with 18O2 and C18O2 in isothermal and temperature-programmed modes using closed and flow reactors and data analysis based on developed model of oxygen diffusion and exchange. For solid electrolytes’ materials (Sc- and Ce-doped zirconia) as well as for proton-conducting materials [Ln5.5(Mo,W)O11.25], the effect of composition heterogeneity on the oxygen mobility was demonstrated. For Ln6 – xWO12 – δ, a strong effect of structure on the oxygen mobility was demonstrated. For oxides with asymmetric structure, where oxygen migration proceeds via cooperative mechanisms [La2(Mo,W)2O9, (Ln,Ca)2NiO4], the doping hampers the cooperative migration, resulting in oxygen mobility deterioration and sometimes forming additional slow diffusion channels. In the PrNi0.5Co0.5O3–Ce0.9Y0.1O2 nanocomposites that are materials of the solid oxide fuel cells’ cathode and functional layer of the oxygen separation membranes, two diffusion channels were observed, where more mobile oxygen corresponds to the fluorite phase and interfaces; less mobile, to the perovskite phase. This is due to special features of cations redistribution between the phases.

Structural and transport properties of doped bismuth titanates and niobates

Cobalt- or Zn-doped bismuth titanates and Mg+Cu-doped bismuth niobate were synthesized by the method of organic-inorganic precursors combustion. Distribution of dopants over the sites of the pyrochlore structure was elucidated by X-ray powder diffraction structure refinement and pycnometric density analysis. Zinc and cobalt cations are mainly located in Bi sites appearing also in Ti sites at their high content. Mg cations are located only in Nb sites, while Cu cations are equally distributed between Bi and Nb sites. Magnetic susceptibility data confirm Сo2+ state of cobalt atoms revealed by NEXAFS. Antiferromagnetic exchange appears when Сo atoms enter the titanium sites. Electronic conductivity of p-type dominates for Co-doped samples in the 160–750°C temperature range. The oxygen mobility and surface reactivity were studied by the oxygen isotope heteroexchange with C18O2 in isothermal and temperature-programmed modes. Mobility of the lattice oxygen can be described by a homogeneous model. The oxygen mobility and surface reactivity are comparable for doped bismuth titanates and niobates correlating with conductivity and being the highest for Co and Cu-doped samples with disordered distribution of dopants between lattice sites. Classification codesA6630LA7280GA8120EA8170JB2130B2520E

Retention modeling and retention time prediction in gas chromatography and flow-modulation comprehensive two-dimensional gas chromatography: The contribution of pressure on solute partition

This study aims at modelling and predicting solute retention in capillary Gas Chromatography (GC) and Flow Modulation comprehensive GC (FM-GCxGC). A new thermodynamic model, taking into account the effects of temperature and pressure, is proposed to describe the variation of the equilibrium partition constant of a solute during its elution. This retention model was challenged with the classical one, and both were applied to: (i) stationary phase film thickness indirect estimation; (ii) retention time (RT) prediction of a set of 11 model polycyclic aromatic hydrocarbons (PAHs) on the SLB-IL60 and DB-35MS columns, in temperature-programmed mode. Film thickness determination led to values about 2 times lower than those indicated by column nominal dimensions, whatever the employed model. Prediction of retention times in GC led to 0.84 and 0.26% mean errors using the classical and the extended models, respectively. Prediction in GCxGC gave 5.5 and 0.44% mean errors in 1st dimension RTs, and 7.3 and 2.2% mean errors in 2nd dimension RTs, using the classical and the extended models, respectively.

Headspace gas-chromatographic determination of formaldehyde in urine

A method is proposed for determining formaldehyde by headspace gas chromatography using an o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) derivatizing agent. Formaldehyde in urine was derivatized to o-(pentafluorobenzyl)oxime and extracted by heating an urine sample with PFBHA in a sealed vial. Gas-chromatographic analysis of the headspace was performed in the temperature-programmed mode using an HP-5 capillary column with a flame ionization detector. The limit of detection is 3.5 μg/L. The accuracy of the procedure, equivalent to an extended relative uncertainty, does not exceed 21%. The examination of the procedure using urine samples of the population of the Irkutsk region has shown that the regional reference levels of formaldehyde concentration are in the range 44–83 μg/L.

Influence of Thermal Treatment of HUSY on Catalytic Pyrolysis of Polypropylene: An Online Photoionization Mass Spectrometric Study

Refuse-derived fuels or chemicals have attracted much attention because of their ecological and economical utilization of municipal solid waste. However, the related conversions involve complex chemical reactions that prevent the use of conventional analytical methods for characterization of the detailed profiles. In this study, the influence of the thermal pretreatment of HUSY on the catalytic pyrolysis of polypropylene (PP) was investigated using online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOFMS). The experiments were carried out in two modes: (1) In temperature-fixed mode, the mass spectra of the pyrolyzates of PP and PP/HUSY at different temperatures were obtained in real time. Moreover, the selectivity of the pyrolyzates and conversion of PP over HUSY zeolites calcined at different temperatures were studied. (2) In temperature-programmed mode, the formation temperatures of the PP pyrolysis products in the presence of HUSY were determined. HUSY zeolites that are pretreated...

Dynamics of topochemical reactions in supported iron Fischer–Tropsch synthesis catalysts during their reduction in flowing CO and CO + H2

The reduction of Fe2O3/SiO2 catalysts in flowing CO and CO + H2 in the temperature-programmed and isothermal modes has been investigated by in situ magnetization measurements. The process takes place in two steps, successively yielding magnetite and Hagg carbide. The carbide concentration in the case of reduction with CO is higher than in the case of reduction with the CO + H2. This is assumed to be due to the formation of Fe2SiO4 in the reduction of the catalysts with CO + H2.

Oxygen mobility and surface reactivity of PrNi1 − xCoxO3 − δperovskites and their nanocomposites with Ce0.9Y0.1O2 − δ by temperature-programmed isotope exchange experiments

Cobalt-doped praseodimium nickelates PrNi1−xCoxO3−δ and Y/(Pr, Y)-doped ceria oxides were synthesized via Pechini route. Nanocomposites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite nanopowders in isopropanol with addition of polyvinyl butyral followed by drying, pressing and sintering at 1000–1100°C. The oxygen mobility and reactivity of perovskites, fluorites and powdered composites obtained by crushing and milling of pellets were estimated by oxygen isotope heteroexchange with 18O2 and C18O2 using both closed and flow (SSITKA) reactors in the temperature-programmed (TPIE) mode. Co-existence of two routes of oxygen bulk diffusion—the fast one and the slow one in perovskites and composites was revealed. In perovskites, the fast oxygen diffusion is a minor channel related to some defects in their structure. In composites, the fast oxygen diffusion is the main route being related to both phases disordered due to cation redistribution between them, first of all, to fluorite-like domains of Pr,Y-doped ceria and perovskite/fluorite interfaces.

Kinetic Investigation of Thermal Decomposition Reaction of Ethane-1,2-Diamine Copper(II) Chloride, C<sub>2</sub>H<sub>8</sub>N<sub>2</sub>CuCl<sub>2</sub>

:The thermal behavior and thermal decomposition kinetic parameters of ethane-1,2-diamine copper (II) chloride (EDCC) in a temperature-programmed mode have been investigated by mean of TG-DTG. There are four stages in the thermal decomposition process. The kinetic parameters of the apparent activation energyEa(130.2, 143.6 and 158.9 KJ·mol–1) and per-exponential factorA(1011.80, 1012.18and 1011.83s–1) in II, III and IV stages were obtained from analysis of the TG-DTG curves by Kissinger method. The values ofEaindicated that the difficulty coefficient of pyrolysis in II, III and IV stages was increased in the order: II < III < IV. The values of the entropy of activation ΔS≠, enthalpy of activation ΔH≠and free energy of activation ΔG≠of the reaction were estimated.

Oxygen mobility and surface reactivity of PrNi1−xCoxO3+δ–Ce0.9Y0.1O2−δ cathode nanocomposites

Cobalt-doped praseodymium nickelate PrNi1 − xCoxO3 − δ (PNCx) and Y-doped ceria Ce0.9Y0.1O2 − δ (YDC) oxides were synthesized via Pechini route. PNCx + YDC composites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite nanopowders in isopropanol with addition of polyvinyl butyral followed by drying, pressing and sintering at 1300 °C. The oxygen mobility and reactivity of powdered PNCx and composites obtained by crushing and milling of dense pellets were estimated by O2-TPD and oxygen isotope exchange with 18O2 and C18O2 using both static and flow (SSITKA) reactors in isothermal and temperature-programmed (TPIE) modes. For PNCx samples sintered at 1300 °C comprised of (Ni,Co)O and Ruddlesden–Popper type phases (Pr2NiO4, Pr4(Ni,Co)3O10), the oxygen mobility and reactivity tend to decrease with Co content. For composites, the oxygen mobility is much higher due to Pr transfer into YDC thus disordering perovskite-like and fluorite-like phases. TPIE C18O2 SSITKA experiments combined with SIMS analysis of the depth profiles of Pr18O and Ce18O suggest that fast oxygen diffusion in composites is provided by domains of disordered perovskite-like phases as well as Pr,Y-doped ceria. For best composites, the value of the oxygen chemical diffusion coefficient estimated by the weight relaxation technique exceeds that of well known LSFC–GDC composite.

Kinetic Investigation of Thermal Decomposition Reactions of 4'-Demethypodophyllotoxin and Podophyllotoxin

The thermal behavior and thermal decomposition kinetic parameters of podophyllotoxin (PPT) and 4-demethypodophyllotoxin (DMPPT) in a temperature-programmed mode have been investigated by means of DSC and TG-DTG. The kinetic model functions in differential and integral forms of the thermal decomposition reactions mentioned above for leading stage were established. The kinetic parameters of the apparent activation energy Ea and per-exponential factor A were obtained from analysis of the TG-DTG curves by integral and differential methods. The most probable kinetic model function of both decomposition reactions in differential form was (1-α) 2. The values of Ea indicated that the reactivity of PPT was higher than that of DMPPT in the thermal decomposition reaction. The values of the entropy of activation ΔS≠, enthalpy of activation ΔH≠ and free energy of activation ΔG≠ of the reactions were estimated.

Preparation, characterization and thermal decomposition mechanism of guanidinium azotetrazolate (GUZT)

Guanidinium azotetrazolate (GUZT) was prepared by replacement reaction using guanidine nitrate (GN) and 5-amino-tetrazole (5-AT) as raw materials. The morphology and structure of the title compound were characterized by scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy and elementary analysis (EA). Thermal behaviors and thermal decomposition processes of GUZT were investigated by means of thermogravimetric thermogravimetric (TG) analysis, differential scanning colorimetry (DSC), condensed phase thermolysis/RSFT-IR and pyrogenetic mass spectrometry in a temperature-programmed mode. The thermal decomposition mechanisms of GUZT were obtained by analyzing its solid-phase thermal decomposition processes and gas-phase thermal decomposition products.

Hydrogen production by catalytic decomposition of selected hydrocarbons and H2O dissociation over CeZrO2 and Ni/CeZrO2

Presented paper deals with the catalytic decomposition of hydrocarbons (methane and toluene) in the aspect of H2 production and types of obtained carbon deposits. The catalyst used in our studies was nickel supported on ceria–zirconia (Ni/CeZrO2). The aim of this work was to investigate the reactivity of obtained carbon deposits with H2O. Both issues are of great importance for determining the mechanisms of carbon deposits formation and their suppression during steam reforming reaction.Our studies revealed that depending on the temperature, CH4 decomposition over Ni/CeZrO2 leads to carbon nanotubes (CNTs), whereas C7H8 decomposition yields pyrolytic carbon. Hydrogen production was found to be higher for CH4 decomposition. Both types of carbon deposits (filamentous and pyrolytic) were found to be very reactive with H2O, which is due to their activation by the ceria–zirconia support.Catalytic tests of hydrocarbon decomposition and H2O dissociation were carried out in the temperature-programmed mode and isotherms, and were supported by the in situ reflectance FT-IR measurements. Obtained carbon deposits were characterized using high-resolution transmission electron microscopy (HRTEM).

Preparation, Characterization and Combustion Catalytic Action of Bismuth/Zirconium Gallate

Bismuth/zirconium gallate(Gal-BiZr) was synthesized from gallic acid,bismuth nitrate,and zirconyl nitrate firstly,and characterized by elemental analysis,X-ray fluorescence(XRF),and Fourier transform infrared(FTIR) spectroscopies.The thermal behavior and decomposition mechanism of Gal-BiZr in a temperature-programmed mode were investigated by thermogravimetric(TG) analysis,differential scanning colorimetry(DSC),and condensed phase thermolysis/FTIR techniques.The decomposition products of Gal-BiZr are Bi2O3,ZrO2,and C.Samples of propellants containing Gal-BiZr were prepared by extrusion technology,and the effect of Gal-BiZr on double-base(DB) propellant combustion properties was investigated.Gal-BiZr exhibits good catalytic behavior in the combustion of DB and Hexogeon(RDX)CMDB propellants.

Prediction of retention times in temperature programmed gas chromatography using the retention equation derived from crystallization behavior of polymer

Based on the enlightenment from the crystallization behavior of polymer, a novel retention equation constructed with only three pairs of isothermal retention data was proposed to predict retention times in temperature programmed gas chromatography (TPGC). The new retention equation worked beautifully in both the single- and the multiple-ramp temperature-programmed modes, yielding the average absolute relative errors of 0.65% for single-ramp TPGC across all the 18 analytes in 3 temperature programmed experiments and 0.30% for multiple-ramp TPGC across all the 8 analytes in 6 temperature programmed experiments tested in this work. Moreover, to compare with the new retention equation systematically and thoroughly, another 13 retention equations determined by two or three parameters were derived from a universal formula lnk=a0+a1/T+a2/T2+a3lnT+a4T+a5T2 as a combination of the relationships between lnk and T with reference to four widely used retention equations in publications. Calculated by these 14 retention equations with the same experimental data, the absolute relative errors of prediction retention times for single-ramp TPGC mode were compared by means of statistical analysis. At a very high significance level of 1%, statistical evidences of paired t-test strictly implied that the new retention equation yielded the best prediction results among all of the 14 retention equations. In addition, remarkably, among the 13 retention equations, three seldom used in publications retention equations also worked nicely, yielding satisfactory average absolute relative errors of 0.78%, 0.81% and 0.84% for single-ramp TPGC.

Deactivation of Ni-YSZ Material in Dry Methane and Oxidation of Various Forms of Deposited Carbon

Carbon deposits are the most probable mode of deactivation for solid oxide fuel cell (SOFCs) using methane or higher hydrocarbons as fuel. The deposition of various carbon allotropes on the anode material was studied under dynamic and isothermal conditions. The results show methane dissociation on Ni-YSZ (nickel-yttrium stabilized zirconia) under the temperature-programmed mode in three general steps. Under isothermal conditions, various carbon species formed depending on the temperature. The presence of amorphous, filamentous, pyrolitic, and graphitic carbon allotropes was determined by quadrupole mass spectroscopy (QMS), X-ray crystallography, field emission scanning electron microscopy (FE-SEM), and infrared spectroscopy (IR). Carbon allotropes were subsequently oxidized in the atmosphere with 20.0 vol% and 0.5 vol% of oxygen in argon. Complex oxidation mechanisms were detected and discussed.