Changes in the composition of synthesis products upon transitioning from self-ignition to combustion

Changes in the chemical composition of condensed products upon switching from synthesis in the self-ignition mode to combustion synthesis is studied by approximate analytical and numerical means for condensed substances that react via competing reaction pathways. It is shown that these different modes of synthesis produce different compositions of the reaction products. The conditions required for transitioning from one mode of combustion initiation (thermal explosion) to another (ignition) are determined. It is found that this transition can occur upon changing the temperature of a heater by just two characteristic intervals. A scaling procedure that allows the calculation results obtained at zero dimensionless temperature of the heater to be used to determine the effect its non-zero dimensionless temperature has on the ignition mode and the composition of the obtained products is proposed. Calculations show that materials with different distributions of the chemical composition along the sample can be obtained by deliberately changing the temperature of the heater.     

Study of the mechanism of the photoisomerization and photooxidation of bilirubin using a model for the phototherapy of hyperbilirubinemia

Liquid chromatography was employed for a study of photochemical degradation of bilirubin in the complex with human albumin, using a model system in the presence of riboflavin. The concentrations of bilirubin, the photoisomers and biliverdin were monitored. The reaction mechanism was verified using a quantified mathematical model and was represented by a reaction scheme. Photoisomerization is the initial process, followed by photooxidation to degradation products of the tetrapyrrole skeleton, with formation of biliverdin as an intermediate. The blood of newborns that were irradiated for treatment of hyperbilirubinemia was studied for the sake of comparison. The effect of some biochemically important substances was followed, in view of possible inhibition of the processes. The experimental results demonstrate that riboflavin acts as a catalyst, even at the concentrations typical for its occurrence in blood. The results are discussed from the point of view of the mechanism of bilirubin degradation during phototherapy.     

From theory of spectra to standardless analysis of molecular objects

The authors discuss the methodology of quantitative analysis of pure substances and mixtures by optical spectra (IR, Raman, UV, etc.) without using samples of standard composition (standardless molecular spectral analysis). An algorithm of quantitative mixture analysis using the reduction of ideal spectra to real ones and computational algorithms of the determination of component concentrations in a mixture taking into account error distribution is proposed. The applicability of the method is estimated using computational experiments parsed and conclusions about the specific features of its work are drawn.     

The feasibility of standardless molecular spectral analysis of mixtures under conditions of photochemical transformations of molecules

The problem of standardless molecular spectral analysis of multicomponent mixtures with allowance for photochemical reactions of their molecules has been formulated and its formal procedure has been developed. The case when the time dependence of the spectrum of the mixture is determined only by the change in the concentrations of the components with time and the spectra of the individual components remain constant has been considered. The corresponding requirements for the physicochemical properties of the molecules have been identified and shown to be met for quite a wide range of polyatomic molecules and their reactions. The exception is molecules with metastable excited electronic states. A large set of model calculations relevant to the actual molecular entities and their experimentally observed photochemical transformations have shown the practicability of the approach. The solution of the problem converges to exact values and is resistant to uncertainty in setting the initial spectral data due to the peculiarities of the experimental procedure, the choice of theoretical models, and other factors. The resulting theoretical values of the concentrations meet the least squares test and their errors are measured by the variance per unit weight. The variance makes it possible to evaluate the noise level of the spectral data used in the analysis and reliability of the result and can serve as a criterion for the correctness of the initial hypotheses about the qualitative composition of a multicomponent mixture.     

Identification of naphthoylindoles acting on cannabinoid receptors based on their fragmentation patterns under ESI‐QTOFMS

‘Herbal highs’ have been advertised as legal and natural substitutes to cannabis, but a detailed examination of these products has revealed that the herbal matrix is laced with synthetic substances that mimic the effects of marijuana. Producers select the ingredients based on the results of scientific studies on the affinities of different chemicals to cannabinoid receptors. Naphthoylindoles have turned out to be the most popular class of substances identified in the products. Legal actions taken in order to tackle the problem of uncontrolled access to one substance have usually resulted in the marketing of derivatives or analogues. In the study, the mass spectral behavior of twelve synthetic cannabinoids from the naphthoylindole family under electrospray ionization (ESI) was investigated. LC‐QTOFMS experiments were performed in three modes (low fragmentor voltage, high fragmentor voltage with/without collision energy), and they enabled the identification of protonated molecules and main ions. A general fragmentation pattern under this ionization method was proposed, and mechanisms of ion formation were discussed. The developed procedure allowed the determination of substituent groups of the core naphthoylindole structure and distinction between positional isomers. The obtained results were used for the prediction of the ESI‐MS spectra for many naphthoylindoles with a high affinity to cannabinoid receptors. Similarities and differences between ESI‐MS and electron impact‐MS spectra of naphthoylindoles were discussed. The developed identification process was presented on an example of an analysis of an unknown herbal material, in which JWH‐007 was finally identified. Knowledge of the fragmentation mechanisms of naphthoylindoles could also be used by other researchers for identification of unknown substances in this chemical family. Copyright © 2012 John Wiley & Sons, Ltd.     

Difficulties with the Determination of Mercury in Foodstuffs by Stripping Voltammetry

The main problems with the determination of mercury in foodstuffs by stripping voltammetry were analyzed. The conditions for obtaining the analytical signal from mercury(II) at a gold–graphite electrode and for reducing the effect of interfering substances were refined. A simplified procedure for the preparation of foodstuff samples by the chemical (HNO₃ + H₂O₂) and photochemical oxidation of samples was proposed. The determination conditions were optimized by the fractional factorial design of the experiments. It was shown that a gas-discharge XeBr excilamp can be used as an alternative to a mercury quartz lamp in the sample preparation of foodstuffs. The conditions for reducing the blank experiment and its contribution to the results of determination were considered.     

Kinetic spectrophotometric determination of hydrocortisone acetate in a pharmaceutical preparation by use of partial least-squares regression

A kinetic spectrophotometric method for the determination of hydrocortisone acetate based on its condensation with isonicotinic acid hydrazide is proposed. The method is applied to the determination of hydrocortisone acetate in a commercially available pharmaceutical preparation, presented as a pomade, that also contains another corticosteroid and additional active compounds. The operating procedure involves dissolving the pomade in chloroform and the addition of the reagent solution directly to the cuvette, in this way avoiding the previous extraction of analytes from the insoluble pomade matrix required by the alternative HPLC procedure. Calibration is performed by partial least-squares regression, using absorbance or first derivative spectra values recorded each minute during the first 30 min of reaction. Use of first derivative spectra overcomes possible scattered light problems produced by excipients precipitating, and produced slightly better results than absorbance data. The relative standard deviation obtained for 11 replicates analysed on different days was approx. 1.5%. The proposed method improves both accuracy and precision of the classical initial rate method and the precision of the HPLC procedure.     

A glucotest for the quality control of food raw materials and products

Ready-to-use indicatory forms shaped as tablets, e.g., glucotests, are proposed for the rapid semi-quantitative determination of glucose in foodstuffs. Their function is based on the classical Fehling’s reaction with the selection of reagents aimed to eliminate the influence of proteins, reduce sample pretreatment to a minimum, and perform the test within 10 min. The glucotests are economical, simple to manufacture, and have no limits for shelf life and storage conditions. If necessary, the glucotests can be applied for the quantitative determination of glucose as well. To do this, a photometric procedure is proposed based on the quantification of unreacted copper acetate in the composition of the glucotest by its intrinsic absorbance. To analyze colorful products, sample pretreatment using an anion-exchange resin in the acetate form is recommended.     

Numerical Simulation of Chemical Equilibria and Photostationary States Using Spectrophotometric and Spectrofluorometric Data

A mathematical model describing series of experimental absorption and fluorescence spectra of multicomponent systems via equilibrium and photostationary concentrations of components of the systems was constructed. With photostationary concentrations, it is possible to take into account changes in the fluorescence spectra resulting from photochemical reactions. Ther unknown equilibrium and kinetic parameters of the model and also the spectra of pure components are found from experimental data by the nonlinear least-squares method. A software for the model was developed, and its applicability to the determination of the composition of multicomponent systems, resolution of the spectra of pure components, and determination of equilibrium and kinetic constants was demonstrated.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 3, 2005, pp. 386–394.Original Russian Text Copyright © 2005 by Rusalov.     

Pre-column (hnu-HPLC) photochemical reaction for the on-line characterization of photoproducts using p-aminobenzoic acid as a model substance

A special photochemical reactor on the basis of high-pressure resistant reaction coils has been constructed, which can be coupled to an HPLC-system before the separation column. By means of the so-called "on-line pre-column (hv-HPLC)"-method, the starting compounds can be irradiated on-line, the conversion products formed can be separated and detected subsequently. The efficiency of the proposed method was proved by using p-aminobenzoic acid (PABA) as a model substance.     

Mass spectrometry determination of the isotopic composition of uranium hexafluoride

An absolute method for the determination of isotopic composition of substances in multiple collector mass spectrometers was proposed. The detailed analysi of terms used in the method for the determination of the isotopic composition of uranium hexafluoride was given. Parabolic nature of occurring constant measurement error, obtained in theory, was proved by test data for ²³⁵U within a wide range of its concentrations. In order to use the method of measurements in practice, the registration of constant error, through linear discriminatory relations Δc _i (c _i ), preliminary determined using uranium hexafluoride with standard isotopic composition, was proposed. The advantages of the developed method in comparison with the traditional analysis procedures, applied at present, were considered.     

Application of photochemical inhibition in flow injection systems: Determination of epinephrine and L-dopa

An automated procedure for the photochemical determination of epinephrine and L-dopa has been developed. It is based on the strong inhibition by these catecholamines on the photochemical reaction between phloxin and ethylenediaminetetraacetic. The proposed flow-injection method allows the fluorimetric determination of epinephrine in the range 1.9-26.4 mug/ml and of L-dopa in the range of 1.5-12.7 mug/ml, with a sampling frequency of 35 samples/hr. The method was applied successfully to the determination of body catecholamines in pharmaceutical preparations.     

Photochemical degradation of flavins. IV. Studies of the anaerobic photolysis of riboflavin

Abstract A variety of substances which quench the fluorescence of riboflavin decrease the rate of anaerobic photobleaching (photolysis) of the flavin at concentrations which have little effect on the fluorescence. A semi-quantitative estimate of the yield of various products of photolysis using thin layer chromatography and autoradiography with C¹⁴-labelled riboflavin shows that at least ten radioactive products are formed. Whereas the yield of most of these is decreased by low concentrations of quenchers, such as phenol, some are decreased only at quencher concentrations high enough to decrease the fluorescence significantly. The effects of quenchers on the phosphorescence and ESR signal induced by light were also observed. It is concluded that lumichrome arises, in part, from degradation of the excited singlet state, but that 9-formylmethylflavin and other products arise through the triplet state. A major product of anaerobic photolysis is a substance moving just ahead of riboflavin on chromatograms and which, like 9-formylmethylflavin, is destroyed by treatment with sodium hydroxide. Effects of varying the nature of the solvent, the flavin structure and the pH are reported, and the kinetics of processes involved in the primary photochemical acts and in quenching are discussed.     

On the perfection and application of the ‘scintillation method’ for spectral analysis of disperse substances

The principles of the “scintillation method” of spectral analysis are described. The method is based on the registration of a spectral-line flash intensity and its variation with time while a disperse substance is introduced into an excitation source. A comparative estimate of the precision of this type of analysis is given and results are presented of the determination of gold in standard reference material with a chemical composition of ores and rocks. The possibilities and application of the method to the analysis of disperse substances are considered.     

Gemini型表面活性剂疏水链中肉桂酸基团的光致二聚反应

采用紫外光谱、 红外光谱和质谱研究了Gemini型表面活性剂N,N'-二对丁氧基肉桂酰胺基胱氨酸钠(SDBCC)溶液的光化学反应过程和产物, 分析了SDBCC浓度对分子疏水链中肉桂酸基团光化学反应产物分布和光致二聚产率的影响. 结果表明, 在不同浓度下, SDBCC分子疏水链中的肉桂酸基团的光化学反应过程均以光致二聚反应为主, 且为分子内光致二聚产物. 但聚集体形成前后在光照过程中肉桂酸基团的光致二聚反应产率随SDBCC浓度的变化趋势不同, 聚集体形成前光致二聚反应产率随浓度的增加而增大, 聚集体形成后光致二聚反应产率随浓度的增大而减小. 综合光化学反应过程、 拓扑条件和聚集体结构进一步分析了胶束和囊泡的存在对SDBCC分子疏水链中肉桂酸基团光致二聚反应的影响.     

Potentials of gas chromatography in the determination of reaction products in the catalytic oxidation of ammonia to nitrogen(II) oxide

Peculiarities of the determination of nitrogen oxides and ammonia in the reaction products of ammonia oxidation are studied by gas chromatography. Particular attention was paid to the sampling problem. It is shown that in the course of the transportation of samples containing nitrogen(II) oxide and oxygen, the oxidation of nitrogen(II) oxide to nitrogen(IV) oxide takes place in the vapor phase. The conditions were found for suppressing or even eliminating the gas-phase reaction of NO oxidation to NO₂. A procedure for the gas-chromatographic analysis of the reaction products in the catalytic oxidation of ammonia is proposed using several samples and packed columns, including the determination of NO with respect to NO₂ formed in the gas-phase oxidation of NO.     

Stoichiographic Methods in the Analysis of Substances of Unknown Composition

A hierarchic structure of concepts of the chemical composition of substances, including substances of unknown composition, was proposed on the basis of physicochemical criteria. The latest achievements in the theory and practice of standardless stoichiographic methods for determining the molecular composition of substances (differentiating dissolution and ion chromatography) were considered.     

Nonuniqueness of solution of reverse problem of chemical kinetics for first-order reactions

The question of nonuniqueness of the solution of the reverse problem of chemical kinetics has been investigated. The subject of investigation is the situation in which some of the substances participating in the reaction are not accessible for measurement (intermediate substances). The investigation was performed on models of first-order reactions, using Laplace transforms. Direct mathematical formulas have been derived, through which an answer can be obtained to the question of how many independent functional combinations of constants, and which combinations, and will specifically admit an unambiguous evaluation on the basis of kinetic information on the initial reactants and the reaction products. The reaction of ethylene chlorination is examined as an illustrative example.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 701–708, November–December, 1985.     

Extraction and determination of biologically active components of St. John’s wort and its pharmaceutical preparations

A procedure is developed for the determination of biologically active substances (BAS) of common St. John’s wort (Hypericum perforatum L.) by HPLC using two columns, Luna C18, 100 Å (for the determination of phenolcarboxylic acids and flavonoids), and Onyx Monolithic C18 (for the determination of phloroglucinols and naphthodianthrones), in the gradient elution mode with diode array detection. The detection limits for analytes are 0.05–0.20 μg/mL. To optimize the conditions, we studied the extraction of biologically active substances from St. John’s wort by a water–ethanol solution at different temperatures and pressures and under the effect of microwave radiation and ultrasound. The maximum amounts of substances were extracted in a dynamic mode under heat and pressure. The procedure was applied to the St. John’s wort samples of different brands and some pharmaceutical products based on it. The components of extracts were identified by retention times, UV spectra, and mass spectra. It was shown that the content of biologically active substances in pharmaceutical samples of St. John’s wort depends on the herb habitat. It was shown that hyperforin decomposed in pharmaceutical formulations based on St. John’s wort during storage.