Influence of measuring conditions on the quantification of spectroscopic signals in TA-FTIR-MS systems

Simultaneous thermal analysis (TA) and evolved gas analysis by mass spectrometry (MS) and/or Fourier transform infrared spectroscopy (FTIR) is a powerful hyphenated technique combining direct measurement of mass loss and sensitive spectroscopic analysis. In the present study the influence of several experimental parameters which may affect the quantification of FTIR signals have been studied using a combined TA-FTIR-MS system. Parameters studied include: sample mass (1-400 mg), carrier gas flow rate (25-200 mL min^-1), resolution of the FTIR spectrometer (1-32 cm^-1), and location of injection of the calibrating gas. MS analysis, which was not significantly affected by the experimental conditions, was used as a reference for assessing the accuracy of quantification by FTIR. The quantification of the spectroscopic signals was verified by the decomposition (NaHCO₃) or dehydration (CuSO₄·5H₂O) of compounds with well-known stoichiometry. The systematic study of the parametric sensitivity revealed that spectral resolution and carrier gas flow rate, which affect the acquisition time in the IR-cell, are key parameters that must be adjusted carefully for reliable quantification. The dependence of the reliability of quantification on these parameters is illustrated and conditions leading to proper quantification are discussed. As an example, for a standard spectral resolution of 4 cm^-1 and a FTIR gas cell volume of 8.7 mL, the carrier gas flow must be lower than 100 mL min^-1 for warranting accurate results (relative deviation <2%). The concentration range of analyzed species is limited but can be extended by proper selection of the wavenumber regions for molecules giving strong IR signals.     

Elucidation of organic reaction mechanisms by direct observation of the intermediate by Raman Laser rapid spectrometry.

We have demonstrated the utility of Raman Laser rapid spectrometry in the detection of an unstable intermediate in an organic reaction by averaging a large number of spectra of the reaction mixture in the range of absorption of a characteristic Raman vibration mode.     

Influence of the grain morphology of V2O5 on its reduction-reoxidation behaviour

The influence of the grain morphology of V₂O₅ on its reduction-reoxidation behaviour has been investigated by means of thermoanalytical methods. X-ray analysis and electron microscopy. Well-developed V₂O₅ platelets exposing predominantly the (010) face exhibited a significantly different reduction profile than poorly defined agglomerates of microcrystalline V₂O₅. Intermediate phases detected during reduction were V₆O₁₃ and VO₂ (rutile). The corresponding reoxidation profiles were found to be only weakly dependent on the grain morphology of V₂O₃. Electron microscopy showed that the original grain morphology of the V₂O₅ samples was not influenced markedly by the reduction-reoxidation cycle.     

Efficient S-alkylation of cysteine in the presence of 1,1,3,3-tetramethylguanidine

The synthesis of S-alkylated cysteine derivatives was carried out successfully in the presence of 1,1,3,3-tetramethylguanidine. Alkylation proceeded in high yields on unprotected amino acids and peptides containing a sulfhydryl group.     

On-column derivatization with o-phthaldialdehyde for fast determination of homocysteine in human urine

A new assay for urinary homocysteine is described. The assay relies on an on-column derivatization with o-phthaldialdehyde, using a reversed-phase HPLC column and detection/quantification by fluorescence. The analysis time for reduced and total homocysteine, including sample work-up, was 5 and 13 min, respectively. Quantification limit was 25 nM.     

Mechanism of the condensation of homocysteine thiolactone with aldehydes

Chemical reactivity of homocysteine thiolactone (HTL) has been implicated in cardiovascular disease. Owing to its aminoacyl-thioester character, HTL undergoes facile electrophilic and nucleophilic reactions at its amino and activated-carboxyl group, respectively. To gain insight into the mechanism of the reactions involving its amino group, the kinetics of the condensation of homocysteine thiolactone with formaldehyde, acetaldehyde, and pyridoxal phosphate, were analyzed in the pH range from 5 to 10. The reactions were first order with respect to HTL, aldehyde, and hydroxide ion concentrations. Of the two ionic species of HTL (pKa=6.67+/-0.05), the acid form HTL+ was approximately 100-fold more reactive than the base form HTL(0). The reactions of HTL with aldehydes involve intermediate adducts. The conversion of the intermediate carbinolamine to a product, 1,3-tetrahydrothiazine-4-carboxylic acid or its 2-substituted analogue, occurs in a two-step reaction. The first step involves hydrolysis of the thioester bond in the intermediate, facilitated by anchimeric assistance by the oxygen of the carbinolamine group of the intermediate. The second step involves an attack of the liberated thiolate on the aldehyde-derived carbon of the intermediate, affording 1,3-tetrahydrothiazine-4-carboxylic acid or its 2-substituted analogue. An unusual feature of these reactions is that the formation of the carbinolamine group increases the reactivity of the thioester bond of HTL approximately 10(4)-fold. The facile formation of tetrahydrothiazines may contribute to HTL elimination from the human body.     

Enol-keto tautomerism of aromatic photochromic Schiff base N,N'-bis(salicylidene)-p-phenylenediamine: ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale

A photochromic symmetric Schiff base, N,N'-bis(salicylidene)-p-phenylenediamine, is proposed as a probe for the study of solvent dependent enol-keto tautomerism in the ground and excited states. The ground state equilibrium between the enol-keto tautomers is found to depend mainly not on polarity but on the proton donating ability of the solvent. Upon selective excitation of each of these tautomers, the same excited state of a keto tautomer is created: in enol, after the ultrafast excited state intramolecular proton transfer (ESIPT), reaction, and in keto tautomer, directly. Then some part (<30%) of excited molecules are transferred to the photochromic form in its ground state. The evidence of another ultrafast deactivation channel in the excited enol tautomer competing with ESIPT has been found. The solvent does not influence the ESIPT dynamics nor the efficiency of the creation of the photochrome.     

A quantitative approach to dynamic and isothermal curing of an epoxy resin modified with oligomeric siloxanes

Journal of Thermal Analysis and Calorimetry - The reactivity of a system comprising commercial epoxy resin Epidian® 6 (E6) and triethylenetetramine (Z-1) hardener was studied. The system was...     

Herstellung der Polydimethylsiloxane mit verschiedenen Molekulargewichtsverteilungen und ihre viskoelastischen Eigenschaften

Ohne Zusammenfassung     

Modeling of a vehicle seat suspension system with pneumatic spring and viscous dampers

The paper contains physical and mathematical model of the passive system of seat suspension, used in trucks and buses. Spring force in this system is performed by air spring, whereas damping force is generated by two viscous dampers. Based on the experimental research of characteristic parts of the suspension system, the parameters of mathematical model are determined. Both results of computer simulations and experiments are presented as power spectral density of acceleration courses, measured on seat, in comparison with courses measured on excitation platform in laboratory. Transmissibility of real system and its model are also presented in the paper. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)