Search Results

You are looking at 31 - 40 of 168 items for :

  • All content x
Clear All

Phenoloids with allelopathic effect (juglone, ctaechin, tannin, gallic acid t-cinnamic acid, caffeic acid, coumarin, thymol, salicin in 1mM concentration) cannot be detected after the absorption from the acceptor plant (bean) by the applied selective analytical method (TLC densitometry). Their localisation can be determined by histochemical reagens (ferrichloride, potassium bichromate, sodium hydroxide). In the foliage leaves and excised bean plants they are present mainly in the parenchymatuos elements of the vascular tissue already on the 3rd day, at the beginning of wilting.             Some substances (tropanes) out of the studied allelopathic alkaloids (atropine, scopolamine, belladonnin, tropine and caffeine) can be detected only in small amounts (7-8%) or only in traces in the leaf. Others (e.g. caffeine) accumulate in substantial amount (almost 200%) in their original form. Alkaloids,as well as phenoloids, can be detected in lesf tissue by histochemical methods (Dragendorff and Meyer reagents).

Restricted access

Abstract  

A new pressure DSC module (Mettler DSC27HP) and its abilities for vapor pressure determination in the range of subambient pressure to 7 MPa are presented. To compare the new to an established method, vapor pressures of caffeine, naphthalene and o-phenacetin have been determined both by pressure DSC and the Knudsen effusion cell method. These results, including the derived heats of evaporation and heats of sublimation, are compared to literature values.

Restricted access

Abstract  

The57Fe Mössbauer spectra of substituted pentacyanoferrate /II/ complexes [/CN/5 FeII L]3– have been obtained for L=adenine, guanine, purine, caffeine, ethylene sulphide, dimethyl sulphoxide, and ammonia. The Mössbauer parameters are utilized to classify the various ligands according to their and bonding abilities. A linear correlation between the Mössbauer isomer shift and the tetragonal distortion caused by the ligand L is proposed. Comparison between this linear correlation and the approximative method of Wentworth and Piper1 is made.

Restricted access

Our article presents the comparison of two methods: high-performance thin-layer chromatography (HPTLC) and pressurized planar electrochromatography (PPEC), implemented for the separation of a test mixture of purine derivatives. The two methods were compared in terms of separation selectivity and separation time. Our results show that PPEC enables the separation of the mixture (azathioprine, caffeine, theobromine, theophylline and acyclovir) which could not be efficiently separated in the HPTLC system, due to the different selectivities of separation. PPEC also enables to obtain a much faster separation, performed on the longer distance, in comparison to HPTLC. This makes PPEC a technique which can be useful in the analysis of purine derivatives and other drugs.

Restricted access

Abstract  

Thermal decomposition of several purine derivatives used in medicine –theophylline, theobromine, caffeine, diprophylline and aminophylline was investigated. The analyses were performed using a derivatograph. It has been established, that the thermal decomposition of purine derivatives occurs via three stages. The stages of dehydratation of hydrate and evaporation of aminophylline are distinctly marked on the thermoanalytical curves, which may be used for the control of composition of the studied compounds. The ranges of temperature, in which the analyzed compounds can be technologically transformed without change of their physicochemical properties, were also established. Moreover, the influence of heating rate and sample size on the thermal decomposition of the examined compounds was evaluated.

Restricted access
JPC - Journal of Planar Chromatography - Modern TLC
Authors: Mirko Prosek, Luka Milivojevic, Mitja Krizman, and Maja Fir

A new on-line TLC-MS interface, with computer-controlled extraction of substances from selected spots on a TLC or HPTLC plate, has been constructed. The controlled collection of the sample and its programmed injection into the mass spectrometer is the advantage of this type of interface. The interface has been tested and validated with a standard solution of caffeine as test substance. The results were compared with those from a previously established and now routinely used off-line TLC-SPE-APCI-MS extraction procedure.

Restricted access

Abstract  

In order to determine the applicability of vapor pressure studies on polymorphic modifications, pairs of enantiotropically related modifications of caffeine, theophylline and carbamazepine were investigated. The studies were performed over a wide temperature range (71 to 191°C) and accordingly over a wide vapor pressure range (0.02 to 400 Pa) using an automatic instrument constructed on the basis of the gas saturation principle. This instrument enables an analytical determination of the main component and the impurities present by the chromatographic separation of the substances transported in the gas flow. Therefore, the real partial pressure of the main component can be measured. Due to the high precision of the applied method it was possible to determine partial pressure curves and the thermodynamic transition temperature — the point at which the vapor pressure of two crystal polymorphs is equal. The thermodynamic transition temperatures of caffeine and theophylline were determined to be 136 and 232°C, respectively. These values are in agreement with experimental or calculated values derived from DSC investigations but are more reliable. Vapor pressure measurements of carbamazepine are only meaningful in the low temperature range due to its decomposition at high temperatures. The thermodynamics, advantages and limits of vapor pressure determinations of polymorphic modifications are discussed.

Restricted access

High performance thin layer chromatography (HPTLC) is a frequently used separation technique which works well for quantification of caffeine and quinine in beverages. Competing separation techniques, e.g. high-performance liquid chromatography (HPLC) or gas chromatography (GC), are not suitable for sugar-containing samples, because these methods need special pretreatment by the analyst. In HPTLC, however, it is possible to separate ‘dirty’ samples without time-consuming pretreatment, because disposable HPTLC plates are used. A convenient method for quantification of caffeine and quinine in beverages, without sample pretreatment, is presented below. The basic theory of in-situ quantification in HPTLC by use of remitted light is introduced and discussed. Several linearization models are discussed.A home-made diode-array scanner has been used for quantification; this, for the first time, enables simultaneous measurements at different wavelengths. The new scanner also enables fluorescence evaluation without further equipment. Simultaneous recording at different wavelengths improves the accuracy and reliability of HPTLC analysis. These aspects result in substantial improvement of in-situ quantitative densitometric analysis and enable quantification of compounds in beverages.

Restricted access

Abstract  

Thermogravimetry (TG), differential thermal analysis (DTA) and other analytical methods have been applied to the investigation of the thermal behaviour and structure of the complexes Mg(pc)(na)3⋅3H2O (I), Mg(pc)(py)2⋅2H2O (II),Mg(pc)(pic)2⋅2H2O (III) and Mg(pc)(caf)2⋅4H2O (IV), where pc=2,6- pyridinedicarboxylate, na=nicotinamide,py=pyridine, pic=γ-picoline and caf=caffeine. The thermal decomposition of these compounds is multi-stage processes. The chemical composition of the complexes, the solid intermediates and the resultant products of thermolysis have been identified by means of elemental analysis and complexometric titration. Schemes of destruction of these complexes are suggested. Heating of these compounds first resulted in a release of water molecules. In complexes I, II and IV the loss of the molecular ligands (na, py and caf) occur (on the TG curves) in one step (-2na, -2py and -2caf) and in complex III in two steps (-pic, -pic). The final product of the thermal decomposition was MgO. The thermalstability of the complexes can be ordered in the sequence: IV<I<III<II. Nicotinamide, pyridine, γ-picoline and caffeine were co-ordinated to Mg(II) through the N atom of the respective heterocyclic ring. IR data suggested a unidentate co-ordination of carboxylates to Mg(II) in complexes I–IV.

Restricted access

Temperature-resolved X-ray diffractometry as a thermoanalytical method

A powerful tool for determining solid state reaction kinetics

Journal of Thermal Analysis and Calorimetry
Authors: M. Epple and H. K. Cammenga

Development and experimental setup of the time-, and temperature -resolved X-ray powder diffractometry are described. This method allows far deeper insight into solid state reactions than conventional thermoanalytical methods like differential scanning calorimetry (DSC) or thermogravimetry. As an example, the dehydration of caffeine hydrate was investigated. We found that in earlier stages the reaction is nucleation controlled, whereas for higher extent of reaction diffusion limitation becomes rate-controlling.

Restricted access