Kinetic analysis of thermogravimetric data obtained under linear temperature programming—a method based on calculations of the temperature integral by interpolation

A new technique, called interpolation method, with general application in the kinetic analysis of processes studied by thermogravimetry (TG) under linear temperature programming is developed. It is based on the linear relationship, with slope 1, between log g() and log I(γ, θ) for the appropriate kinetic function, where I(γ, θ) is the normalized temperature integral, θ the normalized temperature (θ=T/T₀) and γ a dimensionless activation energy (γ=E/RT₀). Values of log I(γ, θ) are calculated by linear interpolations in a pre-built table. This method can easily be programmed and implemented in a personal computer, where the results (kinetic parameters and quality of regressions for the kinetic functions considered) are typically obtained in a very short time. The method is validated by analyzing different simulated thermogravimetric curves and comparing the results with those determined with some classic methods taken from the literature. In addition, the results are compared with the values obtained by a similar method, also developed and explained in this paper, which involves the evaluation of all the values of the temperature integral by numerical integration, therefore, demanding a much larger calculation time. The interpolation method is found to be more accurate than other published methods, particularly in the case of thermogravimetric curves corresponding to processes with low activation energies. The results obtained are always similar to those determined by the integration method, which is taken as reference. Application of the technique to experimental data for various types of reactions shows that the results are in agreement with the published parameters and kinetic laws.     

Synthesis and characterization of some heterocyclic derivatives by cyclization of carbohydrate thiosemicarbazones. Part I

We report the synthesis of 2,3-dihydro-1,3,4-thiadiazoles and 1,3,4-thiadiazoles from 1,2:3,4-di-O-iso-propylidene-α-D-galacto-1,6-hexodialdo-1,5-pyranose thiosemicarbazone. The physical and spectro-scopic characterizations of the heterocyclic derivatives as well as the intermediate product are described. We present the prefered conformation in solution using computational calculations and spectroscopic data. The possibilities of chiral induction of the cyclization reaction are discussed.     

Centrifugation equilibrium for spheres and spherocylinders

The centrifugation equilibrium problem is formulated and solved using a new procedure in which the specified variables are the temperature, system volume, particle dimensions and concentrations, angular speed, cell length, and cell distance from the rotation axis. As a result, we obtain the concentration profiles for all types of particles present in the system, which are considered to be immersed in a fluid. The particles are modeled as hard nonattractive spherocylinders using an equation of state, but the procedure is not restricted to any geometrical shape, and can be used with any equation of state available. The fluid is treated as a continuous medium, responsible for centrifugal buoyancy. We make calculations for colloidal suspensions of silica, often used for separations in biotechnology. Results are in good agreement with experiments and show excellent agreement in comparison with Monte Carlo simulations. Our calculations also predict focusing and shifting phenomena that have been experimentally observed in separations of fine particles.     

Sequential Determination of Hydrocortisone and Epinephrine in Pharmaceutical Formulations via Photochemically Enhanced Fluorescence

The study describes a simple and sensitive method for the sequential determination of hydrocortisone (HDC) and epinephrine (EP) in pharmaceutical formulations. The intrinsic fluorescence of epinephrine was measured before submitting the sample to a photochemical reaction to form a fluorescent product from HDC or HDC esters (succinate and acetate). The sample treatment eliminated the fluorescence from EP, thus minimizing spectral interference on the HDC fluorescence. Limits of detection (3S_b/m) were estimated to be 40ngmL^–1 for HDC or HDC salts and 0.2ngmL^–1 for EP. Recovery experiments, made with commercial and laboratory simulated formulations, gave values between 96 and 103% for both HDC and EP. This method can be easily automated, e.g. via flow-injection techniques.