Capillary GC-MS determination of melatonin in several pharmaceutical tablet formulations

A simple capillary GC-MS method for the determination of melatonin from tablets is proposed. Melatonin (N-acetyl-5-methoxytryptamine) was determined by gas chromatography-mass spectrometry after extraction from ground tablets with ethyl acetate and derivatization with N-methyl-N-trimethylsilyl-heptafluorobutyramide (MTSHFBA). Splitless injection offers good reproducibility with a standard deviation of +/- 2%. The proposed method was applied to analyse the melatonin content from three different tablet formulations.     

Phase stability relations in invariant systems

Except for the trivial case of one-component systems, the conventional Schreinemakers phase stability analysis in invariant systems is shown to be thermodynamically and stoichiometrically inconsistent in that the partition of the stability relations into contributions coming from univariant subsystems is formulated only qualitatively. Although the stability relations in invariant systems are essentially additive (i.e., the stability relations in invariant system may be partitioned into a sum contributions coming from univariant subsystems), the quantitative form of this partition has never been considered. On the basis of a new approach to the stability of chemical species in multiple chemical reaction systems that has been recently developed by us (Fishtik, I. J. Phys. Chem. B, 2005, 109, 3851), we show how the stability relations in invariant systems may be uniquely partitioned into contributions coming from univariant reactions. This finding provides a simple algorithm for the construction of various types of thermodynamically consistent stability diagrams.     

Optimized procedure for clay pillaring with aluminum species used in depollution

This study presents the improved method of smectite type clays pillaring, using aluminum salts. To achieve this goal, natural clay with a percentage of more than 95% montmorillonite was used. In order to analyze the pillaring process, a thermal procedure was used (thermal programmed desorption). The results of the pillaring process show that a controlled distance between the foliar band structures as long as 18·10^–10 m can be obtained. The clays modified by pillaring can be used as molecular sieves in microfiltration processes or as agents in residual water chemical depolluting processes. They also have zeolite properties which make them reusable.     

Probing structure in the polymorphic domain of the L-enantiomer of N-benzoyl-phenylalanine by means of 2D solid-state NMR spectroscopy and DFT calculations

A study of polymorphism using a range of solid-state NMR techniques is presented. We demonstrate the existence of at least six polymorphs in a sample of N-benzoyl-L-phenylalanine. We also present methodology for the characterization of the protonation state, hydrogen bonding, and molecular conformation for the polymorphs, together with results of such a characterization for one of the polymorphs present in our sample. DFT modeling is used to investigate the separate effects hydrogen bonding and molecular conformation have on the chemical shift tensor.     

Simultaneous Determination of As,Bi, Sb,Se, Te,Hg, Pb and Sn by Small-Sized Electrothermal Vaporization Capacitively Coupled Plasma Microtorch Optical Emission Spectrometry Using Direct Liquid Microsampling

The simultaneous determination of chemical vapor-generating elements involving derivatization is difficult even by inductively coupled plasma optical emission spectrometry or mass spectrometry. This study proposes a new direct liquid microsampling method for the simultaneous determination of As, Bi, Se, Te, Hg, Pb, and Sn, using a fully miniaturized set-up based on electrothermal vaporization capacitively coupled plasma microtorch optical emission spectrometry. The method is cost-effective, free from non-spectral interference, and easy to run by avoiding derivatization. The method involves the vaporization of analytes from the 10 µL sample and recording of episodic spectra generated in low-power (15 W) and low-Ar consumption (150 mL min⁻¹) plasma microtorch interfaced with low-resolution microspectrometers. Selective vaporization at 1300 °C ensured the avoidance of non-spectral effects and allowed the use of external calibration. Several spectral lines for each element even in the range 180–210 nm could be selected. Generally, this spectral range is examined with large-scale instrumentation. Even in the absence of derivatization, the obtained detection limits were low (0.02–0.75 mg kg⁻¹) and allowed analysis of environmental samples, such as cave and river sediments. The recovery was in the range of 86–116%, and the accuracy was better than 10%. The method is of general interest and could be implemented on any miniaturized or classical laboratory spectrometric instrumentation.     

Semiclassical analysis of low and zero energy scattering for one-dimensional Schrödinger operators with inverse square potentials

This paper studies the scattering matrix S(E;?) of the problem

−?²ψ^″(x)+V(x)ψ(x)=Eψ(x)     

Slow dynamics in glassy methyl alpha-l-rhamnopyranoside studied by 1D NMR exchange experiments

It is widely known that the ability of sugar glasses to preserve anhydrobiotic systems in nature is important but the process is not yet fully understood. Molecular motions in the glassy state are likely to be important in the process but until now have remained largely uncharacterized. Here we describe the use of 1D 13C NMR exchange experiments using CODEX (centreband only detection of exchange) methods to study the dynamics of the well characterised model glassy monosaccharide, methyl alpha-l-rhamnopyranoside. The glass was prepared by fast cooling of a melt inside an NMR rotor. Molecular motions in the range of seconds to milliseconds were observed in the glass, whereas identical experiments using the crystalline material displayed no observable motions in the time-scales covered by the experiment. At 13 to 14 K above Tg the nature of the motion in the glass changed probably due to the onset of larger scale reorientation. A bimodal distribution of jump angles combined with a broad distribution of correlation times was found to best represent the observed motions.     

Intermediate motions as studied by solid-state separated local field NMR experiments

In this report, the application of a class of separated local field NMR experiments named dipolar chemical shift correlation (DIPSHIFT) for probing motions in the intermediate regime is discussed. Simple analytical procedures based on the Anderson-Weiss (AW) approximation are presented. In order to establish limits of validity of the AW based formulas, a comparison with spin dynamics simulations based on the solution of the stochastic Liouville-von-Neumann equation is presented. It is shown that at short evolution times (less than 30% of the rotor period), the AW based formulas are suitable for fitting the DIPSHIFT curves and extracting kinetic parameters even in the case of jumplike motions. However, full spin dynamics simulations provide a more reliable treatment and extend the frequency range of the molecular motions accessible by DIPSHIFT experiments. As an experimental test, molecular jumps of imidazol methyl sulfonate and trimethylsulfoxonium iodide, as well as the side-chain motions in the photoluminescent polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene], were characterized. Possible extensions are also discussed.     

Low-pressure gas chromatography: Recent trends and developments

In the past few years, low-pressure gas chromatography (LP-GC) has been applied for the fast analysis of various pollutants in different environmental and food matrices. A typical LP-GC set-up involves the use of a short microbore column (typically 0.5–1 m × 0.10 mm internal diameter) at the injector side connected with a zero dead-volume connector to a short megabore column (typically 10 m × 0.53 mm) to be used with higher gas velocities. This set-up maintains atmospheric injection conditions, while the analytical column is operated under low-pressure conditions that are compatible with mass-spectrometer analyzers. Although the use of LP-GC results in a loss of separation efficiency, it offers a 3–5-fold reduction in analysis time for organic compounds and thus increased sample throughput and enhancement of the signal-to-noise ratio leading to improved detection limits. Considering the significance of, and the potential interest in, this topic, this review briefly describes the concept of LP-GC. Furthermore, we explore recent developments and applications of LP-GC, with a focus on the use of various column systems and analyzers. Finally, we critically evaluate the prospects for, and the limitations of, LP-GC.