Theory of reverse scan square-wave voltammetry influenced by the kinetics of reactant adsorption

A model of electrode reaction complicated by slow adsorption of the reactant is developed for square-wave voltammetry with inverse scan direction. The relationship between the dimensionless net peak current and the logarithm of dimensionless rate constant of adsorption is a curve with a minimum and a maximum. For this reason the ratio of real net peak current and the square-root of frequency is a non-linear function of the logarithm of frequency and exhibits either a maximum or a minimum. The frequency of extreme serves for the estimation of the rate constant: log(k _ads /D ^1/2 ) = log(k* _ads )_crit + 0.5 log f _crit , where (k* _ads ) _crit is a critical dimensionless rate constant of adsorption. Square-wave voltammetry is sensitive to the kinetics of adsorption if k _ads < 10² cm s⁻¹      

First application of hexaaquaaluminium(III) tetrafluoroborate as a mild, recyclable, non-hygroscopic acid catalyst in organic synthesis: a simple and efficient protocol for the multigram scale synthesis of 3,4-dihydropyrimidinones by Biginelli reaction

For the first time hexaaquaaluminium(III) tetrafluoroborate has been used as a mild acid catalyst in organic synthesis. A simple method of its preparation based on the reaction of aluminium triisopropoxide and tetrafluoroboric acid in isopropanol afforded catalyst of high purity and activity. The three-component Biginelli condensation of acetoacetate esters, urea and aldehydes catalyzed by 10 mol % of [Al(H₂O)₆](BF₄)₃ in refluxing acetonitrile afforded 3,4-dihydropyrimidonones in good to high yields on multigram scales. The tolerance to acid sensitive reactants such as thienyl and furyl carbaldehydes, applicability to sterically hindered β-ketoesters and simple recyclability without losing catalytic activity make this catalyst as good replacement to literature methods. The mechanism of the reaction includes formation of the so called ‘ureido-crotonate’ rather than corresponding acylimino intermediate as found with Brønsted type catalysts.     

Chip electrospray mass spectrometry for carbohydrate analysis

Currently two types of chip systems are used in conjunction with MS: out-of-plane devices, where hundreds of nozzles, nanospray emitters are integrated onto a single silicon substrate from which electrospray is established perpendicular to the substrate, and planar microchips, embedding a microchannel at the end of which electrospray is generated in-plane, on the edge of the microchip. In the last two years, carbohydrate research greatly benefited from the introduction and implementation of the chip-based MS. In two laboratories the advantages of the chip electrospray in terms of ionization efficiency, sensitivity, reproducibility, quality of data in combination with high mass accuracy, and resolution of detection were systematically explored for several carbohydrate classes: O- and N-glycopeptides, oligosaccharides, gangliosides and glycoprotein-derived O- and N-glycans, and glycopeptides. The current state-of-the-art in interfacing the chip electrospray devices to high-performance MS for carbohydrate analysis, and the particular requirements for method optimization in both positive and negative ion modes are reviewed here. The recent applications of these miniaturized devices and their general potential for glycomic-based surveys are highlighted.     

Theory of square-wave stripping voltammetry with adsorptive accumulation

Summary A model of the anion-induced adsorptive accumulation combined with a square-wave voltammetric stripping is developed. The dependence of peak currents and peak potentials on the reversibility of redox reactions and on the reaction mechanism is demonstrated. The sensitivity of the method is estimated.

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Theorie der Square-Wave Stripping Voltammetrie mit adsorptiver Anreicherung

     

Effect of the size of the charged group on the properties of alkoxylated NFCs

The impact of the size of the charged group on the properties of alkoxylated NFC was studied by two chloroalkyl acid reagents. It was found that the employment of the larger 2-chloropropionic acid reagent leads to improved properties, e.g. higher fraction of nano-sized materials, and significantly better redispersion as compared to when the smaller monochloroacetic acid was employed. The differences in the impacts of the different reagents were hypothesized to be due to a more efficient disruption of the cohesion between the nanofibrils when a larger charged group was employed.     

Zaleplon co-ground complexes with natural and polymeric β-cyclodextrin

In this study, we compared the suitability of parent β-cyclodextrin (βCD) and its water soluble polymeric derivative (PβCD) as co-grinding additives aimed to enhance the solubility of zaleplon (ZAL), a hypnotic drug. Equimolar drug/carrier mixtures were co-ground in a high-energy micromill over different time intervals. Data obtained by differential scanning calorimetry, X-ray powder diffractometry and scanning electron microscopy showed a higher affinity of ZAL for the solid state interaction with PβCD, resulting in powders with lower relative drug crystallinity (RDC) compared to that obtained with natural βCD (RDC = 51.10 and 12.5 % for complexes with βCD and PβCD co-grounded for 90 min, respectively). On the other hand, grinding the drug alone did not result in a significant reduction of the drug crystallinity (RDC = 99.87 % for the sample ground for 90 min). Although ¹H-NMR spectroscopy confirmed that both co-ground products were readily converted into inclusion complexes upon dissolution in water, they presented different dissolution properties. The dissolution velocity of co-ground complex with PβCD was 25 % faster compared to that prepared with the parent βCD and almost double compared to that of the drug alone, irrespective of the pH value of the dissolution media. This clearly demonstrated the suitability of co-ground ZAL/PβCD complex in the development of an immediate release oral formulation of ZAL.     

Voltammetry of Immobilized Particles of Cannabinoids

A new electrochemical method for the detection of cannabinoids in food products is proposed. The method is based on the voltammetry of microparticles at a paraffin‐impregnated graphite electrode. In square‐wave voltammetry, in 0.1 M KNO₃ at pH 7, the responses of delta‐9‐tetrahydrocannabinol, cannabinol and cannabidiol consist of a single peak with the maximum at 0.55 V±0.01 V. This peak is ascribed to the electrooxidation of the phenol group to phenoxy radical. In microparticles of hemp and marijuana cannabinoids can be detected in pure drug as well as in the mixture with foreign insoluble powders.     

Thermodynamic study of inclusion complexes of zaleplon with natural and modified cyclodextrins

The thermodynamics and stoichiometry of zaleplon (ZAL) complexation with different cyclodextrin derivatives [β-CD, hydroxypropyl-β-cyclodextrin (HP-β-CD), randomly methylated-β-cyclodextrin (RAMEB), sulphobutylether-β-cyclodextrin (SBE-β-CD)] in aqueous solution was studied by spectrofluorimetry and ¹H NMR spectroscopy in order to obtain a more general understanding of the driving forces behind the inclusion phenomena. Job’s plot derived from the NMR spectral data and statistical analysis of spectrofluorimetric titration data confirmed the formation of equimolar complexes in all systems tested, excluding the possibility of higher order complex formation. Furthermore, thermodynamic parameters obtained by both techniques gave similar and negative values of ΔG° for all complexes, indicating spontaneous inclusion of drug into CDs. From a thermodynamic point of view, two types of inclusions were determined. One is enthalpy driven ZAL complexation with β-CD, HP-β-CD and RAMEB, while the other is entropy driven complexation observed in the case of SBE-β-CD. The mechanisms behind each type of inclusion were discussed in detail.     

Evaluation of a method for treatment of iron gall ink corrosion on paper

Iron gall ink was the most widely used writing ink for paper from the Middle Ages to the twentieth century. Unfortunately, the ink ingredients contain corrosive transition metal ions and acids that cause severe damage to the paper carrier. New or improved paper conservation methods for iron gall ink stabilization are constantly sought. The aim of the study was evaluation of a recently proposed stabilization treatment, adapted to lower relative humidity, applied to various model and historical paper samples containing iron gall ink. The effect of stabilization treatment on paper samples during artificial thermal aging was followed by the determination of the molecular weight distribution by size exclusion chromatography and colorimetry. Migration of iron and copper compounds from the ink lines was monitored by laser ablation inductively coupled plasma mass spectrometry. The results demonstrate that effective stabilization of iron gall ink-containing paper can be successfully achieved by interleaving with papers impregnated with the antioxidant tetrabutylammonium bromide and alkaline buffer under experimental conditions. Negative side effects of the stabilization treatment, such as migration of iron or copper ions from the ink lines and changes of color after the treatment and after accelerated thermal degradation, were limited, proving that the proposed stabilization treatment can be considered for future use by conservators.