Development and validation of a derivative ultraviolet spectrophotometric method for the determination of darifenacin in tablets using experimental designs and its comparison with chromatographic method

A set of experimental designs was applied to develop and validate a spectrophotometric method using derivative transformation coupled with zero-crossing methodology for the quantification of darifenacin hydrobromide in extended-release tablet form. In the presence of the matricial interference, a central composite (face-centered) design was necessary to reach the best condition without interference in the quantification. The optimal system was confirmed using the function named Derringer’s desirability to assess high precision and low quantification limit. The best condition pointed was the first order to derivative transformation, Δλ = 4, scale factor 150, scanning speed 280 nm/s and anulation point in 239.4 nm as wavelength. From these parameters it was possible to perform the method validation resulting in R ² = 0.999, concentration ranging from 0.10 to 2.50 μg/mL, recovery 98.65% and mean precision 97.67% (RSD = 0.0136). Additionally, robustness was assessed by a Plackett-Burman design, and no significant variability was obtained. The spectrophotometric method was compared with high-performance liquid chromatography method, resulting in no significant difference between the methods.     

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH⁻ and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO₂RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH⁻ and H₂O activity that in turn can possibly affect activity, stability, and selectivity of the CO₂RR. We determine the local OH⁻ and H₂O activity in close proximity to a CO₂-converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO₂RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.     

On the extent of star countable spaces

For a topological property P, we say that a space X is star Pif for every open cover Uof the space X there exists Y ? X such that St(Y,U) = X and Y has P. We consider star countable and star Lindelöf spaces establishing, among other things, that there exists first countable pseudocompact spaces which are not star Lindelöf. We also describe some classes of spaces in which star countability is equivalent to countable extent and show that a star countable space with a dense σ-compact subspace can have arbitrary extent. It is proved that for any ω ₁-monolithic compact space X, if C _p (X)is star countable then it is Lindelöf.     

The Degree of Weakly Discretely Generated Spaces

A space X is discretely generated at a point \({x \in X}\) if for any \({A \subseteqq X}\) with \({x \in \textsf{cl}(A)}\) , there exists a discrete set \({D \subseteqq A}\) such that \({x \in \textsf{cl}(D)}\) . The space X is discretely generated if it is discretely generated at every point \({x \in X}\) . We say that X is weakly discretely generated if for any non-closed set \({A \subseteqq X}\) , there exists a discrete set \({D \subseteqq A}\) such that \({\textsf{cl}(D) \setminus A \neq \emptyset}\) . New results about these properties in the classes of pseudocompact and ?ech-complete spaces are obtained and a theorem of Ivanov and Osipov concerning the ordinal function idc is generalized to the class of ?ech-complete spaces.     

A simple HPLC-DAD method for determination of adapalene in topical gel formulation

A simple stability indicating high-performance liquid chromatography method for the analysis of adapalene in pharmaceutical gel formulation is developed and validated. An isocratic separation is performed using a Merck RP-8 (150 mm × 4.6 mm i.d., particle size 5 m) column and a mixture of acetonitrile water (67:33, v/v, pH adjusted to 2.5 with phosphoric acid) as the mobile phase. The detection is achieved with a photodiode array detector at 321 nm. The specificity of the method is verified by subjecting both the reference substance and the pharmaceutical form to hydrolytic, oxidative, photolytic, and thermal stress conditions. There is no interference from the excipients of the formulation on the determination of adapalene in gel. The response is linear over the concentration range of 8.0-16.0 μg/mL (r > 0.999) with a limit of detection and quantification of 0.04 and 0.14 μg/mL, respectively. The mean recovery is 100.8%. The RSD values for the intra- and inter-day precision studies are < 1.2%. The method is validated by reaching satisfactory results for linearity, selectivity, specificity, precision, accuracy, robustness, and system suitability.     

Polyacrylamide gel electrophoretic studies on the self-association of crotamine: characterization and molecular dimension of n-mer species

Polyacrylamide gel electrophoresis (PAGE) at pH 4.4 was used to study the concentration dependence of absolute mobility of crotamine. Within amounts ranging from 5-65 micrograms the toxin appeared in at least three n-mer species which were characterized by their geometrical mean radius R and molecular weight Mr estimation. The R and Mr values of crotamine bands were obtained from equations described in the literature and by using standard polypeptides and proteins submitted to the same experimental conditions. When amounts of up to 20 micrograms were assayed by PAGE the bands had a monomer molecular weight value of 4650 and R was 1.08 nm. From 20-35 micrograms the toxin migrated as dimer (Mr 10,000) with an R value of 1.42 nm. However, amounts higher than 35 micrograms crotamine were mostly resolved in a "two-band" pattern with R and Mr values corresponding to higher associated species.     

Light-driven horseradish peroxidase cycle by using photo-activated methylene blue as the reducing agent

In this work, the regeneration of native horseradish peroxidase (HRP), following the consecutive reduction of oxo-ferryl pi-cation (compound I) and oxo-ferryl (compound II) forms, was observed by UV-visible spectrometry and electron paramagnetic resonance (EPR) in the presence of methylene (MB+), in the dark and under irradiation. In the dark, MB+ did not affect the rate of HRP compound I and II reduction, compatible with hydrogen peroxide as the solely reducing species. Under irradiation, the dye promoted a significant increase in the native HRP regeneration rate in a pH-dependent manner. Flash photolysis measurements revealed significant redshift of the MB+ triplet absorbance spectrum in the presence of native HRP. This result is compatible with the dye binding on the enzyme structure leading to the increase in the photogenerated MB* yield. In the presence of HRP compound II, the lifetime of the dye at 520 nm decreased approximately 75% relative to the presence of native HRP that suggests MB* as the heme iron photochemical reducing agent. In argon and in air-saturated media, photoactivated MB+ led to native HRP regeneration in a time- and concentration-dependent manner. The apparent rate constant for photoactivated MB+-promoted native HRP regeneration, in argon and in air-saturated medium and measured as a function of MB+ concentration, exhibited saturation that is suggestive of dye binding on the HRP structure. The dissociation constant (KMB) observed for the binding of dye to HRP was 5.4+/-0.6 microM and 0.57+/-0.05 microM in argon and air-saturated media, respectively. In argon-saturated medium, the rate of the conversion of HRP compound II to native HRP was significantly higher, k2argon=(2.1+/-0.1)x10(-2) s(-1), than that obtained in air-equilibrated medium, k2air=(0.73+/-0.02)x10(-2) s(-1). Under these conditions the efficiency of photoactivated MB(+)-promoted native HRP regeneration was determined in argon and air-equilibrated media as being, respectively: k2/KMB=3.9x10(3) and 12.8x10(3) M(-1) s(-1).     

Analysis of semicarbazide in baby food by liquid chromatography tandem mass spectrometry (LC-MS-MS)--in-house method validation

A method for detection of semicarbazide (SEM) in baby food was validated. SEM was extracted with hydrochloric acid and derivatised with 2-nitrobenzaldehyde, using [15N2,13C] semicarbazide as internal standard. The extract was neutralised, purified on a solid phase extraction cartridge and SEM was determined by reversed phase LC-MS-MS. Linearity was demonstrated in the ranges from 0.1 ng ml(-1) to 1 ng ml(-1) and from 2 ng ml(-1) to 80 ng ml(-1). Matrix effects were non significant for meat-based and significant for apple and rice-based baby foods, in both ranges. Mean recoveries ranged from 87.8% to 107.2% with relative standard deviation from 0.2% to 9.1%, considering both ranges. Limits of detection and quantification were 0.1 microg kg(-1) and 0.25 microg kg(-1), respectively. The results of the validation process demonstrated the method suitability for use in food control.     

Electric field gradient and electronic properties of crown thioether compounds

We compare published TDPAC experiments on ¹¹¹Cd in the crown thioether C₆H₁₂S₃AgCl with ab-initio electronic structure calculations performed within the framework of the Density Functional Theory using the Projector Augmented Wave method. We conclude from this comparison that the Cd atom at the very moment of the TDPAC experiment is positively charged, and we point out to a methodological difference between reproducing experimental electric-field gradients in molecules versus solid metals.     

Extrapolating to the one-electron basis set limit in polarizability calculations

We report calculations of polarizabilities using total energies extrapolated to the complete basis set limit. A dual-level scheme has been employed, with the complete basis set limit of the correlation energy determined by the recently reported uniform singlet- and triplet-pair extrapolation method. The finite field approach has been employed, with tensors and averaged polarizabilities for the ground electronic states of H 2, N 2, CO, and H 2O reported and compared with available experimental data in the literature. Exploratory results are also presented for C 6H 4NO 2NH 2.