Determination of nifedipine in dog plasma by high‐performance liquid chromatography with tandem mass spectrometric detection

Nifedipine is a dihydropyridine calcium channel blocker used widely in the management of hypertension and other cardiovascular disorders. In this work, a simple, rapid and sensitive liquid chromatography/tandem mass spectrometry method was developed and validated to determine nifedipine in dog plasma using nimodipine as the internal standard. Chromatographic separation was carried out on a C₈ column. The mobile phase consisted of a mixture of acetonitrile, water and formic acid (60:40:0.2, v/v/v) at a flow rate of 0.5 mL/min. Detection was performed on a triple quadrupole tandem mass spectrometer in selected reaction monitoring mode via an atmospheric pressure chemical ionization source. The method has a lower limit of quantification of 0.20 ng/mL with consumption of plasma as low as 0.05 mL. The linear calibration curves were obtained in the concentration range of 0.20–50.0 ng/mL (r = 0.9948). The recoveries of the liquid extraction method were 74.5–84.1%. Intra‐day and inter‐day precisions were 4.1–8.8 and 6.7–7.4%, respectively. The quantification was not interfered with by other plasma components and the method was applied to determine nifedipine in plasma after a single oral administration of two controlled‐release nifedipine tablets to beagle dogs. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhanced efficiency of recyclable C3-symmetric cinchonine-squaramides in the asymmetric Friedel–Crafts reaction of indoles with alkyl trifluoropyruvate

The highly enantioselective Friedel–Crafts reaction of indoles with trifluoropyruvate catalyzed by a C₃-symmetric cinchonine-squaramide is reported. A wide variety of trifluoromethylated indole derivatives were obtained in high yields and with excellent enantioselectivities (99% and up to >99% ee). Moreover the C₃ catalyst can be easily recovered and was used five times.     

Existence of positive stationary solutions for a diffusive variable-territory prey–predator model

This paper concerns the existence of positive stationary solutions for a diffusive variable-territory prey–predator model, and completely settles an open problem of Wang and Pang (2009). The main result closes a gap in an earlier result (2011) by the authors.     

Determination of limonin in dog plasma by liquid chromatography–tandem mass spectrometry and its application to a pharmacokinetic study

A highly sensitive liquid chromatography–tandem mass spectrometry method was developed and validated for the determination of limonin in beagle dog plasma using nimodipine as internal standard. The analyte and internal standard (IS) were extracted with ether followed by a rapid isocratic elution with 10 mm ammonium acetate buffer–methanol (26:74, v/v) on a C₁₈ column (150 × 2.1 mm i.d.) and subsequent analysis by mass spectrometry in the multiple reaction monitoring mode. The precursor to product ion transitions of m/z 469.4 → 229.3 and m/z 417.2 → 122.0 were used to measure the analyte and the IS. The assay was linear over the concentration range of 0.625–100 ng/mL for limonin in dog plasma. The lower limit of quantification was 0.312 ng/mL and the extraction recovery was >90.4% for limonin. The inter‐ and intra‐day precision of the method at three concentrations was less than 9.9%. The method was successfully applied to pharmacokinetic study of limonin in dogs. Copyright © 2012 John Wiley & Sons, Ltd.     

New mesogen with thermotropic cubic phase: 3,4,5‐tris‐(11,11,12,12,13,13,14,14,15,15,16,16,16‐tridecafluorohexadecyloxy)benzoic acid

A new mesogen, 3,4,5‐tris(11,11,12,12,13,13,14,14,15,15,16,16,16‐tridecafluoro‐hexadecyloxy)benzoic acid, with a thermotropic cubic phase was synthesized. The phase behaviour of the fluorinated compound and its corresponding non‐fluorinated analogue was investigated by differential scanning calorimetry, polarizing optical microscopy, and synchrotron small‐angle X‐ray diffraction. The fluorinated compound, which contains three partially perfluorinated alkoxy moieties and one carboxylic acid group capable of forming hydrogen bonding, exhibits a very interesting sequence of isotropic, cubic and hexagonal columnar phases, whereas its non‐fluorinated compound does not have a liquid crystalline phase. The cubic phase exists over a wide temperature range (from ～200°C to ～30°C on cooling) and is hence amenable to various physical measurements and potential applications. The ability to self‐assemble molecules into ordered superstructure via both partially perfluorinated chains and hydrogen bonding provides new insight for the development of novel liquid crystalline materials.