Anisotropic studies of multi-wall carbon nanotube (MWCNT)-filled natural rubber (NR) and nitrile rubber (NBR) blends

50/50 NR/NBR blends with various MWCNT loadings were prepared by mixing with MWCNT/NR masterbatches on a two-roll mill and sheeted off at the smallest nip gap. Then, the effect of milling direction, machine direction (MD) and transverse direction (TD), on the mechanical and electrical properties of the blends was elucidated. Dichroic ratio and SEM results confirmed that most of the MWCNTs were aligned along MD when MWCNT was less than 4 phr, and the number of agglomerates increased when MWCNT was more than 4 phr. Additionally, anisotropic properties were clearly observed when 4 phr MWCNT was loaded. At 4 phr MWCNT, 100% modulus and tensile strength in the MD were about 1.5 and 1.3 times higher than those in the TD, respectively. Moreover, electrical conductivity in the MD was superior to that in the TD by about 3 orders of magnitude. Results from dynamic mechanical tests also showed that the maximum tan δ in the MD sample was lower than that in the corresponding TD sample. In addition, the storage modulus at 30 °C for the MD sample containing 4 phr MWCNT was 1.15 higher than that of the corresponding TD sample. This stronger reinforcement efficiency resulted from the combination of the greater alignment and dispersion of most MWCNTs in the MD sample.     

Validation and pharmacokinetic application of a method for determination of doxazosin in human plasma by high-performance liquid chromatography

A simple high-performance liquid chromatographic method for the determination of doxazosin in human plasma was developed and validated. Prazosin was used as internal standard. After extraction twice with ethyl acetate, chromatographic separation of doxazosin in human plasma was carried out using a reversed-phase Apollo C18 column (250 x 4.6 mm, 5 microm) with mobile phase of methanol-acetonitrile-0.04 m disodium hydrogen orthophosphate (22:22:56, v/v/v) adjusted to pH 4.9 with 0.9 m phosphoric acid and quantified by fluorescence detection operated with an excitation wavelength of 246 nm and an emission wavelength of 389 nm. The lower limit of quantification (LLOQ) of this assay was 1 ng/mL using 500 microL human plasma. Linearity was established over the range 1-25 ng/mL (r2 > 0.9994). The intra- and inter-day accuracy ranged from 90.5 to 104.4% and the coefficient of variation were not more than 8.6% for both intra- and inter-day precision, over the range of the calibration curve. The absolute recoveries of doxazosin and prazosin from human plasma were more than 91%. Doxazosin demonstrated acceptable short-term, long-term and freeze-thaw stability in human plasma. The assay has been successfully applied to plasma sample ana-lysis for pharmacokinetic study.     

Pre-capillary derivatisation and capillary zone electrophoresis for amino acids analysis in beverages

A simple and rapid method for the simultaneous analysis of amino acids has been developed. Amino acids were derivatised based on pre-capillary derivatisation with 1,2-naphthoquinone-4-sulfonate (NQS) in basic medium (pH 10.0) and developed reaction at 70 degrees C. Their derivatives were analysed by capillary zone electrophoresis (CZE). The parameters affecting CZE separation were investigated including buffer (pH, type and concentration), organic modifier and separation voltage. The optimum condition was 70 mmol L(-1) borate (pH 10.0) containing 10% acetonitrile, separation voltage of 12 kV, and sample injection (0.5 psi, 5s) and on-capillary detection at 240 nm. The separation of seven amino acids was achieved within 17 min. The detection limit was 1.0 mg L(-1) for all studied amino acids. The calibration curves were linear in the concentration range of 1.0-100.0 mg L(-1). The repeatability, intra-day and inter-day analysis were < or = 1.0% and < or = 2.0% for migration time and < or = 5.0% and 6.0% for peak area. The proposed method has been applied to several beverage samples with only a simple dilution and filtration treatment of sample before derivatisation and analysed by CZE.     

Improvement of doxazosin determination in human plasma using high-performance liquid chromatography with fluorescence detection

A simple, sensitive, rapid, and reproducible high-performance liquid chromatographic method is developed and validated for the determination of doxazosin in human plasma without a solvent extraction procedure. This method involves plasma protein precipitation using methanol. The structurally related compound prazosin is used as an internal standard. Doxazosin is detected with high sensitivity using spectrofluorimetry. Over the concentration range 0.5-20 ng/mL, the absolute recovery values are all greater than 98%. The method has a quantitation limit of 0.5 ng/mL. The intra- and interday coefficient of variation and inaccuracy values are all less than 8% and 7%, respectively. Therefore, the method has been applied in pharmacokinetic studies of doxazosin.