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At least 991 records · Page 55Linked to original sources
PubMed1997

Finasteride in biological fluids: extraction and separation by a graphitized carbon black cartridge and quantification by high-performance liquid chromatography.

A simple, specific and sensitive high-performance liquid chromatographic method has been developed and validated for the determination of finasteride in human plasma. A solid-phase extraction procedure was used to isolate finasteride from the biological matrix before quantitative analysis. The analyte was separated on a Symmetry reversed-phase column using acetonitrile-0.04 M orthophosphoric acid (pH 4.0) as mobile phase and quantified by measuring its UV absorbance at 215 nm. The limit of detection for the analyte was 0.005 microg/ml. 4-Androstene-3,17-dione was used as internal standard. The calibration graph of the method was linear from 0.01 to 3.0 microg/ml of finasteride in human plasma, and the coefficient of variation less than 4.5%. This HPLC procedure is simple, precise and accurate for the determination of finasteride in human plasma.

Carbon↗
PubMed2000

Electrooxidation mechanism of biogenic amines at amine oxidase modified graphite electrode.

Amine oxidase (AO, EC. 1.4.3.6) was previously shown to be a very efficient biological recognition element of amperometric biosensors for monitoring biogenic amines. The enzyme was effectively working in both mono- and bienzyme electrode designs, based on either a direct or a mediated electron-transfer pathway. This work focuses on the elucidation of the electron-transfer mechanism of the monoenzymatic unmediated AO-modified biosensor. The observed unmediated catalytic currents were assumed to be caused by (i) a direct electron-transfer process, (ii) the electrooxidation of the formed product, or (iii) their combination. Experiments supporting these assumptions are discussed in detail.

Amine Oxidase (Copper-Containing)↗
PubMed2001

Electrochemical monitoring of proton transfer across liquid/liquid interfaces on the surface of graphite electrodes.

Equilibrium partitioning of HClO4 between aqueous solutions and benzonitrile (BN) or nitrobenzene (NB) was measured and used to evaluate the pKa of the acid in the two organic solvents. The potential drop across the BN/ H2O interface was evaluated from the known potential drop across the NB/H2O interface and the voltammetrically measured formal potential of a ferrocenium/ferrocene redox couple confined within thin layers of the two organic solvents. The voltammetric reduction of tetrachloro-1, 4-benzoquinone in thin layers of BN was used to monitor changes in the concentration of protons in the layer during proton-consuming faradaic reactions. The rate of proton transfer from the aqueous to the nonaqueous phase across the BN/H2O interface was shown to be adequate to sustain proton-consuming reactions at the electrode/BN interface.

Journal Article↗
PubMed1995

Spatial discrimination against background with different optical systems for collection of fluorescence in laser-excited atomic fluorescence spectrometry with a graphite tube electrothermal atomizer.

A single 90 degrees off-axis ellipsoidal mirror fragment was used in a dispersive detection system for electrothermal atomization laser-excited atomic fluorescence spectrometry. The performance of the new optical arrangement was compared with those of optical arrangements that employed a plane mirror in combination with biconvex or plano-convex lenses. All the optical arrangements collected fluorescence in a scheme called front surface illustration. BEAM-4, an optical ray tracing program, was used for calculations of spatial ray distributions and optical collection efficiency for the various optical configurations. Experimentally, the best collection efficiency was obtained by use of the ellipsoidal mirror, in qualitative agreement with simulations done by use of the BEAM-4 software. The best detection limit for cobalt with the new optical arrangement was 20 fg, which was a factor of 5 better than that obtained with conventional optical arrangements with otherwise the same instrumentation. The signal-to-background ratio and the fluorescence collection efficiency were also studied as a function of position of the optical components for the various optical arrangements. For both cobalt and phosphorus, the signal-to-background ratio with the new optical arrangement remained stable within 10-20% during +/- 8 mm shifts in the position of the detection system from the focal plane of the optics. Overall, the new optical arrangement offered high collection efficiency, excellent sensitivity, and facile optical alignment due to efficient spatial separation between the fluorescence signal and the background radiation. The advantages of the new optical arrangement were particularly important during measurements in the presence of high levels of blackbody radiation.

Data Display↗