Development of an HPLC method for determining the alpha 2-adrenergic receptor agonist brimonidine in blood serum and aqueous humor of the eye

A procedure for determining brimonidine [5-bromo-6-(2-imidazolidinylideneamino) quinoxaline] in biological samples using a reversed-phase isocratic HPLC method is described. The application in blood serum and eye aqueous humor of patients treated with the Alphagan ophthalmic solution was carried out by enrichment of samples in brimonidine with solid-phase liquid extraction. Brimonidine reached maximum levels in aqueous humor and serum within 2-2.5 h, whereafter a declining pattern was obtained. An approximate 50% level of brimonidine was identified in serum at 12 h after ocular administration, whereas in aqueous humor this percentage was determined after a period of 4-5 h.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Molar extinction coefficients of some carbohydrates in aqueous solutions

Molar extinction coefficients of some carbohydrates viz. l-arabinose (C₅H₁₀O₅), d-glucose (C₆H₁₂O₆), d-mannose (C₆H₁₂O₆), d-galactose (C₆H₁₂O₆), d(-) fructose (C₆H₁₂O₆) and maltose (C₁₂H₂₄O₁₂) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well.     

Capillary electrophoretic analysis of brimonidine in aqueous humor of the eye and blood sera and relation of its levels with intraocular pressure

The aim of this study was the development of a capillary electrophoretic method for the determination of the levels of the selective alpha(2)-adrenergic receptor agonist brimonidine in aqueous humor of the eye and blood sera and their relation to its efficacy in reducing the intraocular pressure (IOP). Analysis of brimonidine was performed by capillary zone electrophoresis using 20 mM borate, pH 9.3, as operating buffer and detection at 255 nm. Brimonidine levels were determined in aqueous humor and blood sera from seven patients admitted for cataract extraction following ocular administration of the ophthalmic Alphagantrade mark solution. Levels of brimonidine and IOP values were recorded for a 24 h period. Alphagantrade mark administration resulted in a significant reduction of IOP, from within 30 min up to 4-5 h, whereafter a stepwise increase was recorded until 24 h, where mean IOP value returned to that before administration. The IOP reduction was related to the levels of brimonidine in aqueous humor, where maximal levels (80-100%) were obtained within 1-3 h. A 50% amount of the solution was determined after 4-5 h, whereas it reached the minimum level after 12 h. Serum levels reached maximum within 3-4 h, a 50% reduction was recorded in 12 h and minimum level in 24 h. It is concluded that brimonidine administration may significantly reduce IOP in patients when its level is maintained >/=50% of the maximum present in aqueous humor, i.e within a 4-6 h period. Since at this time the level of brimonidine in blood serum has reached maximum value, administration of brimonidine every 6 h may be used to obtain adequate brimonidine levels to maintain a constantly lowered IOP.     

Enhancement of second harmonic generation efficiency: Growth and characterization of magnesium(II)-doped tetrakis(thiourea)nickel(II) chloride crystals

The influence of magnesium doping on the properties of tetrakis(thiourea)nickel(II) chloride crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy. The incorporation of Mg(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The crystal is further characterized by Kurtz powder technique and dielectric studies.