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Periodic Variations (periodic + variation)
Selected AbstractsHydrodynamic Modulation Voltammetry with a Dual Disk Chopped Flow-Microjet Electrode (CF-MJE)ELECTROANALYSIS, Issue 18 2003Nafeesa Simjee Abstract A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase-sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt-Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport-limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ,reference species' (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF-MJE arrangement allows quantitative concentration measurements, down to at least 5×10,7,M. [source] X-ray beam-position monitoring in the sub-micrometre and sub-second regimeJOURNAL OF SYNCHROTRON RADIATION, Issue 6 2005Oliver Bunk It is demonstrated that X-ray beam positions can be extracted from two-dimensional profiles with sub-pixel resolution. Beam-position measurements utilizing a self-designed low-cost two-dimensional detector have been performed at two synchrotron radiation beamlines of the Swiss Light Source. The effective detector pixel size was 4.8,µm and the resolution achieved for the beam position was about 5,nm. At a data rate of 25 frames per second, periodic variations of the beam position could be detected with a frequency resolution below 0.1,Hz. This allowed, for example, the influence of a turbo-pump in the X-ray optics hutch on the beam position to be quantified, and even minute variations related to the electron beam in the storage ring could be detected. [source] Fabrication of Ceramic,Polymer Photonic Crystals by Stereolithography and Their Microwave PropertiesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Soshu Kirihara Three-dimensional photonic crystals with periodic variations in the dielectric constant were fabricated using a stereolithographic rapid prototyping method. The structures were composed of millimeter-scale ordered epoxy lattices in which ceramic particles with high dielectric constants (such as silica and titania) were dispersed. These crystals were designed to reflect microwaves via the formation of photonic band gaps in a gigahertz range. The attenuation of transmission amplitude through the photonic crystals, which was measured as a function of frequency using a network analyzer, clearly showed the formation of band gaps in the microwave range. [source] Cell Separator Operation within Temperature Ranges To Minimize Effects on Chinese Hamster Ovary Cell Perfusion CultureBIOTECHNOLOGY PROGRESS, Issue 6 2007Hans Drouin A cell retention device that provides reliable high-separation efficiency with minimal negative effects on the cell culture is essential for robust perfusion culture processes. External separation devices generally expose cells to periodic variations in temperature, most commonly temperatures below 37 °C, while the cells are outside the bioreactor. To examine this phenomenon, aliquots of ,5% of a CHO cell culture were exposed to 60 s cyclic variations of temperature simulating an acoustic separator environment. It was found that, for average exposure temperatures between 31.5 and 38.5 °C, there were no significant impacts on the rates of growth, glucose consumption, or t-PA production, defining an acceptable range of operating temperatures. These results were subsequently confirmed in perfusion culture experiments for average exposure temperatures between 31.6 and 38.1 °C. A 25,1 central composite factorial design experiment was then performed to systematically evaluate the effects of different operating variables on the inlet and outlet temperatures of a 10L acoustic separator. The power input, ambient temperature, as well as the perfusion and recycle flow rates significantly influenced the temperature, while the cell concentration did not. An empirical model was developed that predicted the temperature changes between the inlet and the outlet of the acoustic separator within ±0.5 °C. A series of perfusion experiments determined the ranges of the significant operational settings that maintained the acoustic separator inlet and outlet temperatures within the acceptable range. For example, these objectives were always met by using the manufacturer-recommended operational settings as long as the recirculation flow rate was maintained above 15 L day,1 and the ambient temperature was near 22 °C. [source] | ||||||||||