Confirmation of identity of aflatoxins

Aflatoxins are detected and determined by analytical procedures based on physical and chemical properties, e.g., ultraviolet absorbance, fluorescence, solubility and Chromatographie retention times during thin layer (TLC) or liquid chromatography. For acceptance of analytical results based on these properties, especially for regulatory purposes, proof of identity of the compound being measured is essential. Numerous tests have been devised for confirmation of identity. Included are tests based on toxicological effects observed in the duckling, zebrafish, chick embryo,Bacillus megaterium and many other species; chemical tests based on formation of derivatives such as the acetates and water adducts; and tests based on color changes of TLC spots after contact with spray reagents, e.g., sulfuric acid. All of the foregoing have inherent uncertainties in interpretation of identity. On the other hand, mass spectrometry (MS) is one of the most specific methods of identification available; however, it has been difficult to apply at the low concentrations at which aflatoxins are routinely detected. In this paper, the confirmation techniques for aflatoxins are placed in historical perspective and are reviewed and evaluated A recently developed procedure for the application of negative ionization MS for the confirmation of identity of aflatoxins in foods or feeds at concentrations as low as 10 ng/g is described. This procedure consists of isolation of the aflatoxin by Association of Official Analytical Chemists’ methods, purification by preparatory 2-dimensional TLC, in situ elution of the aflatoxin TLC spot and analysis of the sample by negative ion chemical ionization MS using a direct insertion probe.     

Extending the low-frequency limits of the compact-range reflector

Traditionally, the compact-range is not considered a viable method for conducting low-frequency (VHF/UHF) antenna or RCS measurements, because of the limited electrical size of the collimating-reflector system. Normally, compact-range measurements are conducted in the extreme near field of the collimating system, where the main reflector size is on the order of 25 to 30 wavelengths minimum, with edge treatments of at least four wavelengths at the operating frequency. This mode of operation limits measurements to the high UHF band (500-800 MHz) and above, for even the largest sized reflector systems. L or S bands are more commonly the low-frequency limit. Recent research with compact ranges indicates that acceptable VHF/UHF measurements can be conducted in the quasi-far-field region of the collimating system, with reflectors as small as five wavelengths, and with electrically short edge treatments. A good user knowledge of this mode of operation is required to maximize its utility. A qualitative measure of acceptable quiet-zone performance must also be established. This paper addresses the theory of operation, practical implementation, and inherent limitations of the non-conventional use of the indoor compact range for conducting low-frequency measurements     

Bird Song Diamond in Deep Space 8k

AI & SOCIETY - The Bird Song Diamond (BSD) project is a series of multifaceted and multidisciplinary installations with the aim of bringing contemporary research on bird communication to a...     

Metachronal waves in the flagellar beating of Volvox and their hydrodynamic origin

Groups of eukaryotic cilia and flagella are capable of coordinating their beating over large scales, routinely exhibiting collective dynamics in the form of metachronal waves. The origin of this behaviour—possibly influenced by both mechanical interactions and direct biological regulation—is poorly understood, in large part due to a lack of quantitative experimental studies. Here we characterize in detail flagellar coordination on the surface of the multicellular alga Volvox carteri, an emerging model organism for flagellar dynamics. Our studies reveal for the first time that the average metachronal coordination observed is punctuated by periodic phase defects during which synchrony is partial and limited to specific groups of cells. A minimal model of hydrodynamically coupled oscillators can reproduce semi-quantitatively the characteristics of the average metachronal dynamics, and the emergence of defects. We systematically study the model''s behaviour by assessing the effect of changing intrinsic rotor characteristics, including oscillator stiffness and the nature of their internal driving force, as well as their geometric properties and spatial arrangement. Our results suggest that metachronal coordination follows from deformations in the oscillators'' limit cycles induced by hydrodynamic stresses, and that defects result from sufficiently steep local biases in the oscillators'' intrinsic frequencies. Additionally, we find that random variations in the intrinsic rotor frequencies increase the robustness of the average properties of the emergent metachronal waves.     

Higher order diffractions from a circular disk

The backscattering from a circular disk is analyzed using the geometrical theory of diffraction (GTD). First-, second-, and third-order diffractions are included in the hard polarization analysis, while first-, second-, and second-order slope diffractions are included for soft polarization. Improvements in the prediction of the monostatic radar cross section (RCS) over previous works are noted. For hard polarization, an excellent agreement is exhibited between experimental and theoretical results, while a very good agreement is noted for soft polarization. To further improve the soft polarization results for wide angles, a model for the creeping wave or circulating current on the edge of the disk is obtained and used to find an additional component of the backscattered field. The addition of this component significantly improves the results for wide angles, leading to excellent agreement for soft polarization also. An axial-caustic correction method using equivalent currents is also included in the analysis.