Type of chromatography using as the stationary phase a thin layer (0.01 inch [0.25 mm]) of a special finely ground matrix (silica gel, alumina, or similar material) coated on a glass plate or incorporated in a plastic film. Solutions of the mixtures to be analyzed are spotted near one edge. Solutions of reference compounds are similarly applied. The edge of the plate is then dipped in a solvent. The solvent travels up the matrix by capillarity, moving the components of the samples at various rates because of their different degrees of attachment to the matrix and solubility in the developing solvent. The components, visible as separated spots, are identified by comparing the distances they have traveled with those of the known reference materials. TLC is useful for biological mixtures, especially lipids in animal or vegetable tissues and isoprenoids and essential oils found in flowers and other parts of plants. The matrices withstand strong solvents and developers better than the paper used in paper chromatography.
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Region in the upper atmosphere, about 6–30 mi (10–50 km) high, with significant concentrations of ozone, formed by the effect of solar ultraviolet (UV) radiation on oxygen and also present in trace quantities elsewhere in earth's atmosphere. Ozone strongly absorbs solar UV radiation, causing atmospheric temperature to climb to about 30°F (0°C) at the top of the layer, and preventing much of this radiation from reaching earth's surface, where it would injure many living things. Chlorofluorocarbons, or CFCs, and some other air pollutants that diffuse into the ozone layer destroy ozone. In the mid-1980s, scientists discovered that a “hole”—an area where the ozone is up to 50percnt thinner than normal—develops periodically in the ozone layer above Antarctica. This severe regional depletion, explained as a natural seasonal depletion, appears to have been exacerbated by the effects of CFCs, and may have led to an increase in skin cancer caused by UV exposure. Restrictions on the manufacture and use of CFCs and other ozone-destroying pollutants were imposed in 1978.
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In fluid mechanics, a thin layer of flowing gas or liquid in contact with a surface (e.g., of an airplane wing or the inside of a pipe). The fluid in the boundary layer is subjected to shear forces. A range of velocities is established across the boundary layer, from zero (provided the fluid is in contact with the surface) to maximum. Flow in boundary layers is more easily described mathematically than is flow in the free stream. Boundary layers are thinner at the leading edge of an aircraft wing and thicker toward the trailing edge; such boundary layers generally have laminar flow in the leading (upstream) portion and turbulent flow in the trailing (downstream) portion. Seealso drag.
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