Each must be executed correctly in order for the analytical result to be accurate. Some analytical chemists distinguish between an analysis, which involves all the steps, and an assay, which is the laboratory portion of the analysis. Sampling During this initial step of analysis, a portion of a bulk material is removed in order to be assayed.
Luminescence In the most common case excitation occurs after the absorption of electromagnetic radiation. The absorption process is identical to that which occurs during absorptiometric measurements.
After ultraviolet-visible absorption, an electron in the analyte molecule or atom resides in an upper electron orbital with one or more vacant orbitals nearer to the nucleus. Emission occurs when the excited electron returns to a lower electron orbital.
The emitted radiation is termed luminescence. Luminescence is observed at energies that are equal to or less than the energy corresponding to the absorbed radiation. After initial absorption, emission can occur by either of two mechanisms.
In the most common form of luminescence, the excited electron returns to the lower electron orbital without inverting its spin—i. This phenomenon, known as fluorescenceoccurs immediately after absorption. When absorption ceases, fluorescence also immediately ceases.
Although it occurs with low probability, the excited electron sometimes returns to a lower electron orbital by a path in which the electron first inverts its spin while moving to a slightly lower energy state and then inverts the spin again while returning to the original spin state in the unexcited electron orbital.
Emission of ultraviolet-visible radiation occurs during the transition from the excited, inverted spin state to the unexcited electron orbital. Because inversion of the spinning electron during the last transition can require a relatively long time, the emission does not immediately cease when the absorption ceases.
The resulting luminescence is called phosphorescence. Both fluorescence and phosphorescence can be used for analysis.
Fluorescence can be distinguished from phosphorescence by the time delay in emission that occurs during the latter. If the luminescence immediately stops when the exciting radiation is cut off, it is fluorescence; if the luminescence continues, it is phosphorescence.
Owing to the arrangement of electron orbitals in molecules and atoms, phosphorescence is observed only in polyatomic species, whereas fluorescence can be observed in atoms as well as in polyatomic species. When fluorescence is observed in discrete, gaseous atoms, it is termed atomic fluorescence.
The apparatus used to make fluorescent and phosphorescent measurements is similar to that used to make measurements of scattered radiation.
The detector is usually placed perpendicular to the path of the incident radiation in order to eliminate the possibility of monitoring the incident radiation. Devices that are used to measure fluorescence are fluorometers, and those that are employed to measure phosphorescence are phosphorimeters.Aug 28, · The Public Inspection page on yunusemremert.com offers a preview of documents scheduled to appear in the next day's Federal Register issue.
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