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1 Ultraviolet Spectroscopy Ultraviolet Spectroscopy The wavelength of UV and visible light are substantially shorter than the wavelength of infrared radiation. The UV spectrum ranges from 100 to 400 nm. ¾ A UV-Vis spectrophotometer measures the amount of light absorbed at each wavelength of the UV and visible regions of the electromagnetic spectrum. ¾ A UV or visible spectrophotometer has the same basic design as an infrared spectrophotometer. ¾ In a standard UV-Vis spectrophotometer, a beam of light is split; one half of the beam (the sample beam) is directed through a transparent cell containing a solution of the compound being analyzed, and one half (the reference beam) is directed through an identical cell that does not contain the compound but contains the solvent. 9 Solvents are chosen to be transparent in the region of the spectrum being used for analysis.
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2 The instrument is designed so that it can make a comparison of the intensities of the two beams as it scans over the desired region of the wavelengths. If the compound absorbs light at a particular wavelength, the intensity of the sample beam (I S ) will be less than that of the reference beam (I R ). Absorption of radiation by a sample is measured at various wavelengths and plotted by a recorder to give the spectrum which is a plot of the wavelength of the entire region versus the absorption (A) of light at each wavelength.
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3 A broad absorption band in the region between 210 and 260 nm. The absorption is at a maximum at 242.5 nm. It is this wavelength that is usually reported in the chemical literature λ max .
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4 Expressions Used in Ultraviolet Spectrometry Expressions Used in Ultraviolet Spectrometry The spectrum shows that the scan is from 200-400 nm. 9 Because absorption by atmospheric carbon dioxide becomes significant below 200 nm, the 100-200 nm region is usually not scanned unless special air-free techniques are employed.
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5 The wavelength of absorption is usually reported as λ max which represents the wavelength at the highest point of the curve. The absorption of energy is reported as absorbance (not transmittance as in infrared spectra). The absorbance at a particular wavelength is defined by the equation : The absorbance by a compound at a particular wavelength increases with an increasing number of molecules undergoing transitions .
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6 Therefore, the absorbance depends on the electronic structure of the compound and also upon the concentration of the sample and the length of the sample cell. Usually, energy absorption is reported as molar absorptivity absorptivity ε (also called molar extinction coefficient) rather as the actual absorbance. The molar absorptivity (usually reported at λ max ) is a reproducible value that makes into account concentration and cell length.
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