Spectrophotometry radiation sources

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

Dilutions are made with 0.05M sodium dihydrogen phosphate buffer, pH 7.00. Copper analyses are made by atomic absorption spectrophotometry the radiation source, cobalt-60 gamma rays, delivers about... 

See also Chemometrics and Statistics Multivariate Calibration Techniques. Optical Spectroscopy Radiation Sources Wavelength Selection Devices Detection Devices. Spectrophotometry Overview. 

Stray-light errors are more likely to be observed near the wavelength limits of an instrument, where the radiation intensity of the source and the efficiency of the optical system are reduced, especially below 220 nm and at the crossover point between the ultraviolet and the visible lamps (about 320 to 400 nm). Errors may become serious where the solvent absorbs strongly or where a strongly-absorbing sample is measured by difference spectrophotometry. 

The carbon lamp is an intense source of monochromatic UV (193.1 nm) radiation (W4). Other monochromatic light sources can be constructed for a limited range of wavelengths by exciting the resonance emission of an atomic vapor (S24). These have a stable wavelength but the emission is unstable. Lasers provide high intensity monochromatic radiation for a number of wavelengths but they are of limited value in absorption spectrophotometry. 

We have seen the relationship between absorption spectrophotometry and spectrofluorometry. A similar relationship exists between atomic absorption spectrophotometry and atomic fluorescence spectrophotometry. In atomic fluorescence, the flame retains its role as a source of atoms these atoms, however, are excited by an intense source of radiation and their fluorescent emission is assayed at an angle of 90° in a manner similar to that of spectrofluorimetry. Lack of sufficiently intense source for many elements has been the limitation of this technique, however, with time instrumental developments are overcoming this problem. High intensity hollow-cathode lamps, or xenon or mercury discharge lamps are used. 

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