Spectrophotometry grating

Sample analyses were carried out by a number of laboratories. We are grateful to Mr. Mark E. Peden and Ms. Loretta M. Skowron of the Water Survey s Analytical Chemistry Laboratory Unit for atomic absorption spectrophotometry, Mr. L. R. Henderson of the Illinois State Geological Survey for X-ray Fluorescence specto-scopy, and Dr. T. A. Cahill of the University of Califomia-Davis for elemental analysis. Mr. R. G. Semonin reviewed the manuscript. This material is based upon work supported by the National Science Foundation under Grant No. ATM-7724294, and by the Department of Energy, Division of Biomedical and Environmental Research, under Contract No. EY-76-S-02-1199. 

A large proportion of spectral data is acquired by dispersive spectrophotometry. The discussion that follows is restricted to instruments that use a diffraction grating as the principal dispersive element. The sense of the following also applies to systems that use a prism. In general, we treat systems using photosensitive detectors and fixed-position slits. Scanning is achieved by rotation of the diffraction grating. 

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. 

Fig. 4.4. Block diagram of optical system for electronic absorption spectrophotometry. L, L, biconvex silica lenses M, Mi, multiple reflexion cell (eight traversals shown) N, deuterium arc P, Hakuto R106 photomultiplier cell Q, wavelength drum and marker (R) S, recorder T, Vibron electrometer amplifier, U, grating. (After ref. 60.)...

In spectrophotometry in the narrower sense the monochromatic light beam of a discrete wavelength is filtered from a polychromatic light beam in an accessory unit (monochromator prism or grating). In addition, monochromatic light may also be produced and used for analytical measurements... 

Light containing only a narrow part of the spectrum. It can be produced from a hollow cathode lamp as in atomic absorption spectrophotometry or by the use of diffraction gratings, prisms and filters to isolate a specific spectral region from a tungsten, hydrogen or other lamp. 

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