Analytical Chem istry - DePauw University

554 Analytical Chemistry 2.0Monochromatic means one color, or onewavelength. Although the light exiting amonochromator is not strictly of a singlewavelength, its narrow effective bandwidthallows us to think of it as monochromatic.radiation at the entrance slit to a monochromatic source of finite effectivebandwidth at the exit slit. The choice of which wavelength exits the monochromatoris determined by rotating the diffraction grating. A narrower exitslit provides a smaller effective bandwidth and better resolution, but allowsa smaller throughput of radiation.Monochromators are classified as either fixed-wavelength or scanning.In a fixed-wavelength monochromator we select the wavelength by manuallyrotating the grating. Normally a fixed-wavelength monochromator isused for a quantitative analysis where measurements are made at one or twowavelengths. A scanning monochromator includes a drive mechanism thatcontinuously rotates the grating, allowing successive wavelengths to exitfrom the monochromator. Scanning monochromators are used to acquirespectra, and, when operated in a fixed-wavelength mode, for a quantitativeanalysis.Interferometers. An interferometer provides an alternative approachfor wavelength selection. Instead of filtering or dispersing the electromagneticradiation, an interferometer allows source radiation of all wavelengthsto reach the detector simultaneously (Figure 10.13). Radiation from thesource is focused on a beam splitter that reflects half of the radiation to afixed mirror and transmits the other half to a movable mirror. The radiationrecombines at the beam splitter, where constructive and destructive interferencedetermines, for each wavelength, the intensity of light reaching thedetector. As the moving mirror changes position, the wavelengths of lightexperiencing maximum constructive interference and maximum destructiveinterference also changes. The signal at the detector shows intensity asa function of the moving mirror’s position, expressed in units of distance ortime. The result is called an interferogram, or a time domain spectrum.fixed mirrorcollimating mirrorlight sourcebeam splittermoving mirrordetectorFigure 10.13 Schematic diagram of an interferometers.focusing mirror