Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods553larger effective bandwidthsmaller effective bandwidthsignalsignalwavelengthwavelengthFigure 10.11 Example showing the effect of the wavelength selector’s effective bandwidth on resolutionand noise. The spectrum with the smaller effective bandwidth has a better resolution, allowing us to seethe presence of three peaks, but at the expense of a noisier signal. The spectrum with the larger effectivebandwidth has less noise, but at the expense of less resolution between the three peaks.different filters. A monochromator is an alternative method for selectinga narrow band of radiation that also allows us to continuously adjust theband’s nominal wavelength.The construction of a typical monochromator is shown in Figure 10.12.Radiation from the source enters the monochromator through an entranceslit. The radiation is collected by a collimating mirror, which reflects a parallelbeam of radiation to a diffraction grating. The diffraction grating isan optically reflecting surface with a large number of parallel grooves (seeinsert to Figure 10.12). The diffraction grating disperses the radiation anda second mirror focuses the radiation onto a planar surface containing anexit slit. In some monochromators a prism is used in place of the diffractiongrating.Radiation exits the monochromator and passes to the detector. As shownin Figure 10.12, a monochromator converts a polychromatic source ofcollimating mirrorfocusing mirrorPolychromatic means many colored. Polychromaticradiation contains many differentwavelengths of light.diffraction gratingλ 1 λ2λ 3entrance slitlight sourceλ 1 λ 3λ exit slit2detectorFigure 10.12 Schematic diagram of amonochromator that uses a diffractiongrating to disperse the radiation.