Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods617(a) (b) (c)Figure 10.50 An europium doped strontium silicate-aluminum oxide powder under (a) naturallight, (b) a long-wave UV lamp, and (c) in total darkness. The photo taken in total darkness showsthe phosphorescent emission. Source: modified from Splarka (commons.wikipedia.org).solid substrate, or trapping the molecule in solution. Figure 10.50 showsan example of phosphorescence.Exc i t a t i o n v e r s u s Em i s s i o n Sp e c t r aPhotoluminescence spectra are recorded by measuring the intensity ofemitted radiation as a function of either the excitation wavelength or theemission wavelength. An excitation spectrum is obtained by monitoringemission at a fixed wavelength while varying the excitation wavelength.When corrected for variations in the source’s intensity and the detector’sresponse, a sample’s excitation spectrum is nearly identical to its absorbancespectrum. The excitation spectrum provides a convenient means for selectingthe best excitation wavelength for a quantitative or qualitative analysis.In an emission spectrum a fixed wavelength is used to excite thesample and the intensity of emitted radiation is monitored as function ofwavelength. Although a molecule has only a single excitation spectrum, ithas two emission spectra, one for fluorescence and one for phosphorescence.Figure 10.51 shows the UV absorption spectrum and the UV fluorescenceemission spectrum for tyrosine.absorbanceabsorbancespectrumfluorescenceemissionspectrum220 260 300 340 380wavelength (nm)emission intensityFigure 10.51 Absorbance spectrum and fluorescenceemission spectrum for tyrosine in apH 7, 0.1 M phosphate buffer. The emissionspectrum uses an excitation wavelength of260 nm. Source: modified from Mark Somoza(commons.wikipedia.org).