Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods643phosphorescencephosphorescent quantumyieldphotoluminescence photon plasmaphotodiode arraypolychromatic protecting agent radiationless deactivationrelaxation releasing agent resolutionself-absorption signal averaging signal processorsignal-to-noise ratio single-beam singlet excited stateslope-ratio method spectral searching spectrofluorimeterspectrophotometer spectroscopy stray radiationtransducer transmittance triplet excited stateturbidimetry vibrational relaxation wavelengthwavenumber10JChapter SummaryThe spectrophotometric methods of analysis covered in this chapter includethose based on the absorption, emission, or scattering of electromagneticradiation. When a molecule absorbs UV/Vis radiation it undergoesa change in its valence shell configuration. A change in vibrational energyresults from the absorption of IR radiation. Experimentally we measurethe fraction of radiation transmitted, T, by the sample. Instrumentationfor measuring absorption requires a source of electromagnetic radiation, ameans for selecting a wavelength, and a detector for measuring transmittance.Beer’s law relates absorbance to both transmittance and to the concentrationof the absorbing species (A = –logT = ebC).In atomic absorption we measure the absorption of radiation by gasphase atoms. Samples are atomized using thermal energy from either aflame or a graphite furnace. Because the width of an atom’s absorptionband is so narrow, the continuum sources common for molecular absorptioncan not be used. Instead, a hollow cathode lamp provides the necessaryline source of radiation. Atomic absorption suffers from a number ofspectral and chemical interferences. The absorption or scattering of radiationfrom the sample’s matrix are important spectral interferences that maybe minimized by background correction. Chemical interferences includethe formation of nonvolatile forms of the analyte and ionization of theanalyte. The former interference is minimized by using a releasing agent ora protecting agent, and an ionization suppressor helps minimize the latterinterference.When a molecule absorbs radiation it moves from a lower energy stateto a higher energy state. In returning to the lower energy state the moleculemay emit radiation. This process is called photoluminescence. One formof photoluminescence is fluorescence in which the analyte emits a photonwithout undergoing a change in its spin state. In phosphorescence, emissionoccurs with a change in the analyte’s spin state. For low concentrations