Analytical Chem istry - DePauw University

612 Analytical Chemistry 2.0per analysis, with maximum sample throughputs of 20–30 determinationsper hour. The cost of a new instrument ranges from between $10,000–$50,000 for flame atomization, and from $18,000–$70,000 for electrothermalatomization. The more expensive instruments in each price rangeinclude double-beam optics, automatic samplers, and can be programmedfor multielemental analysis by allowing the wavelength and hollow cathodelamp to be changed automatically.10EEmission SpectroscopyAn analyte in an excited state possesses an energy, E 2 , that is greater than itsenergy when it is in a lower energy state, E 1 . When the analyte returns to itslower energy state—a process we call relaxation—the excess energy, DE∆E = E 2−E1must be released. Figure 10.4 shows a simplified picture of this process.The amount of time the analyte spends in the excited state—its lifetime—isshort, typically 10 –5 –10 –9 s for electronic excited states and 10 –15 sfor vibrational excited states. Relaxation of an analyte’s excited-state, A*, occursthrough several mechanisms, including collisions with other species inthe sample, by photochemical reactions, and by the emission of photons.In the first process, which is called vibrational relaxation, or nonradiativerelaxation, the excess energy is released as heat.A* → A+heatRelaxation by a photochemical reaction may involve a decomposition reactionA* → X+Yor a reaction between A* and another speciesA * + Z→ X+YIn both cases the excess energy is used up in the chemical reaction or releasedas heat.In the third mechanism, the excess energy is released as a photon ofelectromagnetic radiation.A* → A+hνThe release of a photon following thermal excitation is called emission andthat following the absorption of a photon is called photoluminescence. Inchemiluminescence and bioluminescence, excitation results from a chemicalor biochemical reaction, respectively. Spectroscopic methods based onphotoluminescence are the subject of the next section and atomic emissionis covered in Section 10G.