Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods551Table 10.3 Common Sources of Electromagnetic RadiationSource Wavelength Region Useful for...H 2 and D 2 lamp continuum source from 160–380 nm molecular absorptiontungsten lamp continuum source from 320–2400 nm molecular absorptionXe arc lamp continuum source from 200–1000 nm molecular fluorescencenernst glower continuum source from 0.4–20 mm molecular absorptionglobar continuum source from 1–40 mm molecular absorptionnichrome wire continuum source from 0.75–20 mm molecular absorptionhollow cathode lamp line source in UV/Visible atomic absorptionHg vapor lamp line source in UV/Visible molecular fluorescencelaser line source in UV/Visible/IR atomic and molecular absorption,fluorescence, and scatteringChemical Sources of Energy Exothermic reactions also may serve as asource of energy. In chemiluminescence the analyte is raised to a higherenergystate by means of a chemical reaction, emitting characteristic radiationwhen it returns to a lower-energy state. When the chemical reactionresults from a biological or enzymatic reaction, the emission of radiationis called bioluminescence. Commercially available “light sticks” and theflash of light from a firefly are examples of chemiluminescence and bioluminescence.Wave l e n g t h Se l e c t i o nIn Nessler’s original colorimetric method for ammonia, described at thebeginning of the chapter, the sample and several standard solutions of ammoniaare placed in separate tall, flat-bottomed tubes. As shown in Figure10.9, after adding the reagents and allowing the color to develop, the analystevaluates the color by passing natural, ambient light through the bottomof the tubes and looking down through the solutions. By matching thesample’s color to that of a standard, the analyst is able to determine theconcentration of ammonia in the sample.In Figure 10.9 every wavelength of light from the source passes throughthe sample. If there is only one absorbing species, this is not a problem.If two components in the sample absorbs different wavelengths of light,then a quantitative analysis using Nessler’s original method becomes impossible.Ideally we want to select a wavelength that only the analyte absorbs.Unfortunately, we can not isolate a single wavelength of radiation from acontinuum source. As shown in Figure 10.10, a wavelength selector passesa narrow band of radiation characterized by a nominal wavelength, aneffective bandwidth and a maximum throughput of radiation. The effectivebandwidth is defined as the width of the radiation at half of itsmaximum throughput.Emission Intensity (arbitrary units)200 250 300 350 400wavelength (nm)Figure 10.8 Emission spectrum from a Cuhollow cathode lamp. This spectrum consistsof seven distinct emission lines (thefirst two differ by only 0.4 nm and are notresolved in this spectrum). Each emissionline has a width of approximately 0.01 nmat ½ of its maximum intensity.