Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods599Vis instrumentation ranges from several hundred dollars for a simple filterphotometer, to more than $50,000 for a computer controlled high resolution,double-beam instrument equipped with variable slits, and operatingover an extended range of wavelengths. Fourier transform infrared spectrometerscan be obtained for as little as $15,000–$20,000, although moreexpensive models are available.10DAtomic Absorption SpectroscopyGuystav Kirchoff and Robert Bunsen first used atomic absorption spectroscopy—alongwith atomic emission—in 1859 and 1860 as a means foridentify atoms in flames and hot gases. Although atomic emission continuedto develop as an analytical technique, progress in atomic absorptionlanguished for almost a century. Modern atomic absorption spectroscopyhas its beginnings in 1955 as a result of the independent work of A. C.Walsh and C. T. J. Alkemade. 13 Commercial instruments were in place bythe early 1960s, and the importance of atomic absorption as an analyticaltechnique was soon evident.10D.1 InstrumentationAtomic absorption spectrophotometers use the same single-beam or double-beamoptics described earlier for molecular absorption spectrophotometers(see Figure 10.26 and Figure 10.27). There is, however, an importantadditional need in atomic absorption spectroscopy—we must covert theanalyte into free atoms. In most cases our analyte is in solution form. If oursample is a solid, then we must bring it into solution before the analysis.When analyzing a lake sediment for Cu, Zn, and Fe, for example, we bringthe analytes into solution as Cu 2+ , Zn 2+ , and Fe 3+ by extracting them with asuitable reagent. For this reason, only the introduction of solution samplesis considered in this text.At o m i z at i o nThe process of converting an analyte to a free gaseous atom is called atomization.Converting an aqueous analyte into a free atom requires that westrip away the solvent, volatilize the analytes, and, if necessary, dissociatethe analyte into free atoms. Desolvating an aqueous solution of CuCl 2 , forexample, leaves us with solid particulates of CuCl 2 . Converting the particulateCuCl 2 to gas phases atoms of Cu and Cl requires thermal energy.CuCl CuCl Cu Cl2 ( aq) → () s → ( g) + 22 ( g )What reagent we choose to use dependson our research goals. If we need to knowthe total amount of metal in the sediment,then we might use a microwave digestionusing a mixture of concentrated acids,such as HNO 3 , HCl, and HF. This destroysthe sediment’s matrix and bringseverything into solution. On the otherhand, if our interest is biologically availablemetals, we might extract the sampleunder milder conditions, such as a dilutesolution of HCl or CH 3 COOH at roomtemperature.There are two common atomization methods: flame atomization and electrothermalatomization, although a few elements are atomized using othermethods.13 (a) Walsh, A. Anal. Chem. 1991, 63, 933A–941A; (b) Koirtyohann, S. R. Anal. Chem. 1991, 63,1024A–1031A; (c) Slavin, W. Anal. Chem. 1991, 63, 1033A–1038A.