10: Spectroscopic Methods


An early example of a colorimetric analysis is Nessler’s method for ammonia, which was introduced in 1856. Nessler found that adding an alkaline solution of HgI2 and KI to a dilute solution of ammonia produced a yellow-to-reddish brown colloid, in which the colloid’s color depended on the concentration of ammonia. By visually comparing the color of a sample to the colors of a series of standards, Nessler was able to determine the concentration of ammonia. Colorimetry, in which a sample absorbs visible light, is one example of a spectroscopic method of analysis. At the end of the nineteenth century, spectroscopy was limited to the absorption, emission, and scattering of visible, ultraviolet, and infrared electromagnetic radiation. Since then, spectroscopy has expanded to include other forms of electromagnetic radiation—such as X-rays, microwaves, and radio waves—and other energetic particles—such as electrons and ions.

• 10.1: Overview of Spectroscopy
The focus of this chapter is on the interaction of ultraviolet, visible, and infrared radiation with matter. Because these techniques use optical materials to disperse and focus the radiation, they often are identified as optical spectroscopies. For convenience we will use the simpler term spectroscopy in place of optical spectroscopy; however, you should understand we will consider only a limited piece of what is a much broader area of analytical techniques.
• 10.2: Spectroscopy Based on Absorption
In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation passes through the sample without a loss in intensity. At selected wavelengths, however, the radiation’s intensity is attenuated. This process of attenuation is called absorption.
• 10.03: UV
• 10.4: Atomic Absorption Spectroscopy
Guystav Kirchoff and Robert Bunsen first used atomic absorption in 1859 and 1860 to identify atoms in flames and hot gases. Although atomic emission continued to develop as an analytical technique, progress languished for almost a centurybefore the work of A. C. Walsh and C. T. J. Alkemade in 1955. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique soon was evident.
• 10.5: Emission Spectroscopy
An analyte in an excited state possesses an energy, $$E_2$$, that is greater than its energy when it is in a lower energy state, $$E_1$$. When the analyte returns to its lower energy state, the excess energy, $$\Delta E = E_2 - E_1$$, is released as a photon, a process called emission.
• 10.6: Photoluminescent Spectroscopy
The release of a photon following thermal excitation is called emission and that following the absorption of a photon is called photoluminescence, which is divided into two categories: fluorescence and phosphorescence.
• 10.7: Atomic Emission Spectroscopy
The focus of this section is on the emission of ultraviolet and visible radiation following the thermal excitation of atoms. Atomic emission occurs when a valence electron in a higher energy atomic orbital returns to a lower energy atomic orbital.
• 10.8: Spectroscopy Based on Scattering
The blue color of the sky during the day and the red color of the sun at sunset are the result of light scattered by small particles of dust, molecules of water, and other gases in the atmosphere. The earliest quantitative applications of scattering, which date from the early 1900s, used the elastic scattering of light by colloidal suspensions to determine the concentration of colloidal particles.
• 10.9: Problems
End-of-chapter problems to test your understanding of topics in this chapter.