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10: Spectroscopic Methods

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  • 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 produces a yellow to reddish brown colloid, with the colloid’s color depending 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 its introduction, 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 that we are considering only a limited part of 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.
    • 10.3: UV/Vis and IR Spectroscopy
      The 1930s and 1940s saw the introduction of photoelectric transducers for ultraviolet and visible radiation, and thermocouples for infrared radiation. As a result, modern instrumentation for absorption spectroscopy became routinely available in the 1940s—progress has been rapid ever since.  Frequently an analyst must select—from among several instruments of different design—the one instrument best suited for a particular analysis. In this section we examine several different instruments for mole
    • 10.4: Atomic Absorption Spectroscopy
      Modern atomic absorption spectroscopy has its beginnings in 1955 as a result of the independent work of A. C. Walsh and C. T. J. Alkemade. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident. Atomic absorption spectrophotometers use the same optics described earlier for molecular absorption spectrophotometers, but with an important additional component—we must covert the analyte into free atoms.
    • 10.5: Emission Spectroscopy
      An analyte in an excited state possesses an energy that is greater than its energy when it is in a lower energy state. When the analyte returns to its lower energy state—a process we call relaxation. Relaxation of an analyte’s excited-state occurs through several mechanisms, including collisions with other species in the sample, by photochemical reactions, and by the emission of photons.
    • 10.6: Photoluminescence Spectroscopy
      Photoemission is divided into two categories: fluorescence and phosphorescence. Emission of a photon from the singlet excited state to the singlet ground state—or between any two levels with the same spin—is called fluorescence. Emission between a triplet excited state and a singlet ground state—or between any two levels that differ in their respective spin states–is called phosphorescence. Both fluorescence and phosphorescence can be used for qualitative analysis and semi-quantitative analysis.
    • 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. The emission consists of a series of discrete lines at wavelengths corresponding to the difference in energy between two atomic orbitals.
    • 10.8: Spectroscopy Based on Scattering
      Two general categories of scattering of radiation are recognized: elastic and inelastic. In elastic scattering, the radiation is scattered without undergoing change in the radiation’s energy.  Elastic scattering is divided into two types: Rayleigh, or small-particle scattering, and large-particle scattering. Turbidimetry and nephelometry are two techniques based on the elastic scattering of radiation by a suspension of colloidal particles.
    • 10.E: Spectroscopic Methods (Exercises)
      These are homework exercises to accompany "Chapter 10: Spectroscopic Methods" from Harvey's "Analytical Chemistry 2.0" Textmap.
    • 10.S: Spectroscopic Methods (Summary)
      This is a summary to accompany "Chapter 10: Spectroscopic Methods" from Harvey's "Analytical Chemistry 2.0" Textmap.