Spectrochemical Methods – Introduction
Spectroscopy: The interaction of radiation with matter.
Spectrochemical methods: Application of spectroscopic principles for
chemical analysis.
Electromagnetic radiation of different energies interact with matter in
different ways, giving rise to a variety of spectrochemical methods.
Broad classes of spectrochemical methods:
A. Absorption – measure the magnitude of radiant power from an
external source that is absorbed by the analyte.
B. Emission/Luminescence – measure the magnitude of radiant power
emitted by the analyte.
C. Scattering – measure the characteristics of radiant power from an
external source that is
scattered by the
analyte.
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To understand optical spectrochemical methods, i.e. methods using the
interaction of electromagnetic radiation with matter, a qualitative
understanding of absorption and emission of electromagnetic radiation is
required.
The nature of electromagnetic radiation and its interactions with matter
is discussed in Chapter 6.
The wave model, which you should all be familiar with (Section 6B),
explains phenomena
such as:
 Wave
superposition
 Diffraction
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Also Refraction, Reflection, Transmission, Scattering all discussed in
Section 6B. You are better off consulting your physics text for this
information.
The wave model fails to account for the absorption and emission of
radiant energy; the basis for spectrochemical methods.
Section 6C – quantum mechanical properties of radiation.
EM radiation can also be viewed as a stream of particles called photons,
the energy of which is proportional to frequency. Planck’s Law.
1. Atoms/Ions/Molecules can only exist in certain discrete states
characterized by definite amounts of energy. When a species
changes states, it absorbs or emits energy exactly equal to the
energy difference between the states.
2. The frequency (ν) or wavelength (λ) of energy absorbed or emitted
is related to the energy difference by Planck’s Law.
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Although we will soon begin with a discussion of molecular (UV-Vis)
absorption, look at the vapor phase emission spectrum of Na, along with
its atomic energy level diagram.
Higher energy orbital  Lower
energy orbital: Emission
Lower energy orbital  Higher
energy orbital  Absorption
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The absorption spectra of molecules are very different from the
absorption spectra of atoms.
The energy of an atom Eatom = Eelectronic
The energy of a molecule Emolecule = Eelectronic +
Evibrational + Erotational
Quantitative relationships of major classes of spectrochemical methods:
Since we will start with two absorption spectrochemical methods
(molecular and atomic UV-Visible absorption), a short discussion of the
concentration relationship for this class of methods ensues.
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Block diagram for UV-Vis molecular absorption instrument:
Focusing in on the sample (cuvet) with pathlength b:
Questions/Problems Chapter 6:
1f,g,l,m, 2-4, 7, 8, 14-17
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