Introduction to Spectrochemical Methods

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Chapter 24

• Introduction to Spectrochemical Methods

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Spectroscopy deals with the interactions of
radiation and matter. Spectroscopic analytical
methods are based on measuring the amount of
radiation produced or absorbed by molecular or
atomic species of interest. Spectroscopic
methods are classified according to the region of
the electromagnetic spectrum used or produced
such as G-ray, X-ray, ultraviolet (UV),
visible, infrared (IR), microwave, and
radio-frequency (RF). Spectrochemical methods
are the most widely used tools for the
elucidation of molecular structure as well as the
quantitative and qualitative determination of
both inorganic and organic compounds.
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24A Properties of Electromagnetic
Radiation Electromagnetic radiation is a form
of energy that is transmitted through space at
enormous velocities. Electromagnetic radiation
can be described as a wave with properties of
wavelength, frequency, velocity, and
amplitude. The wave model fails to account for
phenomena associated with the absorption and
emission of radiant energy. For these
processes, electromagnetic radiation can be
treated as discrete packets of energy or
particles called photons or quanta.
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The amplitude of an electromagnetic wave is a
vector quantity that provides a measure of the
electric or magnetic field strength at a maximum
in the wave. The period of an electromagnetic
wave is the time in seconds for successive maxima
or minima to pass a point in space. The
frequency of an electromagnetic wave is the
number of oscillations that occur in 1s. The
unit of frequency is the hertz (Hz), which
corresponds to one cycle per second, that is, 1
Hz 1 s-1. The frequency of a beam of
electromagnetic radiation does not change as it
passes through different media.
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The product of the frequency in waves per unit
time and the wavelength in distance per wave is
the velocity v of the wave in distance per unit
time (cm s-1 or m s-1). v ?? In a vacuum,
light travels at its maximum velocity. This
velocity, c, is 2.99792 ? 108 m s-1. The
velocity of light in air is only about 0.03
less than its velocity in vacuum. c ??
3.00 ? 108 m s-1 3.00 ? 1010 cm s-1
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The wavenumber, , is another way to describe
electromagnetic radiation. It is defined as the
number of waves per centimeter and is equal to
1/?. By definition, has the units of
cm-1. The wavenumber in cm-1 (Kayser) is most
often used to describe radiation in the infrared
region.
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Radiant Power and Intensity The radiant power,
P, in watts (W) is the energy of a beam that
reaches a given area per unit time. The
intensity is the radiant power-per-unit solid
angle. The Particle Nature of Light
Photons The energy of a single photon is related
to its wavelength, frequency, and
wavenumber E h? hc/? hc where h is
Plancks constant (6.63 ? 10-34 Js).
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24B Interaction of radiation and matter The
electromagnetic spectrum covers an enormous range
of energies (frequencies) and thus wavelengths.
Useful frequencies vary from gt1019 Hz (g-ray)
to 103 Hz (radio waves).
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Spectrochemical methods that use not only visible
but also ultraviolet and infrared radiation are
often called optical methods. Spectroscopists
use the interactions of radiation with matter to
obtain information about a sample. Several
chemical elements were discovered by
spectroscopy. The analyte is predominately in
its lowest-energy or ground state. The sample
is then stimulated in some way by applying energy
in the form of heat or electrical energy
(emission spectroscopy) , light, particles, or a
chemical reaction (chemiluminescence
spectroscopy). The stimulus causes some of the
analyte species to undergo a transition to a
higher-energy or excited state. Information
about the analyte is obtained by measuring the
electromagnetic radiation emitted as it returns
to the ground state.
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In absorption spectroscopy, the amount of light
absorbed as a function of wavelength is
measured. Absorption measurements can give both
qualitative and quantitative information about
the sample. In photoluminescence spectroscopy,
the emission of photons is measured following
absorption. The most important forms of
photoluminescence for analytical purposes are
fluorescence and phosphorescence spectroscopy.
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24 C Absorption of radiation Every molecular
species is capable of absorbing its own
characteristic frequencies of electromagnetic
radiation. This process transfers energy to the
molecule and results in a decrease in the
intensity of the incident electromagnetic
radiation. Absorption of the radiation thus
attenuates the beam in accordance with the
absorption law. The absorption law, also known
as the Beer-Lambert law describes quantitatively
how the amount of attenuation depends on the
concentration of the absorbing molecules and the
path length over which absorption occurs.
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The transmittance T of the solution is the
fraction of incident radiation transmitted by the
solution. Transmittance is often expressed as a
percentage and called the percent
transmittance. T P/P0 The absorbance, A,
of a solution is related to the transmittance in
a logarithmic manner.
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Transmittance and absorbance usually cannot be
measured because the solution to be studied must
be held in a container (cell or cuvette).
Reflection and scattering losses can occur at
the cell walls.
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To compensate for these effects, the power of the
beam transmitted through a cell containing the
analyte solution is compared with one that
traverses an identical cell containing only the
solvent, or a reagent blank. An experimental
absorbance that closely approximates the true
absorbance for the solution is thus obtained,
that is, A log P0/P ? log
Psolvent/Psolution
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According to Beers law, absorbance is directly
proportional to the concentration of the
absorbing species, c, and to the path length, b,
of the absorbing medium A log(P0/P)
abc a is a proportionality constant called the
absorptivity. Concentration expressed in moles
per liter and b in cm, gives the proportionality
constant, called the molar absorptivity,
?. A ?bc
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Beers law also applies to solutions containing
more than one kind of absorbing substance.
Provided that there is no interaction among
the various species, the total absorbance for a
multicomponent system at a single wavelength is
the sum of the individual absorbances. Atotal
A1 A2 An ?1bc1 ?2bc2
?nbcn where the subscripts refer to absorbing
components 1, 2, . . . , n.
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An absorption spectrum is a plot of absorbance
versus wavelength (or wavenumber or
frequency). A plot of molar absorptivity e as a
function of wavelength is independent of
concentration. This type of spectral plot is
characteristic for a given molecule and is
sometimes used to aid in identifying or
confirming the identity of a particular species.
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The major atomic absorption transitions occur
when the single outer electron of sodium is
excited from its room temperature or ground state
3s orbital to the 3p, 4p, and 5p orbitals.
These excitations are brought on by absorption
of photons of radiation whose energies exactly
match the differences in energies between the
excited states and the 3s ground state.
Transitions between two different orbitals are
termed electronic transitions.
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Molecules undergo three types of quantized
transitions when excited by ultraviolet,
visible, and infrared radiation. In addition to
electronic transitions, molecules exhibit two
other types of radiation-induced transitions
Vibrational transitions and rotational
transitions. Vibrational transitions occur
because a molecule has a multitude of quantized
energy levels, or vibrational states, associated
with the bonds that hold the molecule together.
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The total energy E associated with a molecule
is E Eelectronic Evibrational
Erotational where Eelectronic is the energy
associated with the electrons in the various
outer orbitals of the molecule, Evibrational is
the energy of the molecule as a whole due to
interatomic vibrations and Erotational accounts
for the energy associated with rotation of the
molecule about its center of gravity.
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There are few exceptions to the linear
relationship between absorbance and path length
at a fixed concentration. Some of these
deviations, called real deviations, are
fundamental and represent real limitations to the
law. Others are a result of the method that we
use to measure absorbance (instrumental
deviations) or from chemical changes that occur
when the concentration changes (chemical
deviations).
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Beers law describes the absorption behavior only
of dilute solutions and in this sense is a
limiting law. At concentrations exceeding about
0.01 M, the average distances between ions or
molecules of the absorbing species are diminished
to the point where each particle affects the
charge distribution and thus the extent of
absorption of its neighbors. Because the extent
of interaction depends on concentration, the
occurrence of this phenomenon causes deviations
from the linear relationship between absorbance
and concentration.
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24 D Emission of electromagnetic
radiation Chemical species can be caused to emit
light by (1) bombardment with electrons (2)
heating in a plasma, flame, or an electric arc
or (3) irradiation with a beam of light.
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Radiation from a source is conveniently
characterized by means of an emission spectrum,
which usually takes the form of a plot of the
relative power of the emit- ted radiation as a
function of wavelength or frequency. Three types
of spectra are superimposed in the figure a
line spectrum, a band spectrum, and a continuum
spectrum. The line spectrum consists of a
series of sharp, well-defined spectral lines
caused by excitation of individual atoms that are
well separated, as in a gas. The band spectrum,
marked bands, is comprised of several groups of
lines so closely spaced that they are not
completely resolved. The source of the bands is
small molecules or radicals in the source flame.
Finally, the continuum spectrum is responsible
for the increase in the background that appears
above about 350 nm.
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A spectral continuum of radiation is produced
when solids such as carbon and tungsten are
heated to incandescence. Thermal radiation of
this kind, which is called blackbody radiation,
is more characteristic of the temperature of the
emitting surface than of its surface material.
The radiant power P of a line or a band depends
directly on the number of excited atoms or
molecules, which in turn is proportional to the
total concentration c of the species present in
the source. P kc
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When excitation and emission wavelengths are the
same, the resulting emission is called resonance
fluorescence. Fluorescence is a
photoluminescence process in which atoms or
molecules are excited by absorption of
electromagnetic radiation. Nonradiative
relaxation and fluorescence emission are two
mechanisms by which an excited atom or molecule
to give up its excess energy and returns to its
ground state.
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• There are two types of nonradiative relaxation
• Vibrational deactivation, or relaxation takes
  place during collisions between excited molecules
  and molecules of the solvent.
• Internal conversion occurs when there is
  relaxation between the lowest vibrational level
  of an excited electronic state and the upper
  vibrational level of another electronic state can
  also occur.
• The Stokes shift refers to fluorescence radiation
  that occurs at wavelengths that are longer than
  the wavelength of radiation used to excite the
  fluorescence.