Download Optical Spectroscopy

Instrumental Chemistry
Chapter 7 and 8
Optical SpectroscopyComponents and Introduction
Brief Introduction
Spectroscopic instruments were developed for use
in the visible region are optical instruments.
 Optical spectroscopic methods are based upon six
phenomena namely:
(1) absorption
(2) fluorescence,
(3) phosphorescence
(4) scattering
(5) emission,
(6) chemiluminescene

ABSORPTION
In atoms, electron transitions of outer-shell
electrons correspond to the absorption or emission
of electromagnetic radiation that is in the
ultraviolet and visible regions. Because
vibrational and rotational energy levels are not
possible in atoms, the absorption and emission of
radiation that occurs in molecules when an
electron travels between the numerous vibrational
and rotational energy states of one electron level
are not possible. Only a single transition between
each set of electron levels is possible. As a
consequence, the bands of emitted or absorbed
radiation are narrow in atoms.
Absorption (cont.)

Atomic-absorption (AA) spectroscopy uses
the absorption of light to measure the
concentration of gas-phase atoms. Since
samples are usually liquids or solids, the
analyte atoms or ions must be vaporized in a
flame or graphite furnace. The atoms absorb
ultraviolet or visible light and make transitions
to higher electronic energy levels. The
analyte concentration is determined from the
amount of absorption.
Light Source
 The
light source is usually a hollowcathode lamp of the element that is
being measured. Lasers are also used
in research instruments. Since lasers
are intense enough to excite atoms to
higher energy levels, they allow AA and
atomic fluorescence measurements in a
single instrument. The disadvantage of
these narrow-band light sources is that
only one element is measurable at a
time.
Atomizer

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AA spectroscopy requires that the analyte atoms be in the gas phase.
Ions or atoms in a sample must undergo desolvation and vaporization
in a high-temperature source such as a flame or graphite furnace.
Flame AA can only analyze solutions, while graphite furnace AA can
accept solutions, slurries, or solid samples.
Flame AA uses a slot type burner to increase the path length, and
therefore to increase the total absorbance (see Beer-Lambert law).
Sample solutions are usually aspirated with the gas flow into a
nebulizing/mixing chamber to form small droplets before entering the
flame.
The graphite furnace has several advantages over a flame. It is a much
more efficient atomizer than a flame and it can directly accept very
small absolute quantities of sample. It also provides a reducing
environment for easily oxidized elements. Samples are placed directly
in the graphite furnace and the furnace is electrically heated in several
steps to dry the sample, ash organic matter, and vaporize the analyte
atoms.
FLUORESCENCE
The absorption of energy from a
radioactive source by atoms can result
in atoms in an excited electron level.
Atomic fluorescence occurs when the
excited atoms emit radiation after
initially being excited by absorption of
photons.
Fluorescence

Atomic fluorescence is the optical emission from gas-phase atoms that
have been excited to higher energy levels by absorption of
electromagnetic radiation. The main advantage of fluorescence
detection compared to absorption measurements is the greater
sensitivity achievable because the fluorescence signal has a very low
background. The resonant excitation provides selective excitation of
the analyte to avoid interferences. AFS is useful to study the electronic
structure of atoms and to make quantitative measurements. Analytical
applications include flames and plasmas diagnostics, and enhanced
sensitivity in atomic analysis. Because of the differences in the nature
of the energy-level structure between atoms and molecules, discussion
of laser-induced fluorescence (LIF) from molecules is found in a
separate document.
Types of Fluorescence



Resonant fluorescence: Occurs when the fluoresced radiation is of the
same wavelength as the absorbed radiation. Resonant fluorescence is
the type that is used most often for quantitative analysis.
Direct-line fluorescence: Occurs when an electron in an excited state
emits radiation upon falling, to an electron level that is above the level
that is above from which the electron originally absorbed radiation.
The wavelength of the emitted radiation is longer than the wavelength
of the absorbed radiation.
Stepwise fluorescence: is preceded by absorption and collision
deactivation to a lower excited electron level. Fluorescence occurs
when the atom emits a photon as the electron or by collision
deactivation.
Types of Fluorescence (cont)


Stokes fluorescence: Is fluorescence in which the fluoresced radiation
has a wavelength that is greater than the absorbed radiation. Further
loss of energy to the around state can occur either by emission of
another photon or by collision deactivation.
Anti-stokes fluorescence: It is a form of fluorescence in which the
emitted radiation has a shorter wavelength than the absorbed radiation.
A fifth type of fluorescence can also occur.. After an atom becomes
electronically excited by absorption, the excited atom transfers some or
all of its energy to an atom of a different element. The atom of the
ground state (or some other lower energy level). Atomic fluorescence
of that type, which is rarely encountered, is sensitized fluorescence.
The Spectrofluorometer
 The
sensitivity of fluorescence is
dependent on both the fluorophore and
the instrument. The response of a
fluorophore will depend on the molar
absorptivity and the quantum yield.
These factors are, in general, beyond
the control of the analyst.
Polarization
 Molecule
of interest is randomly oriented in
a rigid matrix (organic solvent at low
temperature or room temperature polymer).
Plane polarized light is used as the
excitation source.
Phosphorescence
Excitation during both fluorescence and
phosphorescence occurs when radiation is
absorbed by an electron in the ground state
of a molecule causing the electron to be
excited to a higher electron state. During
the absorption the electron does not reverse
its spin. Generally excitation occurs from a
singlet ground state to an excited singlet
state.
Phosphorescence (cont)

The transitions between the excited singlet state and the
excited triplet state or between the excited triplet state and
the ground singlet state are examples of intersystem
crossing. The considerable barrier to spin reversal that
exists in a molecule prevents intersystem crossing from
occurring, as rapidly as singlet transitions. Because of that
barrier, phosphorescence occurs on a much longer time
scale than fluorescence.
SCATTERING
Radioactive Scattering is the
change in direction of motion of an
incident photon as it strikes a
particle of the sample.
CHEMILUMINESCENCE:
 Chemiluminescence
occurs after excitation
of a molecule or ion by the energy emitted
during the chemical or biochemical reaction
in which the excited species is a product.
 In many cases, the chemical excited energy
level of a molecule is identical to the energy
level that could be attained by absorption of
electromagnetic radiation.
Chemiluminescence (cont)

Chemiluminescence can occur in the ultraviolet,
visible, or near-infrared regions. The majority of
chemiluminescent reactions occur in the visible
region. Bioluminescence (BL) is
chemiluminescence that occurs in biological
systems. Perhaps the best-known example of
bioltiminescence is that which occurs when
fireflies emit light.
Components of Spectroscopic
Instruments

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

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A stable source of radiant energy
A transparent container for holding the sample
A device that isolates a restricted re-ion of the spectrum for
measurement
A radiation detector, which converts radiant energy to a
usable signal (usually electrical)
A signal (usually electrical)
A signal processor and readout, which displays the
transduced signal on a meter scale, a cathode-ray tube, a
digital meter or a recorder chart
LASERS
Laser
is an acronym for Light
Amplification by Stimulated Emission
of Radiation.
A laser is a device that emits highintensity coherent (in-phase) radiation
over a narrow (typically 0.001 to 0.0 I
nm) bandwidth.
Components of Lasers
MECHANISM OF LASER ACTION
Laser action can be understood by
considering the four processes:
(a) Pumping
(b) Spontaneous emission (fluorescence)
(c) Stimulated emission
(d) Absorption
Atomic Line Widths

The widths of atomic lines are of considerable
importance in atomic spectroscopy. Narrow lines
are highly desirable for both absorption and
emission work because they reduce the possibility
of interference due to overlapping spectra. Line
widths are of prime importance in the design of
instruments for atomic absorption spectroscopy.
Atomization Methods

In order to obtain optical and atomic mass spectra,
the constituents of a sample must be converted to
gaseous atoms or ionized, which then can be
determined by emission, absorption, fluorescence, or
mass spectral measurements. The process by which
the sample is converted into an atomic vapor is called
atomization. The precision and accuracy of atomic
methods are critically dependent upon the
atomization step and the method of introduction of
the sample into the atomization region.
Absorption
Types of Designs of
Optical Instruments
Temporal
Design
Spatial Design
Multiplex Design
References
 www.anachem.umu.se/cgi/jumpstation.exe?
OpticalMolecularSpectroscopy
 www.anachem.umu.se/jumpstation.htm
 www.anachem.umu.se/cgi/jumpstation.exe?
AtomicSpectroscopy
 www.minyos.its.rmit.edu.au/~rcmfa/mstheo
ry.html
 http://www.chemsw.com
 http://www.scimedia.com/chemed/spec/atomic/aa.htm