Lecture 6.
FT-IR and Raman Spectroscopy
FT-IR
• Analytical infrared studies are based on
the absorption or reflection of the
electromagnetic radiation that lies between
1 and 1000µm.
• Infrared or IR analysis is one of the most
common spectroscopic techniques used
for compound identification and
concentration measurements.
Three forms of IR
• Near IR – (1 – 2.5µm)
• Mid IR – (2.5 - 50µm)
• Far IR – (beyond 50µm)
Principle
• The absorption of light in the near and the
mid IR regions occurs when the chemical
bonds of a sample interact with the
radiation of a light source.
• When the chemical bonds contain different
atoms situated at the two ends of the
bond, they form an electric dipole which
oscillates at a set frequency.
• Due to the asymmetry of the bonds,
interactions between the bonds and
monochromatic light can occur if the
frequency of the light is equal to the
frequency of the dipole.
• Therefore, the electrical component of the
light can transfer its energy to the bond if
the mechanical frequency of the bond and
the electromagnetic frequency of the light
are the same.
Absorptions in the Infrared
• When a range of radiation wavelengths
are used, the generated data or infrared
absorption information is presented in the
form of a spectrum, which is the basic
document issued from a spectrometer.
• The values of wavelengths are substituted
by their equivalent wavenumbers v‾ whose
units are in cm-1.
Optical layout of FT-IR
spectrometer
Optical layout contd.
Sample preparation
• Almost any solid, liquid or gas sample can be analyzed
• Little or no preparation is required, may have to grind
solid into KBr matrix or dissolve sample in a suitable
solvent (CCl4 and CS2 are preferred)
• Water should be removed from sample if possible
• Most samples can be prepared for infrared (IR) analysis
in approximately 1 to 5 min.
limitations
• Minimal elemental information is given for
most samples.
• Background solvent or solid matrix must
be relatively transparent in the spectral
region of interest.
• Molecule must be active in the IR region.
Mid-IR Principles
• Atoms within molecules are in constant
motion and can move in three different
directions. These movements are referred
to as the three classical Cartesian
coordinates.
• All of these movements confer upon each
isolated molecule a combined mechanical
energy.
• The total molecular energy occurs from
the sum of the independent quantified
terms named energy of rotation Erot,
energy of vibration Evib, and electronic
molecular energy Eelec:
Etot = Erot + Evib + Eelec
The values of these energies can be
indepenent from each other based on
Born-Oppenheimer principle.
• For example, a radiation source emitting 1000
cm-1 corresponds to a photon energy E = hcv =
0.125 eV in the mid-IR region.
• If this photon or radiation is absorbed by a
molecule its total energy will be increased by this
energy.
• In theory, the energy of vibration Evib will be
changed but the electronic molecular energy
Eelec will not change due to insufficient transition
energy between two different electronic levels.
• When utilizing IR, samples usually consist
of condensed liquids or solid phases in
pure or diluted solutions.
• For example, in a case study of headbox
analysis of alkaline fine paper, headbox
deposits sample along with spectra of
wood pitch fatty acid salt and the fatty acid
salt of hydrolyzed ASA were taken.
• Deposit spectra was
similar to the reference
material
• The spectra for bentonite
(montmorillonite clay) and
kaolin clay have peaks at
around 3600 cm-1.
• Montmorillonite clay and
kaolin clay had peaks
similar to O-H stretching
• Deposit sample did not
have any peak at
3600cm-1
References
•
Silverstein, M, Robert., Webster, X, Francis.,
Kiemle, D., Spectrometric Identification of Organic
Compounds, John Wiley & Sons; 7 edition