Infrared spectroscopy is a useful method
for matching unknown substances to known substances in order to identify
them. As a characterization tool, IR spectroscopy can provide certain
structural clues to the overall molecular structure of the unknown substance.
However, other methods must be used in conjuction with IR analysis in order
to fully characterize the new substance. Since forensic science typically
deals with substances that are already known, IR is a useful tool in this
The mid-infrared region is a common region used in IR spectroscopy. This
region spans the wavenumbers from about 400 cm-1
to 4000 cm-1. Infrared photons have energies
similar to the vibrational energies of molecular bonds. Since vibrational
energy transitions are quantized, a bond can be caused to vibrate if it absorbs
a photon with a frequency equal to its natural vibrational frequency. This
absorption of IR photons forms the basis of IR spectroscopy.
How does an IR Spectrometer work?
The older version of an IR spectrometer works by shining light through
a sample chamber and through a solvent reference chamber, then measuring
the amount of radiation absorbed by the sample as compared to that absorbed
by the reference. A detector plots the absorbance (or % transmittance)
as a function of wavenumber. This process gives a spectra for the sample
which may be used to learn information about the sample.
As you can see in the picture to the right, the radiation source beam
is split by a mirror in order to pass through both the sample and the reference
chambers. The light is reflected using mirrors into a monochromator
(labeled splitter on the diagram) which only allows light of a single wavelength
at a time to reach the detector. The detector receives the signals from both
the sample beam and the reference beam. This information goes into the processor
which translates the information into a plot
with wavenumber on the x-axis and intensity
on the y-axis. Intensity is measured as the percent transmittance of
the IR radiation with respect to the reference. In other words, a 100%
transmittance means that the sample absorbed the same amount of radiation
as the reference. A 0% transmittance means that the sample absorbed
all of the radiation. The plot shows 100% trasmittance at the top and
0% at the bottom. The result is a plot with several peaks in the downward
direction. These peaks correspond to frequencies of light that were
absorbed by molecules because they matched the frequencies of the natural
vibration of the molecular bonds. Some spectra will use absorbance
values or reflectance values instead of % transmittance. The variables
used will be chosen based on the desired goal of the study.
This older style was replaced by the FTIR or Fourier Transform Infrared
Spectrometer. Fourier transform is a mathematical function that allows
the entire IR spectrum to be analyzed at once. Instead of passing
through a monochromator, the beam passes into an interferometer where the
mathematical calculation is performed to get a spectrum identical to the
one described above. This type of spectrometer works much more quickly
than the older style because the analysis does not have to be performed
in steps. Additionally, there is no reference chamber, so a blank sample
is run and stored in the memory of the computer to correct for air or solvents.
Any bond in a molecule can undergo several
types of motion. Both stretching and bending motions can absorb IR
radiation. Below are six common types of bond motions for bonds around
an sp3 central atom.
In order for a molecule to absorb IR radiation, the electric component
of the radiation must interact with the bond. This can only occur if
the bond has a change in dipole moment as a result of the vibration. The
oscillating electric field of the radiation would cause alternating stretching
and compressing of a polar bond because the field exerts a force on the
positive end in an opposite way to the negative end. As the electric
field oscillates, the bond would vibrate. If the vibration caused
by the oscillating electric field is equal in frequency to the natural vibration,
the bond can absorb the energy. One can consider the vibrational energy
level of a molecule. If the IR photon has an energy equal to the difference
between two energy states, then the molecule can absorb the photon and jump
to the higher of the two vibrational states.
A bond that has a zero net dipole moment can still absorb IR radiation
because it has small portions of time when it is unsymmetrical due to dispersion
forces and molecular collisions. These absorbances are so weak, that
they are not usually considered when analyzing the IR spectrum.
Even within different molecules, the vibrational frequencies of certain
types of bonds are not highly affected
by the structural environment around the bond. These bonds produce characteristic absorption bands
within a specific range on the IR spectrum. Though many molecules share
similar types of bonds, it is unlikely that any two molecules would produce
exactly the same absorption spectra. When taken as a whole, the small
shifts in frequency that ARE caused by the structural environment of the
bond create a unique spectra for each molecule. The "fingerprint region"
of an IR spectrum falls in the 600 cm-1 to 1400
cm-1 range. This region is where most of
the bending vibrations (and some stretching vibrations) occur. Though this
region is not a useful place to obtain structural information, it is still
characteristic for a given molecule. Since many different molecules
may contain one or more of the same common functional groups, it is the fingerprint
region that allows scientists to differentiate between the spectra of two
molecules. Just like a human fingerprint can be used to identify a person,
the IR spectrum can be used to identify a molecule.