FTIR Spectroscopy

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 realm.

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.
IR Spectroscopy Schematic 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.
Symmetrical Stretching Asymmetrical Stretching Scissoring Twisting Rocking Wagging
Symmetric         Asymmetric      Scissoring          Twisting             Rocking            Wagging  
                             Stretch               Stretch

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.

Forensic Spectroscopy


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