Fourier Transform Infrared Spectroscopy (FTIR)

Overview

FTIR is a spectroscopy technique that studies the interaction between Infrared radiation and materials (i.e. samples).  FTIR is often the first technique used to characterize materials thought or known to be organic in nature because the analysis is typically fast, sample prep is relatively easy and often non-destructive and the analysis can provide detailed information on the organic composition.  Further, FTIR instruments are inexpensive relative to many other types of analytical equipment making them fairly common place in labs.  It can provide a ‘fingerprint’ of organic compounds meaning that comparing spectra from a sample of unknown composition with a spectral database often allows the identification of the unknown to be determined.  Further, FTIR is extremely versatile being able to analyze a wide array of materials (solids, powders, liquids, gases).  FTIR has detection limits in the 0.1-1wt% allowing it to detect additives and contaminants present in a sample at suitable concentrations.   

 

Infrared radiation can be subdivided into three regions. 

Near IR = 14,000-4000cm-1 (1.7-0.5eV)

Mid IR = 4000-400cm-1 (0.5-0.05eV)

Far IR = 400-10cm-1 (0.05-0.001eV)

 

The Mid IR region is that most commonly used for IR Spectroscopy.  However, more advanced modern instruments allow for analysis in one or both of the other regions as well. 

 

Molecules within materials naturally vibrate at frequencies that are unique to each specific functional group.  These vibrational frequencies are in general controlled by the bond strength and the mass of the atoms.  It so happens that the energies of the Infrared portion of Electromagnetic Radiation are in the same range as the molecular vibrational energies.  (Energy is, of course, directly proportional to frequency.)  Thus, shining IR light onto a sample results in the material absorbing the frequencies corresponding to the specific vibrational frequencies comprising the sample.  The remaining energy will be transmitted through the sample.  In IR Spectroscopy infrared light interacts with a sample and the amount that is absorbed (or transmitted) at each wavelength is measured.  Peaks that occur correspond to the various molecular bonds present within the sample. 

 

An FTIR instrument consists of an IR source, an interferometer, a sample holder and a detector.  The IR source commonly used for Mid IR (MIR) analysis is a SiC element (i.e. Globar Source).  Other sources are necessary if Near or Far IR analysis is of interest.  The IR light passes into an interferometer consisting of a Beam Splitter that directs a portion of the light onto a Fixed Mirror and the remainder onto a Moving Mirror.  The light is reflected off of the two mirrors and recombined at the Beam Splitter producing repetitive interference signals measured as a function of optical path difference between the mirrors as the Moving Mirror is in motion.  The light then passes through the sample followed by a detector.  Pyroelectric and Cooled Photoelectric Detectors are commonly used for Mid-IR analysis.

 

After passing through the sample, the light contains every IR frequency ‘encoded’ within it.  This is the interferogram and it is a time domain spectrum that records the detector response change versus time within the mirror scan.  The interferogram can be decoded using Fourier Transformation (a mathematical procedure) giving an intensity versus frequency spectrum.  In FTIR spectra are typically displayed as % Transmittance or absorbance versus wavenumber (which is directly proportional to frequency by Wavenumber = 1/ λ= ν/c where λ is the wavelength, ν is the frequency and c is the speed of light).

 

FTIR is most commonly used for organic analysis as the bonds within organic molecules tend to be very IR Active (i.e. they absorb IR energies).  The region from 4000-1500cm-1 in FTIR spectra allows for easy identification of various functional groups (e.g. O-H, C-H, N-H, C=O, C≡C, C≡N) present in the material while the region from 1500-400cm-1 contains intricate patterns which can often be used like fingerprints for identifying compounds. 

 

FTIR is less frequently used for inorganic materials as many do not contain IR active bonds and even when present, many inorganic compounds contain heavy atoms resulting in vibrations that do not fall within the common Mid IR region.  Nevertheless, FTIR can be used to aid in characterization of some inorganic compounds.

 

FTIR instruments are often equipped with a microscope capable of shining the IR Light through areas as small as 5um.  This feature is extremely useful in failure analysis applications where specific defects, particles and features are of interest.  By combining a microscope with a linear or 2-D array detector, mapping/imaging is possible as well which can be useful in many applications such as organic multilayer film analyses and mapping cells and tissues in biological research.

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