What is the FT-IR Spectroscopy?

FTIR stands for “Fourier transform infrared” and it is the most common form of infrared spectroscopy. All infrared spectroscopies act on the principle that when infrared (IR) radiation passes through a sample, some of the radiation is absorbed. The radiation that passes through the sample is recorded. Because different molecules with their different structures produce different spectra, the spectra can be used to identify and distinguish among molecules. In this way, the spectra are like people’s fingerprints or DNA: virtually unique.

FTIR is the preferred method of infrared spectroscopy for several reasons. First, it does not destroy the sample. Second, it is significantly faster than older techniques. Third, it is much more sensitive and precise.

These benefits of FTIR derive from the use of an interferometer, which is the infrared “source” and which allows for the greater speed, and the Fourier transform. The Fourier transform is a mathematical function that takes apart waves and returns the frequency of the wave based on time. The “output” of the interferometer is not the spectroscopy spectrum we use, but a graph known as an “interferogram.” The Fourier transform converts the interferogram into the infrared spectroscopy spectrum graph we recognize and use.

What is FTIR spectroscopy used for?

FTIR spectroscopy is used in organic synthesis, polymer science, petrochemical engineering, pharmaceutical industry and food analysis. In other words, it has a wide array of applications, from monitoring processes to identifying compounds to determining components in a mixture.

How does FTIR work?

A molecule’s covalent bonds will selectively absorb radiation of specific wavelengths, which changes the vibrational energy in the bond. The type of vibration (stretching or bending) induced by the infrared radiation depends on the atoms in the bond. Because different bonds and functional groups absorb different frequencies, the transmittance pattern is different for different molecules. (Transmittance is the flipside of absorbance.) The spectrum is recorded on a graph with wavenumber (cm–1) recorded on the X-axis and transmittance recorded on the Y-axis. (Wavenumber is 1/wavelength and corresponds to the energy of the vibration of the molecular bonds.)

How to read an FTIR spectrum

Reading the spectrum is a matter of determining which groups and bonds correspond to which peaks. Simple reference tables for the various groups can help and full spectra can be located both on our product pages and on NIST.




An example of an IR Spectrum graph for Glycine.

 

FTIR Spectral Interpretation

We have shown that FTIR spectroscopy is a very powerful tool with many applications, however data interpretation is not straightforward.  By nature, the total spectrum generated is a series function of absorbed energy response (hence the Fourier Transform portion of the name).
The absorbed bands presented in the spectrum are only somewhat discrete and degenerative.  The particular “peak” of energy at a certain wavenumber can move around based on other chemical and matrix factors (as well as by the way the incident energy is introduced).  Therefore we do not simply have a “look up” table to say what a particular band of energy will absolutely belong to.  The spectrum must be interpreted as a whole system and therefore probably demands the most experienced analysts in all of the spectrographic techniques in correctly characterizing the functionality presented.  Yes there are libraries which can yield lookup information but these libraries are limited in scope and depth compared to the millions of industrial chemicals used, and also will not account for mixtures of chemicals which can yield erroneous search information.

Although typically a qualitative tool for material identification, FTIR analysis can also be used as a quantitative tool to quantify specific functional groups, when the chemistry is understood and standard reference materials are available.  The intensity of the absorbance will correlate to the quantity of functionality present in the sample.  For instance, we utilize FTIR for quantitative analysis for characterizing the amount of water in an oil sample and the degree of oxidation and nitration of an oil.  We have even developed a method for characterizing how paraffinic or naphthenic an oil sample is.  It must be noted however that FTIR is a “bulk” analytical technique, in that little information can be gained from trace or small concentrations of material in a sample (typically greater than 5% constituent).

FTIR Sample Introduction Techniques

Proper FTIR analysis is only as good as the ability to introduce and observe the energy from a particular matrix.  Fortunately we have many sample preparation and introduction techniques available in the laboratory to properly analyze the sample.  In the early days of infrared spectroscopy, the only available method of analysis was transmission.  For analysis by transmission, the sample needed to be made translucent to the laser and infrared energy, by directly inserting the sample in the optical path, casting a thin film on a salt crystal, or mixing a powder version of the sample with a salt and casting.

Today, however we have the ability to not only use transmission techniques, but reflectance techniques as well.  Because of the ability to focus and manipulate the incident beam with optics, we generally rely on variations of ATR (Attenuated Total Reflectance) techniques to introduce and observe the energy.  ATR involves using a phenomenon of internal reflectance to propagate the incident energy.  The beam is introduced to a crystal at an incident angle which enables internal reflectance “bounces” at the bottom and top of the crystal before it leaves the crystal on the opposite side.  The sample is made to make contact with the crystal at the top such that energy interaction occurs at the crystal and sample interface where the bounce positions are located.  Typically the more bounce positions, the larger the energy transfer (and thus better spectral response), however single bounce systems are used when a very small area needs to be analyzed.