FTIR: Fourier-Transform Infrared Spectroscopy Principles and Applications

Fourier-transform infrared spectroscopy (or FTIR, for short) is a method of exploring the physical properties of solids, liquids, and gases. More specifically, it allows the study of the absorptive and emissive properties of materials. The choice of IR wavelengths in FTIR method is particularly useful for studying chemical bonds that connect atoms. It turns out that these bonds tend to vibrate with frequencies that correspond to infrared-wavelength light, and hence can easily be excited with such a radiation. Commonly, IR wavelength spectroscopy covers wavenumbers ranging from 200 to 4000 cm^-1

Introduction to FTIR Spectroscopy

The basic premise of any infrared or vibrational spectroscopy technique is the observation of how light is scattered or absorbed upon reaching a material. In inelastic collisions between infrared light and molecules (either at the surface of a solid material or as a gas or liquid) some collisions produce characteristic vibrations of varying modes dependent on the nature of the bond itself, but with a characteristic vibrational frequency.

While the full scope of such vibrational modes is vast, the vibrational modes can either be Raman active (i.e. light is scattered due to change in polarizability during vibration), IR active (i.e. light is absorbed due to change in dipole during vibration), or both. For this reason, Raman spectroscopy and IR spectroscopy tend to be complementary techniques, as they provide different spectra depending on the chemical bonds present in the sample. For example, ionic bonds such as C-H tend to appear as strong peaks in IR spectroscopy, and covalent bonds such as C=C tend to appear as strong peaks in Raman spectroscopy. You can read more about Raman spectroscopy and microscopy in one of our prior blog posts here.

How does FTIR Spectroscopy Work?

Basic schematic of Michelson interferometer, courtesy of Leng, “Materials Characterization”

The most important component of FTIR spectroscopy, and where it derives its distinction from typical infrared spectroscopy, is the Michelson interferometer. Whereas in dispersive IR spectroscopy monochromatic light sequentially irradiates a sample spanning an infrared wavelength range, FTIR spectroscopy irradiates a sample with many frequencies of IR light at once, utilizing an interferometer and post-processing of the transmitted light.

Without going too much into details, a beam source of various IR wavelength light is sent through a beam splitter, where half reaches a fixed mirror and half a mirror that moves with a constant velocity. These two split beams are then reflected and recombined (now with a path difference between the beams) to construct an interference pattern reflecting the constructive and destructive interference of the recombination. After, this interference pattern (or interferogram) is sent to the sample, and the transmitted portion of the interferogram is sent to a detector. After comparison with a reference sample beam spectrum in the detector, a Fourier transform is performed to obtain the full spectrum as a function of wavenumber.

While all of this might seem quite arcane when compared to the seemingly simple technique of sequential monochromatic irradiation, Fourier-transform spectroscopy offers a number of key benefits that have made it the dominant method of IR spectroscopy. When information from all wavelengths is collected at once, there tends to be signal-to-noise ratio reduction in the output spectra. Furthermore, the more obvious benefit of performing all wavelength measurements at once is the speed reduction when compared to dispersive wavelength techniques. However, since FTIR has been unquestioned industry standard for many decades now, it is possible that forthcoming technique breakthroughs or advances in dispersive techniques may yet catch up.

FTIR: What is it Good For?

Applications of FTIR truly run the gamut, since just about all materials are able to be analyzed by FTIR spectroscopy to provide valuable information, but it has perhaps found its most frequent use in the analysis of carbon-containing organic compounds, for which there is a comprehensive knowledge base of characteristic vibrational frequencies. As previously mentioned, chemical bonds between atoms vibrate at characteristic frequencies which have been widely tabulated, allowing for determination of the composition of samples or presence of potential contaminants by comparing the peaks of an IR spectra with tabulated characteristic frequencies.

Furthermore, quality analysis of various samples can also be determined using FTIR. For example, deviations in the characteristic spectra from a material can be indicators of oxidation or other contamination. In polymers, deviations from an expected IR spectrum can indicate a different degree of polymerization (curing) or decomposition from a reference sample. For these reasons, FTIR is widely used in such disparate applications as polymer characterization, forensic analysis, pharmaceutical analysis, and food testing.

State-of-the-Art and Future Developments

Because infrared spectroscopy is naturally diffraction-limited in the information it can obtain from any given sample, new methods have been devised to perform IR probing on the nanoscale. Nano-FTIR is a technique utilizing scanning probe microscopy and FTIR in a complementary fashion to provide infrared spectrographic information with a special resolution on the order of 10-20nm.

In Nano-FTIR, the whole FTIR setup as previously described illuminates the atomic force tip of an atomic force microscope probe with broadband infrared light, which illuminates the incoming light, creating a strong near IR field on the surface of the sample. The infrared light back-scattered from the tip of the atomic probe is then collected and an IR spectrum can then be constructed from an area on the order of tens of nanometers. A recent invention first patented in 2005, nano-FTIR has seen widespread use in probing the secondary structure of proteins, nanocomposites, and organic thin-film materials.

Basic diagram of Nano-FTIR setup combining atomic force microscopy and FTIR characterization techniques. Courtesy of Wikimedia commons..

While revolutionary changes in how FTIR is fundamentally operated are likely not in the cards, there are numerous domains in which FTIR technology continues to improve. The continuing development of deep learning techniques may decrease manual lookup of tabulated characteristic vibrational frequency values, while developments in IR source devices and detector devices may continue to increase efficiency. As we continue pushing the lower limits of spatial resolution in material characterization demands, it is likely that more techniques to bring IR spectroscopy principles to the nanoscale will be seen.

On FindLight’s photonics marketplace you can find a variety of FTIR devices and accessories as well as a series of Raman Spectrometers.