FTIR and Raman spectroscopy offer distinct molecular insights. Learn how their differing principles—one based on absorption, the other on scattering—make them complementary tools for material analysis.
Vibrational spectroscopy analyzes how molecules vibrate and interact with light, providing a unique “fingerprint” for different substances. Two prominent methods in this field are Fourier Transform Infrared (FTIR) and Raman spectroscopy. Both techniques identify materials by examining their molecular composition and structure, but each probes these vibrations differently to reveal specific details about chemical bonds.
Fourier Transform Infrared Spectroscopy (FTIR) operates by measuring how a sample absorbs infrared radiation. A beam of IR light containing many frequencies is passed through a substance. The molecules absorb light at specific frequencies that correspond to their natural vibrations, much like a tuning fork resonates at a particular pitch. This selective absorption creates a spectral fingerprint based on the chemical bonds and functional groups present.
At the heart of an FTIR instrument is an interferometer, most commonly a Michelson interferometer, which processes all frequencies of the IR light simultaneously. This device splits the IR beam, sends it down two paths, and then recombines it to create an interference pattern called an interferogram. This interferogram contains all the necessary frequency information.
The interferogram is not directly interpretable as a spectrum. A mathematical process known as a Fourier Transform is required to convert the interferogram into the final IR spectrum. The resulting spectrum displays absorbance or transmittance as a function of frequency, showing the peaks that correspond to the absorbed energy. This allows for the identification of functional groups like C=O (carbonyl) or O-H (hydroxyl) bonds.
Raman spectroscopy relies on the scattering of light rather than its absorption. The process begins when a high-intensity, single-wavelength light source, typically a laser, illuminates a sample. Most of the light that scatters off the molecules does so elastically, with the same energy as the incident laser light, in a process known as Rayleigh scattering.
A very small fraction of the scattered light, roughly one in ten million photons, is scattered inelastically. This phenomenon, the Raman effect, involves a transfer of energy between the laser photons and the sample’s molecules. The scattered photons emerge with either less or more energy than they had initially.
This change in photon energy directly corresponds to the vibrational energy levels of the molecule’s bonds. A Raman spectrum is generated by plotting the intensity of this inelastically scattered light against its energy shift relative to the laser source. This allows the technique to provide information that is complementary to FTIR.
The main distinction between FTIR and Raman spectroscopy is their interaction with molecules: FTIR measures light absorption, while Raman measures inelastic light scattering. This leads to different “selection rules” dictating which molecular vibrations are active. FTIR is sensitive to polar bonds with a change in dipole moment during vibration, such as the O-H bond in water. In contrast, Raman excels at detecting vibrations in non-polar, symmetric bonds, like C-C and S-S bonds, which exhibit a change in polarizability.
This difference has practical consequences for sample analysis. Water is a very strong absorber of infrared light, which can overwhelm the signal in FTIR analysis of aqueous solutions. Raman spectroscopy, however, is largely unaffected by water, making it well-suited for studying biological samples or reactions in water-based solvents. Conversely, some samples can exhibit fluorescence when illuminated by a laser, which can obscure the much weaker Raman signal, a problem that does not affect FTIR measurements.
Sample preparation also differs between the two methods. FTIR often requires samples to be prepared in specific ways, such as being pressed into a potassium bromide (KBr) pellet or analyzed using an Attenuated Total Reflectance (ATR) accessory. Raman analysis can frequently be performed directly on a sample with little to no preparation, even through transparent containers like glass vials. While the Raman signal is inherently weaker, Raman microscopy can achieve higher spatial resolution, allowing for more detailed mapping of a sample’s surface.
The distinct capabilities of FTIR and Raman spectroscopy lend them to a wide array of applications, sometimes used in a complementary fashion. In the pharmaceutical industry, their uses include:
In material science, FTIR is used for identifying polymers and studying their degradation processes. Raman spectroscopy is used for characterizing carbon-based materials like graphene, carbon nanotubes, and diamond-like carbon films due to its sensitivity to symmetric carbon-carbon bonds. It is also used in the semiconductor industry to measure stress and crystal quality.
Environmental scientists use both techniques to identify pollutants. FTIR is applied to monitor atmospheric gases, while both methods are used to identify and quantify microplastics. Forensic science laboratories rely on FTIR and Raman to analyze trace evidence, such as fibers, paint chips, and unknown powders.
These techniques also offer non-destructive analysis for cultural artifacts. In art and archaeology, FTIR and Raman can identify pigments, binding media, and degradation products on paintings and ancient objects. In the biomedical field, researchers are exploring the use of both methods for disease diagnostics, such as identifying molecular changes associated with cancer cells and analyzing the composition of tissues.