Difference Between FTIR and Raman Spectroscopy: A Comparison

Both FTIR and Raman spectroscopy provide molecular fingerprints of samples by using light-matter interactions.

In FTIR spectroscopy, infrared light spanning a spectrum of wavelengths is passed through a sample. Functional groups in the material can absorb specific frequencies of this IR light which excites the molecules into a higher vibrational state. The light that isn’t absorbed is transmitted through the sample. By examining the frequencies attenuated in the resulting IR spectrum, the molecular makeup can be determined.

In Raman spectroscopy, a monochromatic visible, near-IR or near-UV laser light illuminates the sample. Most of the light scatters elastically at the same frequency in a process called Rayleigh scattering. But a very small portion of light can interact with molecules and scatter inelastically at shifted frequencies in what is known as the Raman effect. The shifts in the scattered light relate to the vibrational modes of the molecules, allowing identification.

Key Differences

There are several key differences between these two vibrational spectroscopic techniques:

• FTIR relies on light absorption, whereas Raman relies on light scattering effects.
• FTIR can analyze fundamental vibrational modes, while Raman detects changes in polarizability via subtle vibrational shifts.
• FTIR can readily analyze symmetric, polar bonds like O-H groups. Raman more easily detects nonpolar groups.
• Sample preparation and measurement times tend to be faster for FTIR versus Raman.
• Raman provides higher spatial resolution and can more easily analyze aqueous solutions compared to conventional FTIR.

Determining Which Technique to Use

In summary, FTIR provides a wider overall range and faster speeds, while Raman boasts finer spatial detail and aqueous suitability. Materials with significant polar bonds tend to be better studied by FTIR, while symmetric and nonpolar substances are preferred targets for Raman analysis. The strengths and limitations of the two techniques are complementary. So both FTIR and Raman spectroscopy continue to serve as invaluable tools for elucidating molecular structure and composition across scientific disciplines.