Raman spectroscopy and FTIR spectroscopy are both powerful vibrational techniques used to identify and characterise molecules, but they rely on different light–matter interactions. FTIR measures infrared light absorption caused by changes in a molecule’s dipole moment, while Raman detects inelastic scattering resulting from changes in polarizability. This fundamental difference makes them complementary methods—FTIR is more […]
Raman spectroscopy and FTIR spectroscopy are both powerful vibrational techniques used to identify and characterise molecules, but they rely on different light–matter interactions. FTIR measures infrared light absorption caused by changes in a molecule’s dipole moment, while Raman detects inelastic scattering resulting from changes in polarizability. This fundamental difference makes them complementary methods—FTIR is more sensitive to polar bonds (e.g., C–O), whereas Raman excels with nonpolar or symmetric bonds (e.g., O–O), inorganic compounds, and aqueous samples. Together, they provide a comprehensive understanding of molecular structure and chemical composition.
| Parameter | Raman Spectroscopy | Infrared (IR) Spectroscopy / FTIR |
|---|---|---|
| Basic Principle | Based on inelastic scattering (Raman scattering) of monochromatic light when photons interact with molecular vibrations. | Uses a broad-spectrum infrared radiation source. |
| Source of Radiation | Uses monochromatic visible or near-IR laser light. | Simple and minimal; can analyse solids, liquids, or gases directly. |
| Type of Molecular Change | Involves a change in the polarizability of the molecule. | Involves a change in the dipole moment of the molecule. |
| Interaction with Light | Measures scattered light. | Measures absorbed light. |
| Sample Preparation | Simple and minimal; can analyze solids, liquids, or gases directly. | May require KBr pellet, Nujol mull, or solvent (like CCl₄) for sample preparation. |
| Water Interference | Unaffected by water, since water is a weak Raman scatterer. | Strongly affected by water, as water absorbs in the IR region. |
| Suitable Sample Types | Ideal for aqueous samples, crystals, and biological materials. | Ideal for organic, polymeric, and inorganic solids or liquids. |
| Typical Region Measured | Raman shift: 50–4000 cm⁻¹. | Wavenumber range: 4000–400 cm⁻¹. |
| Instrumentation | Consists of laser source, sample cell, monochromator, detector, and computer. | Consists of IR source, interferometer (FTIR), sample compartment, and detector. |
| Spectral Appearance | Shows sharp, distinct peaks corresponding to symmetric vibrations. | Shows broader peaks for polar functional groups. |
| Sensitivity | Typically less sensitive for low-concentration samples. | Usually more sensitive for polar functional groups. |
| Fluorescence Interference | Fluorescence can interfere with Raman signals. | No fluorescence interference. |
| Complementarity | Detects vibrations inactive in IR but Raman active. | Detects vibrations active in IR but Raman inactive. |
| Quantitative Analysis | Possible, but mainly used for qualitative and structural studies. | Widely used for both qualitative and quantitative analysis. |
| Common Applications | – Material characterization – Crystallinity studies – Pharmaceutical polymorphs – Biological systems | – Functional group identification – Purity testing – Reaction monitoring – Polymer and pharmaceutical analysis |
| Cost and Maintenance | More expensive and sensitive to alignment. | Relatively affordable and easy to maintain. |
| Result Output | Raman spectrum (intensity vs. Raman shift). | IR spectrum (absorbance or transmittance vs. wavenumber). |
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A Raman spectrum is a plot that shows how a sample scatters light when it is illuminated with a monochromatic light source, typically a laser. It displays the intensity of scattered light as a function of Raman shift (measured in cm⁻¹), which corresponds to the vibrational energy levels of the molecules in the sample.
In simple terms, the Raman spectrum provides a molecular fingerprint, revealing information about:
Because each molecule has a unique vibrational pattern, Raman spectroscopy is widely used for identification, structural analysis, and material characterisation.
While both Raman and IR spectroscopy probe molecular vibrations, their fundamental mechanisms and applications differ.
In practice, scientists often use both techniques together to obtain comprehensive chemical and structural information — enhancing accuracy in pharmaceutical analysis, materials science, and molecular research.
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FTIR spectra show peaks due to absorption of infrared light by bonds that change their dipole moment, while Raman spectra show peaks due to scattering of monochromatic light caused by changes in polarizability. As a result, some vibrational modes appear in one spectrum but not the other, making the two techniques complementary.
IR spectroscopy measures light absorption in the infrared region and is sensitive to polar bonds (like O–H or C=O).
Raman spectroscopy measures scattered light from a laser source and is sensitive to nonpolar or symmetric bonds (like C=C or S–S).
FTIR spectroscopy records the absorption of infrared light across multiple wavelengths simultaneously using a Fourier transform, producing a detailed molecular “fingerprint.”
Raman spectroscopy uses a laser to detect shifts in scattered light energy, revealing molecular vibrations. Both are vibrational spectroscopic techniques used for chemical identification and structural analysis.
Raman spectroscopy is generally better for microplastics because it requires minimal sample preparation, can analyze smaller particles (<1 µm), and is unaffected by water. However, FTIR is also widely used for larger particles and quantitative studies. In practice, combining both techniques gives the most reliable identification.
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