Raman vs. FTIR: Selecting the Best Tool
Vibrational spectroscopy stands at the heart of modern materials characterization. Two work-horse techniques—Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy—often compete for bench space in analytical laboratories. Both interrogate molecular vibrations, but the physical principles, sampling considerations, and accessory ecosystems differ enough that choosing the right tool can save days of method development and thousands of dollars in operational costs.
This guide compares Raman and FTIR for polymers, pharmaceuticals, and other industrially relevant samples. We explore:
- Fundamental differences: light scattering vs. absorption
- Sample states and preparation requirements
- Fluorescence challenges in Raman; water interference in FTIR
- Cost of ownership, maintenance, and typical accessories
By the end, you’ll have a practical decision workflow—and direct product links—to select the optimal technique for your application.
1. Fundamental Physics: Scattering vs. Absorption
1.1 Raman: Inelastic Light Scattering
Raman spectroscopy measures the tiny fraction (< 1 in 106 photons) of incident laser light that is inelastically scattered by sample molecules. When photons interact with molecular bonds, most are elastically scattered (Rayleigh scattering) without energy change. A minuscule proportion exchanges vibrational energy with the molecule, emerging shifted in frequency. Plotting intensity vs. shift (cm-1) yields the Raman spectrum.
1.2 FTIR: Broadband Absorption
FTIR relies on absorption of infrared radiation. A Michelson interferometer modulates a broadband IR source; molecular bonds selectively absorb at characteristic frequencies. Fourier transformation converts the measured interferogram into an absorption spectrum.
| Feature | Raman | FTIR |
|---|---|---|
| Interaction Mechanism | Inelastic scattering | Photon absorption |
| Typical Excitation | NIR or visible laser | Mid-IR broadband source |
| Spectral Units | Shift (cm-1) | Wavenumber (cm-1) |
| Selection Rules | Change in polarizability | Change in dipole moment |
The complementary selection rules mean that some vibrations are strong in Raman but weak in FTIR—and vice-versa. For heterogeneous samples such as filled polymers, collecting both spectra can be synergistic.
2. Sample States & Preparation
2.1 Solids
- Raman: Minimal preparation. A fiber-optic probe or microscope objective can interrogate pellets, films, or bulk parts directly—even through transparent packaging.
- FTIR: Requires an optical path free of scattering in most sampling configurations. Attenuated Total Reflectance (ATR) accessories simplify solid analysis by pressing the sample against a crystal, but surface contact is essential.
2.2 Liquids
- Raman: Glass vials are typically transparent to the excitation laser and Raman scatter, enabling non-destructive, in-container measurements. Dilution is rarely necessary.
- FTIR: Water is a strong absorber in the mid-IR. Measurements may need short pathlength cells, D2O substitution, or an ATR crystal with high refractive index (e.g., diamond).
2.3 Powders & Slurries
- Raman: Can analyze through glass or via remote probes submerged in slurries. No pressing or pelletizing.
- FTIR: KBr pellet or diffuse reflectance accessories often used. ATR can still work but may trap air pockets, degrading spectra.
3. Common Pitfalls: Fluorescence vs. Water Absorption
3.1 Fluorescence in Raman
Organic dyes, natural pigments, and some polymer additives fluoresce under visible or NIR lasers, swamping Raman signals. Mitigation strategies include:
- Longer wavelength excitation (e.g., 1064 nm)
- Time-gated detectors
- Photobleaching or sample pretreatment
- Shift to FTIR if fluorescence is insurmountable
3.2 Water Interference in FTIR
Water exhibits broad O–H stretching (∼3400 cm-1) and bending (∼1640 cm-1) bands that mask analyte peaks. Strategies include:
- Use ATR to shorten effective pathlength
- Employ dry-air purging and desiccated accessories
- Select Raman for aqueous samples, as water is a weak Raman scatterer
4. Cost of Ownership & Maintenance
4.1 Instrument Hardware
- Bench-top Raman: New for $35–150 k USD depending on resolution, laser wavelength(s), and microscopy options.
- FTIR Spectrometer: New for $20–80 k USD; adding ATR or gas cells increases cost. White Bear Photonics offers many refurbished FTIR spectrometers to allow more affordable entry at 10-20k.
4.2 Consumables & Wear Parts
White Bear Photonics manufactures and distributes replacement parts for FTIR and Raman systems. Several repair guides are available for common maintenance activities in our resource center. A few parts that need to be replaced with your FTIR or Raman over time to keep them running:
- Raman: Lasers degrade over 5–10 years; fiber probes may need replacement if fouled.
- FTIR: HeNe lasers in FTIR systems degrade over 5-10 years. IR sources become noisier over time and need to be replaced 6 months - 2 years depending on usage.
4.3 Calibration & Service
Both techniques require wavelength calibration and periodic alignment and noise checks. If you need help with this please reach out to us.
5. Accessory Ecosystem
5.1 Raman
White Bear Photonics supports a Raman applications with OEM and DIY systems to allow deep integration between Raman insights with your process applications.
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Raman probes for inline and at-line measurements of liquids and solids. This is the most common Raman system type.
- Raman microscopes measuring small chemical domains in inhomogeneous materials.
- SERS enhanced measurements for trace analysis.
- Custom sampling systems, if you need help designing for constructing a sample system specifically for your samples contact us
5.2 FTIR
White Bear Photonics has numerous new and used IR accessories that can help expand your analytical capabilities.
- ATR, is a low preparation technique for examining solids and liquids. It is the most common implementation of IR. It is often used for material verification, identification and incoming inspection of materials.
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DRIFTS, is primarily for powder analysis.
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Transmission, excels for quantitative work in liquids and gases. It can be used to analyze slipstreams of processes. It also has applications for qualitative identification and quality control for polymer thin films and coatings.
- IR Microscopy and mapping measuring small chemical domains in inhomogeneous materials. We carry have compact, economical IR microscopy systems to help with your analysis.
- Reaction Probes are specialty systems for monitoring chemical reactions with a mid-IR reaction probe.
- Custom sampling systems, if you need help designing for constructing a sample system specifically for your samples contact us
6. Decision Workflow
Use the flowchart below as a quick reference.
1) Is the sample aqueous?
- Yes → Raman preferred (water weak)
- No → Go to 2
2) Is fluorescence likely (dyes, pigments)?
- Yes → FTIR preferred or 1064 nm Raman
- No → Go to 3
3) Need non-contact through container?
- Yes → Raman via glass vial
- No → Either technique viable if there is an active signal.
Still unsure? Our applications team can assist with selecting the correct technique for your your samples.
White Bear Photonics offers turnkey solutions, upgrades, and repair parts for both modalities. Whether you need a laser-stable Raman probe, a replacement FTIR source, or help selecting the correct technique , we’re here to accelerate your spectroscopy.