Low-frequency Raman spectroscopy

Significance of the topic

The application note presents low-frequency Raman spectroscopy as an extension of conventional Raman analysis that accesses vibrational and lattice modes below the common fingerprint cutoff. Extending measurements down to ~65 cm-1 reveals intermolecular, rotational and lattice dynamics that are critical for protein characterization, polymorph and pseudo-polymorph identification, and monitoring solid–liquid and solid–solid phase transitions. This additional spectral information increases specificity and sensitivity for differentiating closely related materials in pharmaceutical development, materials science and quality control.

Objectives and study overview

The document demonstrates the capability of a portable laboratory-grade Raman system (i-Raman Plus 785S) coupled with an E-grade BAC102 probe to record reliable spectra from 65 to 3350 cm-1. Objectives include showing examples of applications where low-frequency data provide diagnostic features not visible in the traditional fingerprint region: amino-acid structural features (L-asparagine), discrimination of pseudo-polymorphs (α-D-glucose vs. α-D-glucose monohydrate), and monitoring phase changes during melting/crystallization (elemental sulfur).

Methodology

Key experimental approaches and conditions used in the examples:

• Instrument: i-Raman Plus 785S Raman spectrometer with 785 nm laser excitation (linewidth <0.2 nm), TE-cooled back-thinned CCD detector, and CleanLaze® technology.
• Probe: BAC102 E‑grade low-frequency probe with optical cut-on starting near 65 cm-1.
• Spectral range and resolution: full measured range of ~65–3350 cm-1 with a spectral resolution of ~4.5 cm-1.
• Laser power and acquisition: typical laser power used ~300 mW; integration times varied by sample and objective (examples: 0.1 s for rapid monitoring of sulfur phase change, 1.2 s for L-asparagine, 10 s for glucose polymorph comparison).
• Sampling modes: non-contact probe sampling (adhesive-sealed quartz window) suitable for solids and surface measurements; heating was applied for the sulfur phase-change experiment.

Instrumentation used

Detailed instrument configuration reported in the application note:

• i-Raman Plus 785S portable Raman spectrometer: high-quantum-efficiency TE-cooled CCD, 785 nm laser, capable of measurements from 65 to 3350 cm-1, spectral resolution ~4.5 cm-1, max laser power ~300 mW, and support for long integration times for weak signals.
• E-Grade BAC102 trigger probe (785 nm): optical cut-on near 65 cm-1, 105 µm excitation fiber (0.22 NA), 200 µm collection fiber (0.22 NA), optical density >6 filters, 1.5 m fiber length, adhesive-sealed quartz window for non-immersive sampling, ~5.4 mm working distance.
• Software: BWSpec acquisition software; optional BWIQ and BWID for multivariate and identification tasks mentioned as compatible.

Main results and discussion

Examples illustrate how low-frequency Raman features add diagnostic power beyond the fingerprint region:

• L-asparagine: Spectra recorded from 65–3200 cm-1 show clear, dominant bands below 200 cm-1 attributable to lattice and low-energy vibrational modes, emphasizing the importance of the low-frequency region for amino-acid structural characterization and intermolecular interaction assessment.
• α-D-glucose vs. α-D-glucose monohydrate (pseudo-polymorphs): Low-frequency spectra (10 s integration) reveal significant differences below 200 cm-1 that distinguish the monohydrate from the anhydrous form. These differences arise from the presence of structured solvent/lattice interactions in the monohydrate and are minimal or absent in the common fingerprint region, demonstrating the utility of low-frequency measurements for polymorph/pseudo-polymorph identification.
• Sulfur phase transition monitoring: Rapid acquisitions (0.1 s integration) during heating showed that a distinct low-frequency peak at ~83.6 cm-1 broadens and shifts when solid α-sulfur melts and transitions toward the λ form. The fingerprint region remained largely unchanged, so low-frequency changes served as a sensitive marker of the phase change.

Overall, the note shows that low-frequency Raman peaks are sensitive to lattice ordering, solvation and phase state, providing complementary information to conventional Raman fingerprint bands.

Benefits and practical applications

Practical advantages and use cases identified:

• Pharmaceutical industry: Improved detection and differentiation of polymorphs, solvated forms and pseudo‑polymorphs that affect drug bioavailability and stability; supports formulation, process monitoring and quality control.
• Biomolecular analysis: Additional structural insights for amino acids and proteins where low-energy modes reflect hydrogen bonding, intermolecular packing and conformational states.
• Materials science and engineering: Characterization of semiconductor lattice vibrations, carbon nanotube modes, solar cell materials, pigments, gemstones and minerals where lattice or low-energy vibrational information matters.
• Process monitoring: Fast acquisition capability allows real-time or near-real-time tracking of phase transitions (crystallization, melting) during processing.

Future trends and potential developments

Anticipated directions and opportunities for low-frequency Raman spectroscopy:

• Integration with multivariate and chemometric tools to automate polymorph classification and to increase sensitivity for subtle lattice changes.
• Broader adoption in process analytical technology (PAT) for inline or at-line monitoring of crystallization and drying steps in manufacturing.
• Miniaturization and ruggedization of low-frequency probe systems to enable more widespread field and industrial deployment.
• Synergistic use with complementary techniques (X-ray diffraction, thermal analysis, infrared spectroscopy) to build robust, orthogonal material characterization workflows.
• Application expansion into advanced materials (2D materials, perovskites, novel nanostructures) where low-frequency phonon modes are linked to functional properties.

Conclusion

The application note demonstrates that extending Raman measurements into the low-frequency region (down to ~65 cm-1) markedly increases the analytical information available for discriminating polymorphs, detecting solvated crystal forms, and monitoring phase transitions. The i-Raman Plus 785S with a BAC102 E-grade probe provides a portable, laboratory-capable solution with sufficient sensitivity and resolution for these tasks, making low-frequency Raman a practical complement to conventional spectroscopic and diffraction methods in pharmaceutical, biological and materials analysis.

References

1. Teixeira, A. M. R.; Freire, P. T. C.; Moreno, A. J. D.; et al. High-Pressure Raman Study of L-Alanine Crystal. Solid State Communications 2000, 116 (7), 405–409.
2. Larkin, P. J.; Dabros, M.; Sarsfield, B.; et al. Polymorph Characterization of Active Pharmaceutical Ingredients (APIs) Using Low-Frequency Raman Spectroscopy. Applied Spectroscopy 2014, 68 (7), 758–776.
3. Golichenko, B. O.; Naseka, V. M.; Strelchuk, V. V.; et al. Raman Study of L-Asparagine and L-Glutamine Molecules Adsorbed on Aluminum Films in a Wide Frequency Range. Semiconductors: Physics, Quantum Electronics & Optoelectronics 2017, 20 (3), 297–304.
4. Smith, E.; Dent, G. Modern Raman Spectroscopy: A Practical Approach, 2nd ed.; John Wiley & Sons, 2019.
5. Pelletier, M. J. Analytical Applications of Raman Spectroscopy, 1st ed.; Blackwell Science: Oxford, 1999.