Low-frequency Raman spectroscopy

Importance of the topic

Raman spectroscopy in the low-frequency region (<200 cm-1, down to ~65 cm-1) reveals lattice, rotational and intermolecular vibrational modes that are not accessible in the conventional fingerprint range (200–3400 cm-1). These low-energy modes carry critical information for solid-state structure, hydrogen-bonding networks, polymorphism, solvation/pseudo-polymorphism, and phase transitions. Accessing this spectral window increases specificity and sensitivity for tasks that are central to pharmaceutical development, protein characterization, materials science and process monitoring.

Goals and overview of the application note

The application note demonstrates the capability of the i-Raman Plus 785S spectrometer coupled with the BAC102 E‑grade low-frequency probe to acquire Raman spectra from 65–3350 cm-1. It presents representative measurements that highlight the analytical value of the low-frequency region: (1) full-range spectrum of L-asparagine showing pronounced low-frequency bands, (2) differentiation of α-D-glucose and its monohydrate (pseudo-polymorphs) using sub-200 cm-1 features, and (3) monitoring of a temperature-driven phase change in sulfur (α → λ) via changes in low-frequency peak shape and position.

Used instrumentation

• i-Raman Plus 785S spectrometer: 785 nm excitation, CleanLaze® laser with <0.2 nm linewidth, maximum 300 mW output, TE-cooled back-thinned CCD, configured spectral range 65–3350 cm-1, spectral resolution ~4.5 cm-1.
• BAC102 E-grade Raman probe (785 nm): low-frequency cut-on start ~65 cm-1, 105 µm fiber (0.22 NA) for excitation, 200 µm fiber (0.22 NA) for collection, optical density >6, 1.5 m fiber length, quartz window for non-contact sampling, ~5.4 mm working distance; not intended for immersion.
• Software and accessories: BWSpec acquisition software; optional cuvette holder, video microscope, XYZ stage, and multivariate/ID software (BWIQ®, BWID®) mentioned as compatible.

Methodology and experimental parameters

Measurements were performed at room temperature (except for the sulfur phase experiment) using the E-grade probe and i-Raman Plus. Typical acquisition settings used in the examples:

• Laser power: 300 mW (100% power for the experiments reported).
• Integration times: ranged from 0.1 s (rapid phase monitoring) up to 10 s (high S/N for polymorph differentiation); one example used 1.2 s for L-asparagine.
• Averages: single-scan acquisitions were demonstrated to highlight fast response capability; longer acquisitions and averaging may be applied depending on sample and S/N requirements.

Key experimental notes: solids were measured directly (e.g., deposited on aluminum for the sulfur experiment); sulfur was heated past its melting point (115.2 °C) to follow the α → λ transition in situ.

Main results and discussion

• L-asparagine: Full-range spectra (65–3200 cm-1) reveal three dominant bands below 200 cm-1. These low-frequency bands correspond to lattice/collective motions and provide structural information complementary to fingerprint region modes used for chemical identification.
• Polymorph/pseudo-polymorph detection (α-D-glucose vs α-D-glucose monohydrate): Significant spectral differences occur in the low-frequency region (below 200 cm-1) that clearly distinguish anhydrous and monohydrate forms. Such differences arise from solvent inclusion and altered lattice dynamics and may be subtle or invisible in the standard fingerprint region alone.
• Phase monitoring (sulfur): Heating from the α-crystalline form into the λ-liquid/phase produces broadening and a shift of a peak near 83.6 cm-1; this change is evident in the low-frequency window while the fingerprint region remains essentially unchanged. Rapid acquisition (0.1 s) demonstrates capability for real-time monitoring of phase transitions.

The data show that low-frequency Raman measurements increase discriminatory power for solid-state structural phenomena and enable fast, sensitive detection of polymorphism, solvation states and phase changes.

Benefits and practical applications

• Pharmaceutical industry: Robust identification and differentiation of polymorphs, hydrates/solvates and pseudo-polymorphs during development, scale-up and quality control; supports regulatory and formulation needs because polymorphic form affects bioavailability and stability.
• Biomolecular research: Complementary structural information for amino acids and proteins, including intermolecular interactions and hydrogen-bonding networks.
• Materials science and industry: Monitoring crystallization and phase transitions in real time (process analytical technology), characterizing lattice vibrations in semiconductors, carbon nanotubes and photovoltaic materials, and analyzing pigments, minerals and gemstones.
• Operational advantages: Non-destructive, rapid measurements; portable instrumentation enabling in-lab and field use; broad spectral coverage reduces need for multiple instruments.

Future trends and applications

• Integration with multivariate and chemometric analysis (PAT workflows) to automate polymorph detection and in-line quality control.
• Real-time, in situ process monitoring with enhanced sampling (mapping, probe automation) for crystallization and formulation control.
• Advances in detector sensitivity and optical filtering to push cut-on frequency lower, improve resolution of closely spaced lattice modes, and reduce required integration times for weak scatterers.
• Combination with temperature-resolved Raman, imaging and machine learning to extract structural dynamics, heterogeneity and to predict material performance from spectral signatures.

Conclusion

The i-Raman Plus 785S with a BAC102 E‑grade low-frequency probe provides reliable, practical access to Raman modes down to ~65 cm-1. Low-frequency Raman extends the analytical reach beyond the fingerprint region, delivering decisive information for polymorph identification, phase-change monitoring and structural characterization of materials and biomolecules. The system supports rapid, non-destructive measurements suitable for laboratory and field applications and can be integrated into advanced workflows for process monitoring 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 Electron. Optoelectron., 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.