Low Frequency Raman Spectroscopy

Importance of the Topic

Low-frequency Raman spectroscopy extends conventional vibrational analysis into the 65 cm⁻¹ region, revealing lattice and intermolecular modes that are invisible in the typical fingerprint range. This capability is critical for detailed characterization of polymorphic forms, protein structures and phase transitions in pharmaceuticals, materials science and nanotechnology.

Study Objectives and Overview

This application note presents the performance of B&W Tek’s i-Raman Plus 785 nm spectrometer coupled with the BAC102 E-grade probe. The goal is to demonstrate how low-frequency detection down to 65 cm⁻¹ enhances sensitivity and selectivity for molecular and solid-state analyses. Three case studies—amino acid fingerprinting, polymorph differentiation and phase monitoring—illustrate the method’s practical value.

Used Instrumentation

• i-Raman Plus portable Raman spectrometer
  • 785 nm CleanLaze® laser, < 0.2 nm linewidth, up to 300 mW output
  • TE-cooled, back-thinned CCD detector
• BAC102 series E-grade probe
  • Spectral coverage 65 cm⁻¹ to 3200 cm⁻¹, resolution ~4.5 cm⁻¹
• Acquisition parameters
  • Integration times 100 ms to 10 s
  • Room-temperature measurements at full laser power

Main Results and Discussion

• L-Asparagine analysis revealed three dominant low-frequency Stokes bands below 200 cm⁻¹, complementing the classic 200–3200 cm⁻¹ fingerprint region.
• Polymorph detection: α-D-glucose and α-D-glucose monohydrate exhibit distinct peaks in the 65–200 cm⁻¹ window, enabling unambiguous differentiation of pseudo-polymorphic forms.
• Phase monitoring: Sulfur’s α-crystalline to λ-liquid transition was tracked by the broadening and shift of the 83.6 cm⁻¹ band, while fingerprint features remained unchanged.

Benefits and Practical Applications

• Enhanced polymorph and solvate identification for API quality control
• Real-time monitoring of crystallization and phase behavior in process development
• Protein conformational studies and lattice dynamics investigations

Future Trends and Opportunities

Low-frequency Raman spectroscopy is poised to impact:

• Semiconductor lattice characterization
• Carbon nanotube and graphene research
• In-situ monitoring of photovoltaic materials
• Mineral and gemstone structure analysis

Conclusion

The integration of low-frequency detection into a portable Raman platform offers a cost-effective route to capture critical molecular and solid-state information. By extending sensitivity to 65 cm⁻¹, the i-Raman Plus and BAC102 E-grade probe enable advanced applications in pharmaceuticals, materials science and beyond.

References

1. Teixeira AMR et al. High-pressure Raman study of L-Alanine crystal. Solid State Commun. 2000;116(7):405–409.
2. Larkin PJ et al. Polymorph characterization of APIs using low-frequency Raman spectroscopy. Appl Spectrosc. 2014;68(7):758–776.
3. Smith E, Dent G. Modern Raman Spectroscopy – A Practical Approach. John Wiley & Sons; 2005.
4. Pelletier MJ. Analytical Applications of Raman Spectroscopy. Blackwell Science Ltd; 1999.