Quantification of Crystallinity Using Transmission Raman Spectroscopy

Importance of the Topic

The crystallinity of active pharmaceutical ingredients influences properties such as solubility, bioavailability, and long-term stability. Accurate measurement of crystalline content in drug products is essential for quality control and ensuring consistent patient outcomes. Bulk analysis methods that are rapid and nondestructive address common challenges in pharmaceutical manufacturing and research.

Objectives and Study Overview

This study evaluates the capability of transmission Raman spectroscopy to quantify low levels of crystalline API in amorphous spray-dried dispersions. Nine powder mixtures containing 0 to 9.4% w/w crystalline API were analyzed to establish detection limits, compare performance with other solid-state techniques, and demonstrate potential for routine application in pharmaceutical workflows.

Methodology and Used Instrumentation

Transmission Raman measurements were performed using a system in which the laser illumination and signal collection occur on opposite sides of the sample, ensuring bulk representativity. Chemometric modeling via partial least squares regression correlated spectral features to known crystalline percentages. Calibration and validation sets were employed to assess model accuracy and sensitivity.
Used instrumentation

• Agilent TRS100 transmission Raman spectrometer for bulk analysis
• Partial least squares regression software for quantitative modeling

Main Results and Discussion

The PLS model achieved a correlation coefficient of R² = 0.99 between predicted and measured crystalline content. The root mean square error of calibration (RMSEC) was 0.91% w/w, and the root mean square error of cross-validation (RMSECV) was 1.33% w/w. The calculated limit of detection was 0.9% w/w crystalline API. Compared to powder X-ray diffraction and solid-state NMR, transmission Raman offers lower detection limits, faster analysis times, and eliminates the need for destructive sample preparation.

Benefits and Practical Applications

Transmission Raman spectroscopy delivers rapid, nondestructive bulk quantification of crystallinity that is representative of the entire sample. Key advantages include

• High-throughput analysis with typical measurement times of 1–5 minutes
• Low cost per test compared to alternative solid-state methods
• No requirement for sample grinding or rotor packing, preserving sample integrity
• Potential for at-line implementation in manufacturing and stability studies

Future Trends and Opportunities

Advances in chemometric algorithms and detector sensitivity will further enhance method performance. Integration into continuous manufacturing environments and real-time quality monitoring represents a significant opportunity. Combining transmission Raman with complementary spectroscopic or imaging techniques may provide deeper insights into solid-state transformations and streamline formulation development workflows.

Conclusion

Transmission Raman spectroscopy using the TRS100 platform provides a reliable, fast, and nondestructive approach for quantifying low levels of crystalline API in amorphous formulations. With a 0.9% detection limit and excellent correlation to reference values, TRS is a viable alternative to powder X-ray diffraction and solid-state NMR for routine pharmaceutical analysis.

References

• Matousek P, Everall N, Littlejohn D, Nordon A, Bloomfield M Dependence of signal on depth in transmission Raman spectroscopy Applied Spectroscopy 2011 65 724-733
• Kumar A, Joseph L, Griffen J, et al Fast non-destructive detection of low level crystalline forms in amorphous spray-dried dispersion using transmission Raman spectroscopy and comparison to solid-state NMR spectroscopy American Pharmaceutical Review 2016