Quantification of Crystallinity Using Transmission Raman Spectroscopy

Importance of the Topic

Accurate quantification of crystalline content in pharmaceutical solids is essential because crystallinity influences dissolution rate, bioavailability, and stability of active pharmaceutical ingredients (APIs). Traditional solid‐state methods often suffer from surface bias, lengthy analysis times, or destructive sample preparation, making them less suited for routine quality control and stability monitoring. Transmission Raman spectroscopy (TRS) addresses these limitations by enabling fast, non‐destructive, and bulk‐representative measurements.

Objectives and Study Overview

This study evaluates the Agilent TRS100 Raman system for absolute quantification of low‐level crystallinity in spray‐dried solid dispersions. Nine powder mixtures containing 0–9.4 % w/w crystalline API in an amorphous matrix were analyzed. The key goals were to establish calibration models, determine the limit of detection (LOD), and compare TRS performance to powder X-ray diffraction (pXRD) and solid‐state nuclear magnetic resonance (ssNMR).

Methodology and Instrumentation

Transmission Raman geometry was employed, where the laser illuminates one side of the sample and the Raman signal is collected on the opposite side, ensuring bulk analysis and minimizing surface bias. Spectra were acquired in 1–5 minutes per sample with the Agilent TRS100. Chemometric modeling using partial least squares regression (PLS) was applied to relate spectral features to known crystallinity levels. Key instrumentation details:

• Agilent TRS100 Raman spectrometer
• Laser excitation and opposing‐side collection for transmission geometry
• PLS chemometric software for calibration and prediction

Key Results and Discussion

Spectral regions sensitive to crystalline‐amorphous transitions were identified between 200–1600 cm⁻¹. The PLS model, built with three latent variables, achieved R² = 0.99, root‐mean‐square error of calibration (RMSEC) = 0.91 % w/w, and cross‐validation error (RMSECV) = 1.33 % w/w. The LOD for crystalline API was determined to be 0.9 % w/w. Compared to pXRD (LOD 2–5 %) and ssNMR (LOD 0.3–1 %), TRS offers rapid analysis, low cost, and non‐destructive bulk sampling.

Practical Benefits and Applications

• Non‐destructive analysis of tablets, capsules, and powders without grinding
• Representative bulk measurements reduce sampling errors from surface heterogeneity
• Short measurement times enable at‐line quality control and high throughput screening
• Low cost per test and minimal sample preparation requirements

Future Trends and Potential Applications

Future developments may integrate TRS with automated sampling systems for real‐time monitoring of crystallinity during manufacturing. Advances in chemometric algorithms and instrumentation sensitivity could further lower detection limits and expand application to complex multi‐component formulations. TRS may also support long‐term stability studies by repeatedly measuring the same samples over time without destruction.

Conclusion

Transmission Raman spectroscopy using the Agilent TRS100 proves to be a highly effective alternative for quantifying low‐level crystallinity in pharmaceutical solids. Its non‐destructive nature, rapid data acquisition, and strong correlation with known standards establish TRS as a valuable tool for quality control and formulation development.

References

1. 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.
2. 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.