Multi-phonon quantitation of rivaroxaban by broadband THz spectroscopy.

Accurate quantification of low-dose crystalline active pharmaceutical ingredients (APIs) remains a critical challenge. Conventional methods, including high-performance liquid chromatography with ultraviolet detection, powder X-ray diffraction, and transmission Raman spectroscopy, often suffer from limited lattice specificity, insufficient sensitivity, or destructive measurement. Here we present a multi-phonon quantitation strategy based on broadband THz spectroscopy (6-13 THz), using rivaroxaban as a representative low-dose API. Periodic DFT calculations on an XRD-validated crystal structure, complemented by isolated-molecule DFT calculations, to establish the lattice and molecular motions underlying the observed THz peaks. The simulated THz spectrum reproduces the experimental result with a mean absolute deviation of 0.06 THz. The three dominant peaks are assigned to physically distinct vibrational modes: an intramolecular vibration (8.93 THz) and two intermolecular lattice phonons (10.49 and 11.82 THz), with H-bond oscillation amplitudes of 0.069-0.125 Å. With these results, peak-area fusion (ΣA) across these modes achieves a limit of quantification (LoQ) of 98.1 μg, which is 25.3% lower than that of the best individual peak-area model and 10.6-fold lower than that of transmission Raman spectroscopy. Peak-height ratios among the three modes showed limited variation with analyte mass under the present measurement conditions, providing an internal consistency check. Overall, this study demonstrates that physically resolved THz phonons can be combined to enhance quantitative sensitivity in a crystalline API model system.