Comparison of Low-, Mid-, and High-Frequency Raman Spectroscopy for an In Situ Kinetic Analysis of Lipid Polymorphic Transformations

The performance of selected spectral ranges in the low-frequency (10-150 cm-1), mid-frequency (1350-1500 cm-1), and high-frequency (i.e., C-H stretching; 2800-2950 cm-1) domains of Raman spectroscopy was evaluated for the kinetic in situ analysis of lipid polymorphic transformations. Tristearin was used as the primary model lipid and its spray-congealed formulations containing 5% w/w isopropyl myristate (IM), oleic acid (OA), and ethyl oleate (EO) were used due to their ability to differentially modulate the tristearin phase transformation from metastable α-form to the stable β-form. The behavior of bulk samples was interrogated under different isothermal conditions (35, 40, 45 and 50 °C) with Raman microscopy providing complementary particulate-level information for specific conditions, including dispersed state within an aqueous environment. Overall, a clear rank order was observed between the lipid additives (IM > EO > OA) for accelerating the conversion to β-form, best exemplified by the low-frequency Raman (LFR) domain. This spectral range also showed superior characteristics over the more commonly utilized mid-frequency and C-H stretching domains to detect faster onset times for the polymorphic transformations that were attributed to its intrinsic structural sensitivity.