Continuous Gradient Temperature Raman Spectroscopy of Unsaturated Lipids from –100 to 50°C Provides Detailed Vibrational Analysis and Rapid, Nondestructive Graphical Product Authentication

Abstract

Gradient Temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur at or near phase transitions. 20 Mb three-dimensional data arrays with 1.0 or 0.2°C increments and first/second derivatives allow complete assignment of solid, liquid and transition state vibrational modes, including low intensity/frequency vibrations that cannot be readily analyzed with conventional Raman. Phase transitions observed with DSC can be directly correlated to shifts in Raman frequency and intensity, as well as the complete appearance/disappearance of modes. GTRS can identify and differentiate specific carbon chain sites, finally allowing Raman analysis to explain why the long-chain polyunsaturated fatty acids (LC-PUFA) exhibit such extreme functional differences despite minimal changes in chemical structure. Individual sites on the polyunsaturated fatty acid chain are nonequivalent, and each LC-PUFA molecule has an individual, specific three-dimensional structure incorporating torsion. In addition, the entire set or any subset of the any of the contour plots can be utilized to create a graphical standard to quickly authenticate a given source. We compared GTRS and DSC data for five commercial fish oils that are excellent sources of docosahexaenoic acid (DHA; 22:6n-3) and eicosapentaenoic acid (EPA; 20:5n-3). Each product has a unique, distinctive response to the thermal gradient, which graphically and spectroscopically differentiates them.