|Broadhurst, C - University Of Maryland|
|Qin, Jianwei - Tony|
|Chao, Kuanglin - Kevin Chao|
Submitted to: Eastern Analytical Symposium
Publication Type: Proceedings
Publication Acceptance Date: 9/28/2015
Publication Date: 11/17/2015
Citation: Broadhurst, C.L., Schmidt, W.F., Kim, M.S., Nguyen, J.K., Qin, J., Chao, K., Bauchan, G.R., Shelton, D.R. 2015. Continuous gradient temperature Raman spectroscopy of unsaturated fatty acids . Eastern Analytical Symposium. Somerset, NJ on November 16-18, 2017..
Technical Abstract: A new innovative technique gradient temperature, Raman spectroscopy (GTRS), identifies Raman frequency shifts in solid or liquid samples, and correlates them with specific temperature ranges within which flexible structures absorb heat. GTRS can easily detect changes that occur within one celcius temperature increments; more precise measurements can be made utilizing shallower gradients over specific temperature ranges of interest. Thus, GTRS provides a very rapid and straightforward technique to identify molecular rearrangements that occur just prior to or at phase transitions. We analyzed the unsaturated fatty acids oleic (OA, 18:1n-9) and linoleic (LA, 18:2n-3) from -100 to 50 Celcius . 20 Mb three-dimensional data arrays with 0.2 C increments and first/second derivatives allowed complete assignment of solid, liquid and transition state vibrational modes. For OA, large spectral and line width changes occurred in the solid state gamma to alpha transition near -4 C, and the melt (13C) over a range of only 1 C. For example, below -7 C, the methyl rocking vibrational mode at 895 cm-1 is strong; from -7 C to 13 C, intensity increases to very strong; above 13 C, intensity falls precipitously and a new, very sharp frequency arises at 858 cm-1. For LA, major intensity reductions from 200-1750 cm-1 and some peak shifts marked one solid state phase transition at -50 C. A second solid state transition (-33 C) had minor spectral changes. Large spectral and line width changes occurred at the melt transition (-7 C). For example, below -7 C, CH2 twisting occurs primarily on the carbonyl sided chain: HOOC-(CH2)7- and extends into CH wagging op at C10. Above -7 C, concurrently CH2 rocking at C11 becomes markedly stronger, C=CH rocking ip at C10 and C12 intensifies, and C=CH wagging op at C9 and C13 becomes undifferentiated. Melting initiates at the diene structure and then progresses towards the ends of the molecule.