NMR analysis in depth

We visited the uniersity of Siena the last Monday and Daniela explained us in detail how an NMR spectrometer works and its utility to determine the quality of the olive oil. The olive oil is categorized according to its acidity. The main acid present in olive oil is oleic acid. Oleic acid is a monounsaturated fatty acid and its content is also very high in the human adipose tissue. The quality of the oils goes from extra-virgin olive oil with less than 0.8% of acid to Lampate oil that is not recommended for cooking puposes due to its high acidity. We also saw the NMR machine and learned about all its components. We had to take off our watches and anything containing metal before due to the strong magnetic field surronding the NMR. It was a wonderful experience, we always have studied NMR and learned very well how to determine compunds looking at the NMR spectrum but I have never seen the actual machine before. What called my atention was the fact that the sample had to be in liquid state to be analyzed. The university of Siena does not possess the equipment for solid-state NMR, but Emory University does, one more motive to be proud of.


All of the analytical techniques have its limitations and its advantages. Liquid or solution-state NMR requires that the sample be soluble at some concentrations but when the analyte´s molecules weight passes the 30kDa the techniques becomes very complicated. There is a size limitation for this NMR technique. Solid NMR does not require the sample to be in solution, liquid state or any different structure, this allows for much more efficient and direct analysis. Solid NMR permits to analyze samples bigger than 100kD. Solid NMR, as every technique, also has disadvantages when compared to solution-state NMR. The main difference between the two techniques is the random motion, mobility and rotation of the sample in the liquid state. Solids lack these rotational and translational movements even though some molecules can still have some sort of rotation and movement, like the rotation of the methyl groups or the flip of the rings.


The advantage of solid NMR is that since the molecule are mainly fixed in the space, clear and defined lines appear in the NMR Spectrum. The disadvantage is that information regarding the orientation of the molecule in the space is not avaliable. This disadvantage somehow limits the analysis of biological macromolecules.
In Solid-state NMR anisotropic nucelar interactions (directionally dependent interactions) influence the behavior of the system, of the nuclear spins. There are two anisotropic interactions characteristic in solid NMR: chemical shift anisotropy and internuclear dipolar coupling.