SCALPEL: Dissecting Spectra

University of Manchester develops new method for testing complex mixtures

Sample Analysis

Analysing complex mixtures efficiently is a recurring problem within the scientific community. No one test gives a detailed assessment about a given material. Each test is a different indicator towards its chemical structure or physical properties. Often materials are too complex to analyse as a whole. They are fragmented and worked on, bit by bit. 

Problems can arise for several reasons. Separating and breaking down a mixture can be a time-consuming process. Also, physically separating samples doesn’t guarantee a precise reconstruction. A team, at the University of Manchester, unveiled a new technique within NMR spectroscopy, enabling precise testing of complex mixtures, without physical separation. 

What is NMR Spectroscopy?

Spectroscopy is the study of the interaction of light with matter. For NMR spectroscopy (Nuclear Magnetic Resonance), as the name suggests, this involves magnets. 

To simplify, we can think of the atoms, making up a molecule, as compasses. A compass has a needle which points to the north, south, east or west. Next, we can think of a magnet, which has a north pole and a south pole. There is a magnetic field created by these two poles. A force which starts from the north, goes up, then back down to the south pole.

If the magnet is placed next to the compass, the needle of the compass will orient itself with the magnetic field. In other words, opposites attract, both north poles point in the same direction and the south needle of the compass will point down towards the southern pole. The compass has aligned itself with the magnetic field.

North pole of the magnet pointing towards the compass. Credit to David Darling.

NMR is quite difficult to simplify. The best way to explain it, is to think of the atoms inside the molecule, as these compasses. The NMR aspect of the experiment is applying a magnetic field to the molecule. When doing this, the components inside the atom will spin in a certain direction, like the needle. The way these nuclei spin, tells us what surrounds them – what their chemical environment is. From this, a spectrum is obtained, allowing for the chemical structure to be determined. 

New Approach: SCALPEL

NMR spectroscopy is a really useful technique. It is used widely, but until now, it was difficult to study complex materials. Large molecules, have many spin states which make for challenging spectrums. The overlapping signals are difficult to attribute, often giving approximate answers. The only way to disentangle components of a spectrum is to use multiple samples or a single sample that is time-varying. 

“Efficient, practical, and nondestructive analysis of complex mixtures is vital in many branches of chemistry. “

G. Dal Poggetto, et al., 2019.

To solve this, in 2019, Mathias Nilsson and his team developed SCALPEL (Spectral Component Acquisition by Localized PARAFAC (parallel factor analysis) Extraction of Linear components). This approach allows for individual components to be extracted from the spectrum and not the mixture itself. Instead of breaking down the sample, we’re breaking down the spectrum.

Image taken from original article, G. Dal Poggetto, et al., 2019.

This new approach combines NMR methods, to select specific spin states at a time. The paper used stout beer to retrieve clear spectra for individual glucose components. Detailed spectra can be obtained by a non-invasive test, from single samples. This progress in NMR spectroscopy enables the identification of complex mixtures more easily than ever before.


Sources

Original Article:

G. Dal Poggetto, L. Castañar, R. W. Adams, G. A. Morris, M. Nilsson, Journal of the American Chemical Society, 2019, 141 (14), 5766-5771, DOI: 10.1021/jacs.8b13290.

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