Separating mixtures in an NMR machine

Canadian researchers have devised a method of extracting the complete proton NMR (nuclear magnetic resonance) spectra for a component of a mixture without the need for physical separation.

NMR spectroscopy has become the identification method of choice for organic chemists and the ability to quantify and identify components in reaction mixtures and formulations without having to conduct a prior separation step is an extremely attractive idea.A new method that permits the extraction of a complete NMR spectrum for a single component of a mixture has been described by researchers from the University of Toronto in an early view article in the journal Analytical Chemistry.

"Whether a synthetic organic chemist faced with a mixture of starting materials and products or a pharmaceutical researcher confronted with the complexities of drug decomposition, unravelling mixtures is a reality of modern chemistry," write the authors.In some cases compound separation by chromatographic techniques can lead to compound degradation due to the compound reacting with the separation media or the separation media disrupting key molecular associations.Led by Dr Andre Simpson, the team developed a viscous, oil-based (proton-free) solvent system whose viscosity can be manipulated by controlling the temperature.

By increasing the viscosity of the solvent, molecular tumbling is reduced generating a large negative shift in the nuclear Overhauser effect (NOE) as well as increasing dipolar interactions providing a "framework for the efficient propagation of spin diffusion that results in spin population disturbance throughout the entire molecule.""If the viscosity of the NMR solvent can be controlled, conditions are established that allow all proteins in a single molecule to correlate with all other protons in the same molecule," write the authors.This should enable researchers to isolate a pure spectrum for a product even in the presence of overlapping starting materials, by-products or both.

Indeed, the researchers managed to devise a method that allowed the separation of spectra from a mixture containing phenanthracene and hexadecanophene that would normally overlap and prohibit accurate data interpretation.A "practical guide" to conducting the experiments will be available through the Analytical Chemistry website once the article has gone to press.Hardware requirements are minimal, needing only an NMR instrument that can conduct experiments at temperatures down to 253K, with the researchers using a Bruker Avance 500 MHz spectrometer, fitted with a 1H-BB-13C TBI probe fitted with a actively shielded Z-gradient and a Bruker variable temperature control unit.

The researchers note that: "ultimately the biggest drawback of the approach described here is mixture solubility as the mixture has to be dissolved in an 80:20 oil mixture."

By Dr Matt Wilkinson
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