Paramagnetic Relaxation Enhancement and the Inconspicuous Effect of Impurities on PEG-Based Metallic Systems

Abstract
NMR relaxometry and, particularly, the analysis of the NMRD curves of paramagnetic systems is far from being a trivial task. In previously published studies it required dealing with the existence of different spin populations and the variation of the diffusion coefficient with the applied external magnetic field.

Generally speaking, paramagnetic relaxation enhancement (PRE) is a complex process, which can arise via inner or outer sphere mechanisms and more often than not, via a combination of these two. For this reason, it is not hard to fit a theoretical model to an enhanced NMRD with a combination of parameters that seems very reasonable but is, in fact, very far from what should be the correct interpretation.

This is what happened when we tried to analyze a set of PEG-based systems where paramagnetic species (“impurities”), initially unaccounted for but present in the acid ligand (the major component of the mixtures), clearly interact with the cobalt-based complex.

This raised further questions about whether these impurities would also interact with the gadolinium and manganese complexes, although this could not be immediately seen in the raw data. In addition, these data could even be fitted with parameters having an apparently plausible values, but based on the erroneous assumption that the different paramagnetic species did not interact with each other.

After purifying the reference and using it in mixtures with the same concentration of metal complexes, it was possible to answer some of these questions.