Abstract

The presence of paramagnetic impurities, including molecular oxygen, in a sample causes line broadening in the high-resolution nuclear magnetic resonance (NMR) spectrum. Some of these paramagnetic impurities can sometimes be eliminated by adapting the conventional methods, such as filtering the sample solutions for insoluble impurities and shaking water-immiscible solutions with dilute hydrochloric acid for soluble impurities. The recommended procedures for removing dissolved oxygen from NMR samples involve vacuum distillation, out-gassing at the boiling point of the solution, or flushing. with oxygen-free nitrogen. Multiple freeze–pump–thaw cycles, which exclude oxygen rigorously, are also in use. Removal of this dissolved impurity is particularly desirable when dealing with compounds such as olefins and aromatic hydrocarbons which tend to form charge–transfer complexes with oxygen. Recently, it was claimed that shaking the sample solution with 10% sodium thionate (Na₂S₂O₄) solution eliminates oxygen, as was evidenced by considerable improvement in the resolution.

Green optical dissolved oxygen sensor based on a chlorophyll–zinc complex extracted from the plant Brassica oleracea L.

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