Continuousflow Experiments

With this mode of operation the output of the chromatographic separation is recorded at the same time as the 1H NMR spectra. In most cases the flow

Figure 4 Contour plot (600 MHz) of the separation of tocopherol isomers.

rate of the mobile phase is decreased to yield an NMR scan accumulation of one chromatographic peak. The acquisition of online continuous-flow NMR spectra results in a two-dimensional contour plot of 1H NMR signals of the separated compounds (x axis = 1H chemical shift) versus retention time (y axis). Figure 4 shows the contour plot of the separation of tocoph-erol isomers on a C30 stationary phase in methanol together with 2% CD3OD. A time interval of 1 s was applied to presaturate the methyl group signal of the solvent. A total of 128 experiments with a time resolution of 27.3 were recorded at a flow rate of 0.3 mLmin-1. The chromatogram along the y axis was reconstructed by co-addition of all proton resonances between 0.8 and 1.9 p.p.m. The resolution of this 1H NMR chromatogram suffers from the small number of 128 data points, but the separation of all five tocopherol isomers is readily apparent.

The structure of the separated compounds can easily be assigned by the methyl group 1H NMR signals between 2.0 and 2.2 p.p.m. and by the aromatic resonances between 6.4 and 6.5 p.p.m. (Figure 5). For instance, the continuous-flow 1H NMR spectrum of ¿-tocopherol shows one signal for the methyl group at C8 and two aromatic resonances with a small splitting due to the meta coupling between H5 and H7. Thus, unequivocal structural assignment of all different isomers is possible by considering chemical shifts, coupling constants and integration ratios.

Solar Panel Basics

Solar Panel Basics

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