A Triple Layer of Immiscible Solvents for NMR Sample Preparation: Enhanced Sensitivity and Reduced Deuterated Solvent for Observation of Heteronuclei
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Organic chemistry labs routinely perform NMR in a standard 5 mm NMR tube. NMR sample is prepared by filling the lowermost 4 cm length of a regular 5 mm 0.d. tube that holds approximately 0.55 ml of a deuterated solvent. This is actually a sample dilution procedure as the signal mainly comes from the central part, i.e. 1.8 cm length sample that fits the typical coil length of 1.8 cm in regular NMR spectrometers. The diluted top and bottom part of the sample is away from the coil and contributes less signal. This dilution procedure amplifies the requirement of expensive deuterated solvent and lowers sensitivity. The present study explores a new way of sample preparation which involves sandwiching a small amount of the deuterated solvent (D2O) of length 1.8 cm containing the analyte between two non-deuterated solvents (CCl4 at bottom of length ≈ 1 cm and similarly C6H6 at top), which are immiscible with the former using a regular 5 mm o.d. NMR tube in such a way that total length is still 4 cm as demanded by the magnetic field homogeneity considerations. The analyte now being closer to NMR coil dissolved in 1.8 cm solvent, improves sensitivity and reduces deuterated solvent hitherto required. 13C and 15N spectra from such a set-up display two to three-fold higher signal to noise ratio and hence four to eightfold savings in experimental time or faster data collection.