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Tuesday, May 20, 2008

Effect of 1H Tuning on the Signal-to-Noise Ratio in 13C NMR Spectra

Most liquid state 13C NMR data is collected with 1H decoupling so that all of the 13C resonances (not coupled to nuclei other than protons) will appear as singlets. Since 13C is only 1.1% naturally abundant and often the chemist is limited by the quantity of sample, acceptable signal-to-noise ratios in 13C NMR spectra are often difficult to obtain in a reasonable period of time. Any hints to improve the signal-to-noise ratio are welcome. One way to ensure that the signal-to-noise ratio is what it should be, is to make sure that the proton channel of the probe is well tuned. A poorly tuned proton channel will lead to incomplete decoupling resulting in broad lines or residual splittings and hence a low signal-to-noise ratio. The reason for this is that when the proton channel is poorly tuned the sample receives less power from the proton channel of the spectrometer and the decoupler pulses in the decoupling scheme (usually WALTZ 16) are no longer calibrated for proper decoupling. This is illustrated in the figure below.For the spectrum in the left-hand panel, the 1H channel was well tuned. In the right-hand panel, the 1H channel was poorly tuned. One can see that the spectrum acquired with the poorly tuned 1H channel has a lower signal-to-noise ratio. There are even more problems with pulse sequences (such as DEPT experiments) which require hard pulses from the 1H channel. This sort of problem can also be seen in proton detected experiments with poorly tuned 13C channels when 13C decoupling is required (eg. HMQC/HSQC experiments)

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