To start, I collected 1D 1H spectra using a 30o pulse (the standard parameters) for standard samples dissolved in methanol-d4, DMSO-d6 and chloroform-d. In each case the probe was tuned and matched before data acquisition and the 90o pulse calibrated. Next, the probe was tuned and matched to the standard sucrose sample which contains 90% H2O/10% D2O. Then, without changing the probe tuning, I collected a second series of 1D 1H spectra on the standard samples using the parameters used previously. Signal to noise was calculated in all spectra using the largest peak. Dividing the S/N in the untuned spectrum by that in the tuned spectrum gives the signal reduction due to not tuning and matching the probe.
|Solvent||S/N tuned||S/N untuned||untuned/tuned|
The sensitivity losses were much smaller than I expected; 1-2% for methanol and DMSO, and just under 20% for chloroform. I expected chloroform to show the greatest sensitivity loss since its tuning and matching is farthest from that of water, but I thought the loss would be greater. It is likely that by using a 30o pulse instead of a 90o pulse the impact of not tuning and matching the probe is reduced, but I chose to collect spectra using a 30o pulse because this is the standard method used on our spectrometers.
The small sensitivity losses are good news for automated data collection as it shows that not tuning and matching the probe will not reduce signal intensity too much. However, these results are only valid for simple, single pulse experiments, such as 1D 1H and and 1D 13C spectra. Any multipulse experiments such as 2Ds, DEPTs or 1D selective experiments are likely to suffer much more if the probe is not tuned and matched. These experiments may have to be run outside of the automation.