Thursday, October 8, 2015

How tuning and matching impacts sensitivity

Tuning and matching is the process of optimizing the frequency and resistance of the probe to suit your sample. Our probes need to be tuned and matched manually by turning the rods at the bottom of the probes. With the 1.7 mm probe the sample volume is small enough that the tuning and matching does not vary much from sample to sample and you can get away without doing it. For the 5 mm probe, however, the larger volume results in a significant difference in the tuning and matching when switching between aqueous and organic solvents. To automate data acquisition, ideally we would use an automated tune and match module to enable the computer to  adjust the rods, but these are expensive. As a workaround we could just skip the tuning and matching and accept a loss in sensitivity. I decided to measure the loss in sensitivity by acquiring spectra of organic solvents when the probe was tuned and matched for an aqueous sample.

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
methanol-d4 15560.7 15206.2 0.977
DMSO-d6 9234.9 9133.7 0.989
chloroform-d 59714.7 48651.4 0.815

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.

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