Sunday, July 2, 2017

Sensitivity vs receiver gain

Recently I have noticed many users recording spectra with low values for the receiver gain. The receiver gain is a scaling factor for the FID signal that ensures spectra are not distorted if the signal is too large, or resolution is lost if the signal is too small. Typically the receiver gain is set automatically by the command "rga", but users should pay attention to the value obtained as it is a good indicator of how successful the experiment will be. To demonstrate this I measured sensitivity at different receiver gain values.

The sample I used contained 1% chloroform in acetone-d6 in a 1.7mm NMR tube. I recorded a simple 1D 13C spectrum with decoupling during acquisition using 16 scans. Spectra were recorded with receiver gain values ranging from 1 to 2050. Two signals were obtained, one from the acetone carbonyl and the other from the acetone methyl groups. The chloroform was not detected. The signal to noise ratio was measured on both peaks using a 10,000 Hz region in the center of the spectrum for the noise. Below, the S/N is plotted against receiver gain.

Both signals show the same trend, a linear increase in S/N until the receiver gain nears 100, after which it remains constant. This demonstrates that for maximum sensitivity the receiver gain should be at least 100. Running an experiment with the receiver gain set to 2.56, for example, reduces the sensitivity by a a factor of 5 and would need to be run 25 times as long as an experiment with the receiver gain set above 100. If you run "rga" and the receiver gain is set to a small value then you should consider changing something to increase the receiver gain.

The data above were obtained from 1D 13C spectra. The most common cause of low receiver gain settings with this type of experiment is a large spike at the start of the FID. This spike is caused by acquisition starting immediately after the last pulse. The pulse takes a finite time to return to zero and some of it may be recorded as part of the FID. Cryoprobes are particularly sensitive to this type of problem. Using a different type of pulse sequence, one which includes a delay between the last pulse and the start of acquisition, is the best solution. Such pulse sequences include the APT, DEPT, DEPTQ and INEPT discussed in the previous post.

For 1D 1H spectra with large solvent peaks it is sometimes difficult to use large receiver gain values without clipping the FID and introducing artifacts. In these cases, the solvent suppression needs to be optimized, either by improving the shimming to narrow the peaks being suppressed, moving the center of the spectrum if using presaturation, or adjusting gradient strengths if using excitation sculpting or watergate.

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