IV. Signal-to-Noise Measurement - II

Choice of a noise region must be consistently applied for standard samples, and for 0.1% ethylbenzene (ETB), we have chosen 5 to 3.5 ppm. It is also critical that the noise be amplified if necessary in order to make an accurate measurement using a ruler. There is not much point in

trying to compare the ratio of noise to a few millimeters high to a peak 20 cm high; plot the noise such that it is several centimeters high. If the instrument performs the signal-to-noise measurement and calculation, the relative intensities are not as critical. Both the Unity+300 and VXR-S 400 NMR spectrometers perform the S/N calculation by software commands.

The last point concerning sensitivity is demonstrated during the ¹³C{¹H} part of the checkout. The checkout requires that you acquire three spectra, using 1, 4, and 16 transients. Signal-to-noise increases as the square root of the number of transients; to double the signal-to-noise you must take four times as many transients.

When using a concentrated sample such as 30% menthol for ¹³C, or when running routine ¹H spectra, the number of transients or scans is quite small, so the point discussed above may not seem important. However, suppose you are in the following situation: you have only a few mg of research sample, and after collecting a ¹³C{¹H} spectrum on the Unity+300 for 2 hours, you get peaks with S/N of only 5:1. Since the peaks are barely visible above the noise ( and you may have missed quarternary carbons), you want to re-collect the spectrum to get a S/N of 50:1, a value most typical for carbon NMR. Unfortunately, this will take 10x10x2 = 200 hours! In such cases, you should consider using a higher field strength spectrometer, which will be able to give you a spectrum with good S/N in much less time.