IV. Signal-to-Noise Measurement - I

               The first spectrum described in the checkout involves determining a signal-to-noise measurement, or S/N. S/N is an important criterion for accurate integrations (see section V), and is also one of the best ways to determine how sensitive an NMR spectrometer is. In general, a higher S/N specification means that the instrument is more sensitive.

A typical result for the Unity+300 checkout is S/N = 87 and is shown in Figure IV-1, while a typical result for he VXR-S 400 checkout is S/N = 109 and is shown in Figure IV-2. If you measure S/N for the ethylbenzene standard that is significantly lower than these values, write your observations in the spectrometer logbook and inform S. Chandrasekaran .

S/N measurements for proton spectra are always determined using a sample of 0.1% ethylbenzene in CDCl₃ (ETB). It is important that the spectrum be acquired under standard conditions:

1. 90⁰ pulse width

2. Line Broadening = 1.0 Hz

3. Spectral Width = 15 to 5 ppm

4. A sufficient relaxation delay (at least 5xT₁)

5. A sufficient digital resolution (less than 0.5 Hz/point)

6. One transient acquisition

Optimum signal-to-noise ratio for any sample is achieved using a line broadening equal to the linewidth at one-half height. When this line broadening is applied, the linewidth at half-height doubles, i.e., it is the sum of the natural linewidth at one-half height plus the line broadening applied. The equation used for calculating S/N is:

S/N = (2.5 x A) / N_pp

A = height of the chosen peak N_pp = peak-to-peak noise

Peak-to-peak noise means exactly that - a measurement from the most positive to the most negative positions for the noise.

As shown below, the wildest differences occur with low probability, so there seems to be a large gap between them and what appears to be the main part of the noise; will-power must be exercised in order to truly measure this gap and not to dismiss it in our minds as “spikes” rather than the incoherent noise it is. True instrumental defects resulting in spikes can be dismissed, but only after several measurements have proven a spike is always present.