University of Ottawa NMR Facility Web Site

Please feel free to make suggestions for future posts by emailing Glenn Facey.



Monday, November 5, 2007

How Many Scans Should I Collect?

Many students ask this question. You should collect enough scans until the signal-to-noise ratio is adequate for your needs. For NMR measurements (where saturation is not a problem) the signal -to-noise ratio increases as the square root of the number of scans. In order to double the signal-to-noise ratio in a particular spectrum one must collect 4 times as many scans as were collected in the original spectrum. Take a look at the figure below where spectra are plotted on an absolute intensity scale against the number of scans collected.
If the signal-to-noise ratio is only 2 after 1 hour, it will be < 7 after 12 hours and only 16 after 64 hours. If you do not see a signal in 1 hour it is probably not worth running the sample overnight!

2 comments:

Anonymous said...

I probably did not understand the example: after 64 hours I expect sqrt(64) * original SNR = 8 * 2 = 16 ! and not 8 as written in the example. Am I right?

Glenn Facey said...

anonymous,

You are absolutely correct. I have corrected the error by editing the post. Thank you very much.

Glenn