Department of Chemistry,
University of Alberta December 2002
NMR News 2002-05
News and tips for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca http://nmr.chem.ualberta.ca
There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.
Other content of this NMR News is no longer meaningful and has been removed April 2010.
|new feature in the on-line reservation system|
|FAQ 2002-05.1: baseline: why is it not straight (why is it rolling)?|
feature in the on-line reservation system
2002-05.1: baseline: why is it not straight (why is it rolling)?
This all breaks down when the parameter gain is inappropriate for the given sample concentration. gain is not a very sensitive parameter - it is quite "forgiving". However, if too high, the baseline becomes a rolling baseline with ups and downs and, worse, artifacts appear as well (see example below). The only remedy in such cases is to reduce gain and re-record the spectrum (diluting the sample would be the more time-consuming alternative!). This is illustrated below with 5 spectra with gain ranging from 48 to 0. The receiver gain follows a logarithmic scale, much like lock power, and hence a change of +/- 6 units doubles/halves the gain.
Two questions arise in this
2) Why not set gain to a very small value so that it fits all cases? When the gain is lowered more and more, the signal intensity and the signal-to-noise ratio are reduced (see below; how much depends on the spectrometer). Consequently, there is an optimum gain for a given sample concentration, but, again, it is not worth worrying too much about finding the optimum, as long as gain is not too high.
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