12 - NMR Theory 3

12 - NMR Theory 3 - More practical stuff More basics of...

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1 More practical stuff More basics of data collection • Why bother with FT NMR? Signal averaging. Important for low concentrations, for low γ nuclei, and for nuclei with low natural abundance. 13 C has a 1.1% natural abundance and is ¼ the γ of 1 H. Signal to noise (S/N) scales with NS (number of scans). That is, the signals add coherently and the noise adds randomly; so to double the S/N, you must quadruple the number of scans.
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2 The change in S/N with NS The flip angle debate •I s a 9 0 ° pulse the best to achieve the highest S/N in a given time period? You would think that placing the maximum signal possible into the x-y plane (where it is digitized) would be desirable. True, if NS=1. However, for 13 C, it turns out that a 30-45 ° pulse works best with d1~1s. Why? consider the angles: for a 90 ° pulse, all of the magnetization must return along the Z-axis. For a 30 ° pulse, there is 0.5 of the signal in the x-y plane (sin30= 0.5) and 0.87 (cos30= 0.866) on the Z-axis. There is something called the Ernst angle : cos α E
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3 Receiver gain The analog to digital converter limits both the range of
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12 - NMR Theory 3 - More practical stuff More basics of...

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