| PROCESSING YOUR NMR DATA |
| SOFTWARE-INDEPENDENT TUTORIAL |
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There are a number of steps involved in converting the time-dependent signal recorded by an NMR spectrometer into an interpretable spectrum. While the exact software commands used to effect these operations varies from system to system, the rationale for each step is the same. This section will describe each of these operaitons, in the order in which they are done, during the normal processing of NMR data. The first few pages deal with manipulations performed in the time-domain (i.e., on the original signal); the remainder, with processing of the transformed spectrum. |
| Characteristics of the free-induction decay (FID) |
| An NMR signal, or free-induction decay (FID) is composed of one or more damped oscillations. (in rare instances, it can simply look like a decaying exponential). The signal is a plot of the voltage (y-axis) induced in the spectrometer's receiver versus time (x-axis). For this reason, an NMR signal is said to be in the time domain. |
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| Note that the oscillation is characterized by only one frequency across the signal. An FID this simple indicates that there is only one type of distinguishable species present in the sample. More commonly, the FID will be composed of several different components of varying frequencies, intensities, and decay constants, giving rise to very complex signals that cannot be interpreted by visual inspection alone. In such cases, the Fourier Transform is used to sort out the components. |
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If two components of slightly different frequency, but equal intensity and decay constant, such as those shown below, are present, the observed FID will be their sum. To view the summed signal, move your mouse into and out of the area below. |
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| If one component is much stronger than the other, as is the case for the red versus the blue below, then the summed signal looks mostly like the dominant component. To see this, move your mouse into and out of this figure: |
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| If, on the other hand, the two components are of equal intensity, but unequal decay constant, then the resulting FID has more intensity at short times, and the envelope of the signal is not a smooth curve. |
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| The results of transforming these different cases will be discussed in a later section. |
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