In a carbon-13 experiment, a large number of scans is required in order to observe the signals from these relatively rare nuclei. These multiple scans are added together to give the final spectrum.
One problem experienced in carbon-13 spectroscopy is that the carbon-13 nuclei have to "relax" back to the ground state before the next scan is taken. Highly substituted carbon nuclei (tertiary and quaternary carbon atoms) relax very slowly (on the order of several minutes). If the delay time between scans is too short, not all of these nuclei will relax, and therefore will not be detected. Integration of the carbon-13 peaks will not always provide an accurate count of each type of carbon atom in the structure.
This problem can be overcome by setting quite a long delay time between scans. However, this procedure will usually make the acquisition of a carbon-13 spectrum take several hours. A compromise is usually made: the delay time is set to about 30 seconds (this will allow most nuclei to relax), and the spectroscopist understands that integration of the peaks will not be possible.
To be frank, integration of carbon-13 spectra is not very important. Peaks due to one carbon atom are generally half the height of peaks due to two identical carbons, and so on. Please be aware that this system does not always work well, because quaternary carbon atoms give unusually weak signals that can be easily misinterpreted.
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