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Using our rule of thumb that 13C has a y value four times smaller than 1H and that 15N has a y value 10 times smaller than 1H, we can see that the FID signal will be 43 = 64 times less with 13 C and 103 = 1000 times less with 15N when compared to XH with the same number of identical nuclei (N) in the sample. But even at the same sample concentration we do not have the same number of nuclei because for 13C only about one in 100 carbon atoms is 13C and for 15N only about one in 300 nitrogen atoms is 15N. Accounting for this smaller value of N, the signal strength (sensitivity) is 5670 times less than XH for 13 C and 260,000 times less than 1H for 15N at natural abundance. For this reason, commercial continuous wave NMR spectrometers could only detect 1H. With pulsed Fourier transform NMR it became possible to detect13 C with long experiments (1 h or more) and concentrated samples (30 mg or more of a typical organic molecule). Detection of 15N is still very difficult without isotopic labeling of 15N in the sample. Biological NMR experiments (proteins, nucleic acids, etc.) now typically involve preparation of uniformly13 C and 15N labeled samples by biosynthesis (e.g., protein expression in E. coli) on labeled media (e.g., 15NH4Cl and U-13C-glucose). We will see that NMR tricks can also allow us to avoid the disadvantage of the first y (e.g., the DEPT experiment, Chapter 7), or even to avoid the disadvantage of all three gammas (e.g., 1H-detected two-dimensional experiments, Chapter 11). Without isotopic labeling, however, there is no trick that can overcome the disadvantage of low isotopic abundance.

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