Natural abundance

Most elements have several isotopes, one or more of which are NMR-active because the nuclear spin I>0, and the natural prevalence of these nuclides varies. For example, the predominant isotope of carbon is ¹²C, with a natural abundance of nearly 99%; however this is a spin-0 nuclide. The NMR-observable form is ¹³C (I=1/2), which is 1.1% abundant. This means that, during an NMR experiment, only about 1% of the carbon atoms in the sample are able to contribute to the signal. Obviously, this implies that it will take much longer to acquire a ¹³C spectrum than, say a ³¹P spectrum (natural abundance of ³¹P = 100%), given the same number of atoms in the samples. For radioactive nuclides, the concept of natural abundance has no meaning, since the preponderance of these isotopes changes with time.