But we can do better than this: we can also work out what sort of chemical environment the carbon atoms are in. All 13C spectra can be divided into four major regions: saturated carbon atoms (0–50 ppm), saturated carbon atoms next to oxygen (50–100 ppm), unsaturated carbon atoms (100–150 ppm), and unsaturated carbon atoms next to oxygen, i.e. C O groups (150 to about 200 ppm).

The spectrum you just saw is in fact that of lactic acid (2-hydroxypropanoic acid). When you turned the last page, you made some lactic acid from glucose in the muscles of your arm—it is the breakdown product from glucose when you do anaerobic exercise. Each of lactic acid’s carbon atoms gives a peak in a different region of the spectrum. But hang on one moment, you may say—don’t we only see signals for carbon-13 nuclei and not carbon-12, which make up most of the carbon atoms in any normal sample of lactic acid? The answer is yes, and indeed only about 1.1% (the natural abundance of ¹³C) of the C atoms in any sample are ‘visible’ by ¹³C NMR. But since those ¹³C atoms will be distributed more or less randomly through the sample, this fact does not affect any of the arguments about the appearance of the spectrum. What it does mean, however, is that ¹³C NMR is not as sensitive as 1H NMR, for example, where essentially all of the H atoms in the sample will be ‘visible’.