Figure 18. Carbon-13 nuclear-magnetic-resonance (¹³C NMR) spectra of coals of increasing rank (see figure 17); ¹³C is a stable isotope of carbon. Diagram shows changes in certain organic compounds brought about by increased coalification. The peaks represent varieties of organic compounds. As rank increases, the variety of compounds decreases and the remaining compounds become structurally tighter and denser. The compounds lost are those that are high in hydrogen and oxygen. Peaks marked with a single asterisk are lignin (woody tissue) indicators; those marked with a double asterisk represent carbohydrates (sugars); and those marked with a triple asterisk represent aliphatic substances (such as waxes and resins). The diagram shows that the carbohydrates are eliminated early in the coalification process, whereas the lignins, waxes, and resins persist to become the principal constituents of higher ranked coals. The scale at the bottom of the figure is a standard ¹³C NMR scale in ppm (parts per million) representing distance, in frequency units, from a standard, which is assigned a position of zero on the scale. The position of peaks is used to interpret the chemical structure of the substance being analyzed, in this case coal. Data from W.H. Orem (U.S. Geological Survey).