Figure 3.

Relation between HOM and volatility classes in carbon–oxygen space. On the basis of the criterion of minimum of six oxygen atoms (see section 2.1.2), potential HOM are easily depicted in this space. However, the other criteria also need to be met in order to justify the assignment as HOM. The positions of the volatility classes can only be roughly estimated, as the precise functionalities will determine the volatility of the molecules. We also note that most of the top left part of the figure depicts purely hypothetical molecular compositions, but the red shaded area is not restricted for simplicity. As HOM often contain other heteroatoms than C and O, such restrictions would depend on what molecules can be considered plausible.