Although the functional groups were similar, the TC was low because some compounds had cyclic alkyl moieties and others had linear alkyl moieties. This is a limitation of using the TC to evaluate similarity. The F cluster, excluding inorganic substances that are often assigned to it, exhibits a structure with aliphatic hydrocarbons similar to those of the E and G clusters (Figures ). It is characterized by a region with a relatively high baseline compared with clusters E and G.