Unresolved complex mixture (UCM), or hump, is a feature frequently observed in gas chromatographic (GC) data of crude oils and extracts from organisms exposed to oil.
The reason for the UCM hump appearance is that GC cannot resolve and identify a significant part of the hydrocarbons in crude oils. The resolved components appear as peaks while the UCM appears as a large background/platform. In non-biodegraded oils the UCM may comprise less than 50% of the total area of the chromatogram, while in biodegraded oils this figure can rise to over 90%. UCMs are also observed in certain refined fractions such as lubricating oils  and references therein.
One reason why it is important to study the nature of UCMs is that some have been shown to contain toxic components, but only a small range of known petrogenic toxicants, such as the USEPA list of 16 polycyclic aromatic hydrocarbons (PAHs), tend to be routinely monitored in the environment.
Analysis of the hydrocarbon fraction of crude oils by GC reveals a complex mixture containing many thousands of individual components.  Components that are resolved by GC have been extensively studied e.g.  However, despite the application of many analytical techniques the remaining components have, until very recently, proved difficult to separate due to the large numbers of co-eluting compounds.  Gas chromatograms of mature oils have prominent n-alkane peaks which distract attention from the underlying unresolved complex mixture (UCM) of hydrocarbons often referred to as the ‘hump’. Processes such as weathering and biodegradation result in a relative enrichment of the UCM component by removal of resolved components and the creation of new compounds.  It has been shown that both resolved and unresolved components of oils are subject to concurrent biodegradation, i.e. it is not a sequential process, but due to the recalcitrant nature of some components, the rates of biodegradation of individual compounds greatly varies.  The UCM fraction often represents the major component of hydrocarbons within hydrocarbon-polluted sediments  (see reference therein) and biota e.g.  A number of studies has now demonstrated that aqueous exposure to components within the UCM can affect the health of marine organisms, including possible hormonal disruption, and high concentrations of environmental UCMs have been strongly implicated with impaired health in wild populations.