The fact that a peak cannot be explained by Mass Frontier, because the corresponding fragment was probably formed by a compound specific mechanism, can in some cases be surprisingly helpful in the identification of characteristic structural groupings that give rise to the peak. For example, the phthalates produce a characteristic ion with m/z 149, which is formed by a highly specific mechanism. The peak at m/z 149 can be easily recognized as a contaminant from elasticized polymers. Mass Frontier is not able to explain this peak since its corresponding fragment is formed by an unusual hydrogen rearrangement and cyclization, which is not supported. To distinguish between a “randomly” unexplained peak and a compound-specific peak you need to find some examples in the library. Using a substructure search, you can retrieve compounds that contain a phthalate group as a common substructure. After the generation of fragments and mechanisms of the retrieved examples, the prominent peak corresponding to the phthalate group remains unexplained in the majority of cases. For example, a phthalate with a functional group at position 3, 4, 5, or 6 will have its prominent peak shifted to higher masses by the mass of this functional group. Such an unexplained, prominent peak, present in the spectra of structurally similar compounds, can be a strong indicator of a compound specific fragmentation process. This information can serve as evidence toward the identification of a substructure under investigation.