Metabolite identification using automated comparison of high-resolution multistage mass spectral trees

Multistage mass spectrometry (MSn) generating socalled spectral trees is a powerful tool in the annotation and structural elucidation of metabolites and is increasingly used in the area of accurate mass LC/MS-based metabolomics to identify unknown, but biologically relevant, compounds. As a consequence, there is a growing need for computational tools specifically designed for the processing and interpretation of MSn data. Here, we present a novel approach to represent and calculate the similarity between high-resolution mass spectral fragmentation trees. This approach can be used to query multiple-stage mass spectra in MS spectral libraries. Additionally the method can be used to calculate structure−spectrum correlations and potentially deduce substructures from spectra of unknown compounds. The approach was tested using two different spectral libraries composed of either human or plant metabolites which currently contain 872 MSn spectra acquired from 549 metabolites using Orbitrap FTMSn. For validation purposes, for 282 of these 549 metabolites, 765 additional replicate MSn spectra acquired with the same instrument were used. Both the dereplication and de novo identification functionalities of the comparison approach are discussed. This novel MSn spectral processing and comparison approach increases the probability to assign the correct identity to an experimentally obtained fragmentation tree. Ultimately, this tool may pave the way for constructing and populating large MSn spectral libraries that can be used for searching and matching experimental MSn spectra for annotation and structural elucidation of unknown metabolites detected in untargeted
metabolomics studies.