Mass Spectrometry Fingerprints of Small-Molecule Metabolites in Biofluids: Building a Spectral Library of Recurrent Spectra for Urine Analysis

A large fraction
of ions observed in electrospray liquid chromatography–mass
spectrometry (LC–ESI-MS) experiments of biological samples
remain unidentified. One of the main reasons for this is that spectral
libraries of pure compounds fail to account for the complexity of
the metabolite profiling of complex materials. Recently, the NIST
Mass Spectrometry Data Center has been developing a novel type of
searchable mass spectral library that includes all recurrent unidentified
spectra found in the sample profile. These libraries, in conjunction
with the NIST tandem mass spectral library, allow analysts to explore
most of the chemical space accessible to LC–MS analysis. In
this work, we demonstrate how these libraries can provide a reliable
fingerprint of the material by applying them to a variety of urine
samples, including an extremely altered urine from cancer patients
undergoing total body irradiation. The same workflow is applicable
to any other biological fluid. The selected class of acylcarnitines
is examined in detail, and derived libraries and related software
are freely available. They are intended to serve as online resources
for continuing community review and improvement.