Gas chromatography with methane chemical ionization and quadrupole-time of flight mass spectrometry obtains molecular ion species to automatically assign elemental formulas
RATIONALE: Gas chromatography–mass spectrometry (GC-MS) usually employs hard electron ionization, leading to extensive fragmentations that are suitable to identify compounds based on library matches. However, such spectra are less useful to structurally characterize unknown compounds that are absent from libraries, due to the lack of readily recognizable molecular ion species.
METHOD: We tested methane chemical ionization on 367 trimethylsilylated (TMS) derivatized metabolites using quadrupole time-of-flight detector (QTOF). We developed an algorithm to automatically detect molecular ion species and tested SIRIUS software on how accurate the determination of molecular formulas.
RESULTS: The automatic workflow correctly recognized 316 (86%) of all detected, derivatized standards. Specifically, strong [M-CH3]+ fragments were observed in all 290 derivatized. Using Sirius software, correct elemental formulas were retrieved in 87% cases within the top-3 hits. When investigating the cases for which the automatic pattern analysis failed, we found that several analytes showed a previously unknown adduct [M+TMS]+, formed by rearrangement.
CONCLUSIONS: Methane chemical ionization with GC-QTOF mass spectrometry is a suitable avenue to identify molecular formulas for abundant unknown peaks.
Chemical ionization, quadrupole-time of flight, unknown metabolite annotation