Separation of mycolic acid isomers by cyclic ion mobility-mass spectrometry

Mycobacterial species contain high concentrations of mycolic acids in their cell wall. Mycobacteria can pose a threat to both human health and the environment. Mass spectrometry lipidomic characterization can identify bacterial species and suggest targets for microbiological interventions. Due to the complex structures of mycolic acids and the possibility of isobaric isomers, multiple levels of separation are required for complete characterization. In this study, cyclic ion mobility (cIM) mass spectrometry (MS) was used for the analysis, separation and fragmentation of mycolic acids isomers from the bacterial species Gordonia amarae and Mycobacterium bovis.

Mycolic acid isomers were interrogated from cultured G. amarae biomass and commercially available M. bovis mycolic acid extracts. These were infused into a cIM-enabled quadrupole time-of-flight MS. Ions of interest were non-simultaneously selected with the quadrupole and passed around the cyclic ion mobility device multiple times. Fragment ion analysis was then performed for the resolved isomers of the quadrupole-selected ions.

Repeated passes of the cIM device successfully resolved otherwise overlapping MA isomers, allowing isomer isolation and producing an ion-specific post-mobility fragmentation spectrum without isomeric interference.

Mycolic acids (MA) isomers from G. amarae and M. bovis were resolved, resulting in a high mobility resolution and low interference fragmentation analysis. These revealed varying patterns of MA isomers in the two species: G. amarae's most abundant ion of each set of MA has 1-2 conformations, while the MA + 2 m/z the most abundant ion of each set has 3-6 conformations. These were resolved after 70 passes of the cyclic device. M. bovis' most abundant ion of each keto-MA set has 2 conformations, while the keto-MA + 2 m/z has 1-2 conformations. These were resolved after 75 passes.