Isomer separation of iron oxide cluster cations by ion mobility mass spectrometry.

Geometrical structures of iron oxide cluster cations have been analyzed by ion mobility mass spectrometry. The series of (FeO)n(+) and FenOn + 1(+) cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into a drift cell. Arrival time distributions in the ion mobility spectrometry indicate that two structural isomers coexist for the (FeO)n(+) clusters at n ≥ 5. By comparison of experimental collision cross sections determined from the arrival times with theoretical ones, two-dimensional ring and sheet structures were assignable for (FeO)n(+) (n = 3-8). In addition to these isomers, compact three-dimensional structures were also found to be stable at (FeO)n(+) (n ≥ 6). Thus, the two-dimensional and three-dimensional structural isomers coexist for (FeO)n(+) (n = 6-8).