Tandem mass spectrometry dissociation of large oligomer ions produced by argon cluster ion beams secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) using gas cluster ion beams on organic molecules such as phospholipids and amino acids produces large protonated oligomer ions. Oligomers containing up to 17 molecules have been observed for amino acids like arginine and tyrosine. In the case of distearoylphosphatidylcholine (DSPC), tetramers are detected at m/z 3159. A cluster ion SIMS system coupled with a quadrupole time-of-flight tandem mass spectrometry (Q-TOF MS/MS) instrument was developed at Kyoto University. Collision-induced dissociation of these large oligomers plays a crucial role in elucidating the stability and structure of secondary ions. MS/MS analyses of DSPC dimers, trimers, and tetramers reveal strong noncovalent interactions, with dimers further stabilized by proton bridging. Among amino acids, the most stable oligomers are observed for arginine, followed by tyrosine and phenylalanine. This trend is attributed to hydrogen bonding between functional groups, particularly the guanidinium group in arginine, which forms especially strong interactions. Unlike conventional ToF SIMS instruments that operate under ultrahigh vacuum, this system operates at relatively high helium pressures (in the pascal range). The background gas pressure significantly influences the stability of secondary ions through gas-phase cooling. In the MS/MS cell, ions undergo collisions with argon gas. Interestingly, the DSPC dimer dissociation collision energy decreases with increasing argon pressure due to multiple collisions but then increases again at higher pressures as a result of gas cooling. This cooling effect reduces the ions’ excitation time, leading to an increase in the appearance energy of certain fragmentation pathways—a phenomenon known as kinetic shift.