Photo-induced dissociation of electrospray generated ions in an ion trap/time-of-flight mass spectrometer

Abstract

Laser photo-induced dissociation (PID) is an attractive alternative to collision-induced dissociation (CID) in probing structural features of biomolecules, such as peptides, by mass spectrometry. We report a new experimental setup for PID studies of biomolecules. It involves the use of an ion trap/time-of-flight mass spectrometer for the detection of PID products. The intact molecular ions are produced by electrospray ionization and introduced into a quadrupole ion trap. The ions stored in the trap are then dissociated by using a 266 nm laser beam from a pulsed Nd:yttrium–aluminium–garnet laser. After a short delay, all the fragment ions are extracted out of the trap and mass analyzed by a linear time-of-flight (TOF) mass spectrometer. The applications of this instrument for the PID studies of several peptides are demonstrated. The PID spectra are compared to those obtained by CID in the same instrument. It is shown that the amount of photoenergy deposited for fragmentation can be controlled by laser absorption property of the ions, laser power, number of laser pulses, and ion trap buffer gas pressure. Energetically optimal PID process results in spectra providing structural information similar to that obtained from CID. An excessive amount of photoenergy deposited to the fragmenting ions favors the formation of deep fragmentation products at the cost of sequence related ions. It is demonstrated that in some cases the PID technique has the potential in probing structural features of peptides that cannot be fragmented in ion trap CID. Finally, it is shown that the combination of the ion trap with the TOF detector provides a unique capability for fast detection of ions formed via PID. The PID products generated after a time delay from 1.5 μs to several milliseconds following the dissociation laser pulse can be characterized.