ABSTRACT
The search for alternatives to collision-induced dissociation (CID) for the structural characterization of ions has led to the development of photodissociation (PD), an activation method in which a laser is used to energize ions. The accumulation of internal energy via absorption of one or multiple photons offers an efcient, tunable strategy that couples well with ion trap mass spectrometry. Both continuous wave (cw) and pulsed lasers, with wavelengths ranging from the infrared to ultraviolet (UV), have been used for PD. The energization processes promoted by collisional activation and photoactivation are fundamentally quite different, although the desired outcome is the same: deposition of sufcient energy to cause bond cleavages that lead to interpretable, diagnostic product ions for elucidation of ion structures. Collision-induced dissociation has remained the most widespread ion activation method in ion trap mass spectrometers, in large part because of its rich history and relatively well-understood mechanism that entails conversion of translational energy of mass-selected precursor ions into internal energy through collisions with an inert target gas, typically helium [1]. CID is accomplished by the application of a supplemental RF voltage, matched to the ion’s secular frequency, to the end-cap electrodes [1]. In a conventional quadrupole ion trap, the ion trap is operated nominally at ca 1 mTorr of helium pressure which enhances the sensitivity and resolution of the ion trap due to collisional cooling of ions to the center of the trap, thus focusing the ions
20.1 Introduction................................................................................................. 827 20.2 IRMPD Applications .................................................................................. 831
20.2.1 Small Molecules/Drugs ................................................................ 831 20.2.2 Peptides and Proteins ................................................................... 832 20.2.3 Oligosaccharides .......................................................................... 836 20.2.4 Nucleic Acids................................................................................ 837
20.3 UVPD Applications .................................................................................... 838 20.4 Conclusions ................................................................................................. 841 Acknowledgments .................................................................................................. 842 References .............................................................................................................. 843
for storage and ejection [2].* This pressure is also favorable for CID experiments where activating collisions over a 20-100 ms period are desirable. Since the very nature of CID is collision-based and because CID depends on the excitation of an ion’s kinetic energy in order to encourage more energetic collisions with the target gas, there exists the opportunity for some ion losses due to scattering events leading to ion trajectories that are either unstable or exhibit excursions beyond the connes of the ion trap, especially when exploiting conditions that maximize energy deposition. Moreover, the m/z-range of ions stored in the ion trap is determined by the magnitude of the RF voltage applied to the trapping electrodes, and it is this same RF voltage that denes the energy transfer during CID based on the energetics of the ion/target collisions. Collision-based energy deposition is maximized at the expense of trapping the lower m/z-range. Typical CID conditions in an ion trap result in truncation of the lower quarter to third of the m/z-range, thus resulting in loss of some potentially diagnostic product ions. CID is also operated generally in a resonant mode, meaning that the frequency of the supplemental RF voltage is tuned to match the frequency of motion of a selected precursor ion. As a consequence, any product ions produced will not be activated further because their secular frequencies will overlap neither with the frequency of the selected precursor ion nor with that of the applied supplementary RF voltage. MSn strategies may be employed to energize and dissociate any uninformative or dead-end product ions.
