J. Phys. Chem. 1996, 100, 5205-5209
5205
Selective Photodissociation of Trapped Ions after Ion Cloud Manipulation with an Impulsive Quadrupolar Electric Field C. D. Cleven, M. Nappi, and R. G. Cooks* Purdue UniVersity, West Lafayette, Indiana 47907-1393
A. W. Garrett, N. S. Nogar, and P. H. Hemberger* Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: July 28, 1995; In Final Form: January 4, 1996X
Trapped ions of different masses can be separated in space within a quadrupole ion trap, making it possible to perform efficient mass-selective photodissociation on a mixture of ions. This method employs an axial quadrupolar dc pulse (1 µs) to force all ions into coherent radial motion; ions of the same mass-to-charge ratio are either in phase or phase-shifted by exactly 180°. After this activating pulse, the ions continue to oscillate at their secular frequency in a coherent fashion so that ions of the same mass-to-charge ratio simultaneously cross the z-axis (r ) 0) twice per secular cycle. At this specific moment, a single-pulse (15 ns) laser beam, aligned collinear with the z-axis, can be used to improve the photodissociation efficiency by irradiating these ions that are radially focused along the z-axis and within the confines of the laser beam. The length of the delay time between the dc pulse and the laser pulse is critical, as it controls the phase, and hence the spatial position, of the ions when the laser is fired. Under the given operating conditions, this method improves the photodissociation efficiency from 9% to 35%. The photodissociation efficiency steadily decreases with longer delay times as the oscillating ions undergo increasing numbers of collisions with the helium buffer gas and lose coherence. Since the secular frequencies of trapped ions are mass-dependent at a fixed rf amplitude, ions of different mass-to-charge ratios will cross the z-axis at different times. Massselective photodissociation is illustrated for a mixture of benzoyl-h5 and -d5 cations by appropriately adjusting the delay time between the dc pulse and the laser pulse. Simulations using the program ITSIM were used to design this experiment, and the data which describe the ion motion are provided.
Introduction Laser-based experiments have been used with quadrupole ion trap and ion cyclotron resonance mass spectrometers for many types of experiments. Laser desorption has been used to perform internal and external ionization in these devices.1-5 Photodissociation has been used to deposit high internal energies into trapped ion clouds of biomolecules,6 to acquire spectroscopic information7-9 and to probe the spatial and temporal distributions of the ion cloud.10-12 In addition, resonanceenhanced multiphoton ionization (REMPI) has been used to investigate fragmentation mechanisms.13-15 All of these experiments involve irradiation of a selected portion of the trapped ion cloud, with the exception of those experiments that handle this problem by reflecting the beam through the device multiple times.6,16 When utilizing photodissociation or other laser-based experiments in an ion trap mass spectrometer,17 it is desirable to achieve the highest efficiency possible. Since the laser beam is often irradiating only a portion of the ion cloud, a multipass6,16 or repetitive irradiation procedure is required to increase the population of the photofragment of interest. Repetitive irradiation, however, can also cause further fragmentation of those desired photofragments. A solution to this problem is to manipulate the ion cloud so that the ions are forced to simultaneously intersect the laser beam in a mass-selective fashion. One way to perform this experiment is to carefully position the laser beam so that the oscillatory motion of the ions brings them within the confines of the laser beam at the moment it is pulsed through the trap. X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-5205$12.00/0
The oscillatory motion of an ion stored in a quadrupole ion trap mass spectrometer is described by a characteristic set of frequencies. The secular frequencies (fu) imposed on the ion by a pure quadrupole electric field are described17 in terms of the parameter βu and are given as
(
fn,u ) n +
(
fn,u ) - n +
)
βu f 2 RF
)
βu f 2 RF
0en
