Selective Photodissociation of Trapped Ions after Ion Cloud Manipulation with an Impulsive Quadrupolar Electric Field

Abstract

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-h 5 and -d 5 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.

Ion Mobility Measurement by Dc Tomography in an Rf Quadrupole Ion Trap

“…The initial distribution widths were equilibrium values obtained from preliminary simulations and are consistent with experimentally determined values, 16,19 if the somewhat different ion mass is taken into account. Subsequently, the ions were subject to ac resonance excitation fields and dc pulses using equivalent parameters to those used in the experiment.…”

“…These slots affect the fields experienced by the trapped ions and, among other consequences, are known to cause mass shifts. 19 As an alternative to the use of laser photodissociation to monitor the position of an ion cloud, Weil et al 21,27 showed that dc tomography could be used to characterize ion motion. In this experiment, the ion population being studied is subjected to a short dipolar or monopolar dc pulse of appropriate amplitude.…”

“…The coherent oscillation of an ion cloud with a finite spread in ion velocities can thus be monitored by measuring the number of ions remaining in the trap after application of the dc probe pulse. Since the spatial and velocity distribution functions for trapped and cooled ion populations are nearly Gaussian, 16,19 the relationship between secular velocity of the coherent ion cloud and number of remaining ions is nonlinear and the range of ion velocities detectable by this method is on the order of the spread of velocities. Larger velocities can be measured by increasing or decreasing the dc pulse voltage and monitoring just the velocity maxima or minima.…”

Laser Ablation Ion Trap Mass Spectrometry—Storage Field Suppression and its Effect Upon Analytical Performance

“…Direct, high-power, non-specific ablation clearly has drawbacks. Not only can space-charge effects induced by extraneous ions degrade mass accuracy and resolution, 25 but the RF suppression described here can degrade trapping performance. A selective ionization scheme for direct analysis of solid materials, such as resonant laser ablation, seems to be a logical coupling of technologies in that analyte ions are selectively created in the absence of bulk matrix ions, reducing or eliminating space-charge effects.…”

Real-Time Broad Spectrum Characterization of Waste by Membrane Introduction Mass Spectrometry

“…By adjusting the DC and RF potentials on the linear quadrupole, we can select the ions of a particular mass-to-charge ratio (m/z) prior to their introduction to the ion trap [8]. In this fashion, we can reduce space charge effects, which will lead to improved mass spectral quality in terms of peak shape and mass measurement accuracy, [9] and mass selective storage will provide lower detection limits by allowing the selective accumulation of only those ions of analytical interest [10]. While mass selection of ions can be effected by applying broad-band waveforms to the ion trap to resonantly eject unwanted ions, it is our experience that external mass isolation leads to higher mass spectral quality than allowing all ions to enter the ion trap and then be ejected by resonant excitation methods.…”