The use of fast DC pulses with a quadrupole ion trap mass spectrometer

Stephen Alan Lammert, Purdue University

Abstract

The use of a fast DC pulse in conjunction with a quadrupole ion trap allows the rapid acceleration and translation of ions stored in this device. The home-built pulser generates a short $(<$5 $\mu$s), fast-rising $(<$20 ns rise time), high voltage (0-950 volt) DC pulse which is applied to either or both endcaps of a standard Paul-type quadrupole ion trap. The initiation of the pulse with respect to the phase of the fundamental RF drive signal can be controlled. Experiments which utilize the rapid acceleration and position changes provided by the pulse are explored. The DC pulse method of activation uses the increased kinetic energies provided by the pulse to deposit large amounts of internal energy to trapped ions. Large pulse amplitudes result in the ion colliding with the electrode surface and sufficient internal energy is acquired in this collision to cause high energy fragmentation of relatively intractable molecular ions. High internal energy deposition is achieved as evidenced by (m/z 91)/(m/z 92) fragment intensity ratios in excess of 20 for dissociation of the n butylbenzene molecular ion and by complete dissociation of the tungsten hexacarbonyl molecular ion (W(CO)$\sb6\sp{+.})$ to W$\sp+.$ Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electron volts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main rf drive voltage at the point when the pulse is initiated. Dissociation efficiencies are less than those typically observed for collision-induced dissociation in the ion trap and range from 50% at low internal energy deposition to 5% at maximum internal energy deposition. When small pulse amplitudes are used, the ions are moved away from the center of the ion trap and subsequent delayed probing, by laser photodissociation, of the ion position after the pulse provides a time-varying photodissociation fragment ion signal. Contained within this signal are the fundamental ion frequencies which can be extracted by Fourier analysis. The changes in relative magnitudes of these fundamental frequencies can be monitored as the storage conditions of the trap are varied.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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