Performance characteristics and resonant AC dipolar excitation for ion motion control in the Orbitrap mass analyzer

Qizhi Hu, Purdue University

Abstract

This thesis describes the prototype form of a powerful mass spectrometer, the Orbitrap, based on a new type of mass analyzer invented by Makarov. Features of the Orbitrap at its present stage of development include high mass resolution (up to 150,000), high mass accuracy (2-5 ppm), a mass/charge range of at least 6000, and a dynamic range greater than 103. Applications based on electrospray ionization are described, including characterization of transition-metal complexes, oligosaccharides, drugs, peptides, and proteins. Use is also made of the high-resolution capabilities of the Orbitrap to confirm the presence of metaclusters of serine octamers in ESI mass spectra and to perform H/D exchange experiments on these ions in the storage quadrupole. Desorption electrospray ionization (DESI) is implemented on the Orbitrap mass spectrometer. The ion source is described and applications which utilize the high resolution capabilities of the Orbitrap are emphasized, including the characterization of peptides and active ingredients in pharmaceutical tablets. Measurements are made in less than 1 second at a resolution of 60,000. The implications of the data for the mechanisms of DESI ionization are discussed. Besides presenting performance characteristics and applications, this thesis demonstrates capabilities to control ion motion in the Orbitrap mass analyzer using resonant ac dipolar waveforms. A dipolar ac signal applied to the split outer electrode of the Orbitrap at the axial resonance frequency causes excitation of ion axial motion and either eventual ion ejection from the trap, if applied in phase with ion motion, or de-excitation, if applied 180° out of phase. Both de-excitation and excitation may be achieved mass-selectively. Degradation of resolution and mass accuracy due to space charge either in the storage quadrupole or during ion injection can be recovered by application of dipolar ac excitation. Rephasing of the ion population after long trapping times in the Orbitrap has been demonstrated and such a capability offers the potential to achieve tandem mass spectrometry in the Orbitrap. Attempts to accomplish tandem mass spectrometry in the Orbitrap by surface-induced dissociation, using collision with the outer electrode of the Orbitrap, were not successful; these experiments are also described.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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