Li Ding

A simulation study of the digital ion trap mass spectrometer

Abstract The theoretical foundation for the digital ion trap (DIT) is presented in outline where the study of ion motion in the DIT employed the a, q, β parameter framework of conventional ion trap study. A user program for SIMION has been developed to simulate the DIT operation in real environment allowing the higher-order fields to be represented. Three-dimensional random ion buffer gas collisions were modelled where Maxwellian velocity distribution of the buffer gas was considered. A mass-selective ejection scan method is described and simulated, where both the trapping quadrupole field and dipole excitation field are driven digitally and their frequencies are scanned proportionally. The performances of the stretched ion trap and a new field-modified ion trap are simulated and compared. Using a field-adjusting electrode near the end-cap aperture, the new ion trap is able to achieve higher mass resolution for both forward and reverse mass scans. The simulation studies also the relation between the ejection time and the parameters of digital dipole excitation, the influence of space charge, duty cycle modulation as an alternative ejection scan method, and ion introduction into the ion trap.

Mapping the stability diagram of a digital ion trap (DIT) mass spectrometer varying the duty cycle of the trapping rectangular waveform

In a digital ion trap the βr and βz boundary lines of the stability diagram are determined experimentally using an innovative approach. In the rectangular waveform-driven digital ion trap (DIT) manipulation of the waveform duty cycle allows introduction of a precisely defined DC quadrupole component into the main trapping field. Variation of the duty cycle can be controlled at software level without any hardware modification. The data generated use peptide ions, which produce stability diagrams in good agreement with the theoretical stability diagrams previously determined by simulation studies.

High-efficiency MALDI-QIT-ToF mass spectrometer

A novel mass spectrometer comprising a Matrix Assisted Laser Desorption/Ionization (MALDI) source, a Quadrupole Ion Trap (QIT) and a Reflectron Time of Flight (reToF) Mass Spectrometer (MS) has been developed. Computer simulations have been carried out to model different methods for efficiently introducing MALDI ions into the QIT and for subsequently extracting ions from the QIT to a dual-stage gridless reToF. The parameters for the system were optimized to get near 100% trapping efficiency and the highest mass resolution. A prototype instrument was built to realize the performance predicted from the computer simulations. Methods for the near orthogonal laser irradiation of a sample, fast switching of large amplitude Radio Frequency (RF) voltages and a gridless dual-stage reflectron were developed. These new developments were combined with a number of other already known techniques to provide the facility to perform high efficiency ion trapping and MS/MS and higher (MSn) analysis. High mass spectra for m/z up to 16,000 have been obtained, and MS3 experiments have been successfully carried out. A sensitivity of 0.1 fmol has been achieved at a mass resolution exceeding 3,000.