Dissertation Abstracts from the Laude Group

Eric Schmidt
Observation and Control of Ion Motion in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(May 1999)

            The application of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer to the observation of ion motion during excitation is detailed in this dissertation. This new capability, simultaneous excitation/detection (SED), is achieved with an externally adjustable capacitive matching network that is attached directly to the trapped-ion cell. This circuit permits a reduction in the RF signal that is coupled to the detection circuitry during excitation by 13-fold. The spectrometer is shown to generate real-time excitation profiles for the evaluation of axial and radial ion loss from the cell. Each profile can be acquired in 2 seconds compared to a 15 minute acquisition time by conventional methods. Experimental parameters such as trapping potential, neutral pressure, and delay time before excitation are evaluated in 1 hour by the SED method. The conventional method would have required more than 8 hours to collect and analyze the same data. The experimental parameters affecting magnetron growth within the trapped-ion cell are observed. A method is demonstrated for consistently inducing coherent magnetron growth by direct excitation at the magnetron frequency. This technique yields a 63% improvement in the reproducibility of magnetron growth initiation. The ability to produce a stable magnetron radius for an ion cloud permits the quantitative evaluation of quadrupolar excitation (QE) for minimizing radial ion loss. A new approach is shown for the evaluation of QE in which the magnetron expansion of small mass ions is monitored directly. For example, benzene molecular ions were retained from a large orbital radius with an efficiency of 87%. The 13% ion loss was attributed to ions other than the molecular ion which were not axialized by the QE event. The optimized QE parameters were then applied for the first successful remeasurement of a small mass ion; a 94.7% remeasurement efficiency was achieved for more than 50 scans of benzene at 5x10-8 Torr. Broadband QE is then demonstrated to allow for the continuous remeasurement of a single population of small mass ions during a gas-phase reaction. With this technique the self-chemical ionization reaction of the same toluene ions with toluene neutrals was observed for several hundred seconds.

Theresa Torres
Development of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for the Study of Ion-Ion Reactions
(August 1998)

            Presented in this work is the first study of a proton transfer ion-ion reaction in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. To perform ion-ion studies, the 3 Tesla FTICR system was modified to include multiple ionization source capability and a novel ion cell design. The five-electrode trapped ion cell is computationally modeled by SIMION to characterize potential well profiles. Experimental results show a three-fold increase in observed ion abundance for trapping in the outer potential well as opposed to the inner potential well. SIMION modeling results confirm a 2.5-fold increase in centerline potential, a two-fold increase in potential well depth and a 90% enhancement in depression width when trapping with the outer electrodes. Further, SIMION characterization of a "double" potential depression formed in the five-electrode cell is evaluated to enhance simultaneous collection of positive and negative ions. Simultaneous trapping of positive and negative ions is possible with the five-electrode cell configuration, but proves to be quite complex in experimental applications. An optimized experimental sequence is presented for simultaneous trapping of negative electrospray ions and positive ions generated by electron ionization. It was discovered that in order to induce an ion-ion reaction within the trapped ion cell, methods to promote overlap of the ion clouds are necessary. Three techniques for inducing movement of the ion cloud along the z-axis, including trap electrode gating, high frequency coupling and resonant excitation are evaluated for selective mixing of trapped ion clouds. Lastly, a continuous electrospray ion beam method is presented as a successful means of inducing and retaining ion-ion product within the five-electrode cell.

Jessica Robinson
Examination of Gas-Phase Conformation of Oligonucleotides Using Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(August 1998)

Carla Harper
Development and Application of an On-Line Liquid Chromatography Interface for the Analysis of Biomolecules by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(August 1998)

Jared Drader
Improved Instrumentation for Magnetic Field Focusing Electgrospary Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(May 1998)

Manuel Ventura
Performance Enhancement of a Quadrupole Ion Trap Mass Spectrometer with High Transmission End-Cap Electrodes (August 1997)

Advantages of high conductance quadrupole ion trap end-cap electrodes, culminating in the design of a hyperbolic mesh end-cap are evaluated. It is found that ion trap dynamic range is significantly enhanced by the ability for ions at any radial position to pass through these electrodes to the external detector. Increased throughput is also exploited in developing a superior means for external ion injection through a mesh entrance end-cap. The high transmission end-caps are compared to the traditional end-cap for a collection of analytical figures of merit. In all cases, superior performance is realized with high transmission electrodes.

A solid exit end-cap perforated with four sets of holes of increasing radius is used to examine signals from radii undetectable with the standard ion trap structure. Signals are measurable from radii 5-6 mm from the center axis of the trap. A half-width increase in ion radial distribution of 0.16 mm is measured for the Coulomb expansion arising from an increase in trapped ion population.

The perforated electrode is employed to address the dynamic range limitation due to space charge in the ion trap in a static pressure experiment. This electrode enhances the ability to transmit large amplitude ions and limits space charge conditions in the trap, demonstrated by reduced peak shape distortion. Under conditions in which peak base width is observed to increase by nearly 1 dalton with a standard end-cap system, base width remains unchanged using the modified exit end-cap. Enhanced dynamic range properties are extended to GC/MS for several analytical figures of merit, including chromatographic peak area (2.5-fold increase), consistent mass position (two-fold increase), and isotopic resolution (five-fold increase).

The calibration with the mesh end-cap revealed dynamic range increases of 2.5-fold for GC peak area, five-fold for mass shift, and ten-fold for isotopic resolution. The mesh electrode’s enhanced ability to trap externally generated ions is demonstrated with laser desorption in which it is found to trap over twice the number of ions as a 7 hole entrance end-cap and four times that measured from a single hole entrance end-cap system.

Scott Anderson
Development and Evaluation of a Low Homogeneity Magnet for Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(December 1996)

Vicki Campbell
Techniques for High Performance Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Analysis of Biomolecules
(December 1995)

Victor Vartanian

Chris Hendrickson
Observation and Control of Ion Trapping and Detection Efficiency in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
(May 1995)