Masters Theses

Date of Award

12-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemistry

Major Professor

John E. Bartmess

Committee Members

James Chemseis

Abstract

Ion Cyclotron Resonance Mass Spectrometry is relatively a young analytical technique. Therefore instrumental development is a large focus of research. In this thesis chapter 2 will focus and discuss modification of the analyzer cell to a different geometry. In chapter 3, a relatively new method of trapping ions, quadrupolar excitation will be investigated.

Signal intensity is directly related to the number of ions trapped in an ICR mass spectrometer. Therefore it is postulated that a larger cell should produce signals of greater intensity than a small cell. There are limitations in the construction of such a cell: the z-dimension is limited due to the gap of the electromagnet, and the receive plates must be relatively close to the ions for appreciable signal linearity at small excitation levels. This leaves only the third dimension for expansion.

A cell has been built four times the length of the standard cubic cell. In this elongated cell, a uniform excitation rf field gradient is achieved by utilizing segmented trapping plates with appropriate fractions of the excitation rf on them. Experiments have been conducted to test the signal dependence on emission current, excitation rf field, trapping plate voltage, grid pulse length, transmit plate voltage, and receive plate voltage.

This new cell has achieved 433% greater signal output for comparable conditions to a standard cubic cell. The cell shows a non-linear dependence on the emission current, but increased signal output can be achieved with much longer grid pulse lengths than normally used in ICR spectrometry. Time trapping studies have been done for the three section and cubic cells. These studies prove that the kinetics of ion loss for the three section cell are similar to those of the cubic cell. It was found that more ions filled the three section cell compared to the cubic cell under similar conditions.

Anions are sometimes formed by a dissociative attachment reaction. Some of the molecules of interest that undergo dissociative attachment, such as water and ammonia, have a low cross section for the process. Therefore it is necessary to carry out the reaction under relative high pressure (5 x 10-6 torr). There is a pressure limit of approximately 10-5 torr in ICR due to collisional loss of ions from the cell. If high pressure is necessary to generate anions, there is not much pressure range left for other gases to be added to carry out the desired chemistry. A high pressure pulse of reactant gas during the electron beam event would be the solution to the problem but the ions will have the tendency to diffuse to the walls of the analyzer cell during the high pressure event. It is possible to trap ions of interest using quadrupolar excitation and prevent ion loss from the cell.

In quadrupolar excitation an additional sinusoidal voltage is used to generate a quadrupolar electric field with a symmetry axis parallel to the magnetic field. This can be done by applying one phase of excitation signal to the pair of plates which function as the transmit plates in normal ICR configuration, while applying the inverse phase of the signal to the receive plates. This excitation signal, which causes the interconversion of the cyclotron and magnetron motion in conjunction with collisional relaxation of the cyclotron orbit, slows the decay of the cyclotron radius and shrinks the magnetron radius. As a result, the ions are centered in the trap.Modifications were done to the UTK ICR mass spectrometer to equip it with the capability of pulsed high pressure quadrupolar excitation. Studies were done to test that ions were trapped in the cubic cell under high pressure conditions. Samples of benzene and water were studied and during each study the conditions were optimized. A proton exchange study was attempted between toluene and hydroxide.

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