Abstract:
Development of instrumentation to enhance the mass resolution and the ion
separation capabilities of plasma desorption mass spectrometers (PDMS) and matrix
assisted laser desorption/ionization (MALDI) mass spectrometer are presented in this
thesis. As an introduction to the thesis, the historical development of different mass
spectrometry techniques are briefly summarized in first two chapters. Although the
time-of-flight mass spectrometry technique was introduced in late forties, the interest
within the mass spectrometry community was quite poor until the development of
new ionization techniques such as fast heavy ion bombardment, laser desorption
ionization, and electrospray ionization.
The first part of the thesis is devoted to describe the construction and some
preliminary characteristics of newly built PDMS TOF mass spectrometer with an ion
mirror. As the experience in building mass spectrometers at Colombo University is
limited, the description of the design and construction work of the new PDMS
system is somewhat detailed with the aim that this work could be used as a sourcebook
for future building projects. The main difficulties encountered in the
construction have been the high cost and the scarcity of suitable materials, and this
was partly overcome by employing used stainless steel materials and other available
materials in the local market. The standard of performance of the mass spectrometer
is found to be comparable to similar instruments operating at any laboratory. The
experiments performed with the mirror and the related simulations studies suggest
that some of the ions with non zero initial axial kinetic energies are particularly
responsible for the delayed ion formation in the plasma desorption process.
As a part of an instrument development, an electrostatic deflector has also
been designed and constructed that can be used in any MALDI time-of-flight mass
spectrometer with delayed extraction and an ion mirror to select ions of a particular
mass. The deflector consists of an interleaved set of parallel deflection electrodes.
As a new concept, thin metal ribbons instead of wires or plates have been used for
the deflection electrodes. Properly timed reversing electric field was used for the
operation of the device. In the operation, a resolving power of approximately 5200
(FWHM) was obtained for an isotopomer of PEG 6000 (m/z ~ 6000) which is far
better than that could be obtained with a commercial instrument.
Another instrumental development presented in the thesis is a construction of
high resolution multiple reflection time-of-flight mass spectrometer which was
designed using two electrostatic mirrors, mounted symmetrically on the same optical
axis facing each other. Mirrors used in the non-compensating mode, were located
between a MALDI ion source and a stop detector. The ions produced in the MALDI
source have been pulsed into the region between the two mirrors using delayed
extraction technique and trapped by successive reflections of opposite electric fields
in the mirrors for a pre-determined period of time before being detected. The
extension of the flight path due to multiple reflections has been used to increase the
mass resolution in time-of-flight spectra. Mass resolutions of 55,000 for substance-P
and 31,000 for bovine insulin were obtained for single laser shot spectra. The
stability of protonated, sodiated and potassiated substance-P ions have also been
investigated.