Abstract:
Imaging mass spectrometry can be used to reveal the relative distribution of lipids,
peptides and proteins present on or near the surface of materials with micro scale spatial
resolution. MALDI imaging mass spectrometry is a recent development, but the field has
grown dramatically in the last few years. Most of the research in MALDI imaging involves a
time consuming sampling technique ( spot-to-spot laser raster sampling ) coupled with axialinjection
TOF instruments. Recently, orthogonal-injection TOF mass spectrometry has been
used with continuous laser raster sampling to speed up the MALDI imaging process with a
higher spatial resolution and sensitivity. This thesis is based on improving the performance of
MALDI imaging in orthogonal-injection TOF mass spectrometry with continuous laser raster
sampling.
The research group at the TOF laboratory, University of Manitoba has developed a
novel MALDI ion source for imaging, in which the MALDI target is oriented from the typical
direction such that, the laser incident angle is perpendicular to the sample surface. This
modification reduced the size of laser spot on the MALDI target and hence improved the
spatial resolution of ion imaging. One of the objectives of this thesis is to study the optimum
conditions of parameters (pressure and de-clustering voltages) of the novel ion source for
protein imaging, minimizing cluster ions.
The quality of MALDI imaging highly depends on the matrix application protocol.
The application of matrix prepared in liquid base disturbs the actual chemical distribution
pattern on tissue samples and affects on spatial resolution of ion images. A spray of matrix
droplets produced by an artistic air brush is one of the popular matrix application techniques
used in MALDI imaging. To maintain a higher spatial resolution of MALDI ion images
produced with continuous laser raster sampling, it is important to have a coating of tiny
matrix droplets over the sample. To achieve this requirement, the sprayer parameters
(spraying time / distance and pressure on the liquid) have been empirically selected in present
matrix applications in imaging. Here a method is proposed to optimize the matrix sprayer
parameters to obtain smaller size of droplets over the sample and applied in an orthogonalinjection
TOF mass spectrometer coupled with continuous laser raster sampling.
Very recently, vacuumed sublimation technique which is traditionally used in
chemistry was introduced for matrix applications in MALDI lipid imaging. Conventional
glass ware system used in chemistry and high vacuumed evaporator used in electron
microscopy for sublimation are generally used for the matrix sublimation in MALDI imaging.
An attempt is made here to develop a dedicated vacuumed sublimation system for matrix
deposition in MALDI lipid imaging.
In continuous laser raster sampling, the laser continuously moves over the sample
surface while firing at a high repetition rate. The influence of sample stage velocity on ion
intensity is studied for different laser spot sizes with a Nd-YAG laser at 500 Hz repetition
rate. According to the results, the homogeneously prepared thin layer of Glyceryl trioleate
(885.43 Da) samples and human brain tissue sections show an increment of lipid ion intensity
with the sample stage velocity up to a certain value and then a decrement at a slower rate.
In orthogonal-injection TOF mass spectrometers, quadruples are used as ion
transmission channel between the ion source and orthogonal injector. It is assumed that the
spatial resolution of ion images produced by continuous laser raster sampling will be effected
due to the mixing of ion packets while they are traveling through the quadrupoles. This ion
packet mixing is investigated by studying their width at the ion detector for different laser
firing frequencies and axial voltage gradients of the quadrupoles. It is found that the ion
packets which are generated with consecutive laser shots are mixed together at the higher
laser frequencies and lower axial voltage gradients.
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