Biomedical Engineering Reference
In-Depth Information
diameter. A mouse brain section was cut and mounted on a glass slide and stained
with hematoxylin and eosin for anatomical visualization. A layer of the finely
ground DHB was disbursed over the tissue section through 20-m m stainless steel
sieve. Goodwin et al. [
135
] also deposited the finely ground CHCA over a rat brain
section for the quantitation of a small molecular drug. Hankin et al. [
136
] described
a matrix sublimation method for imaging MS. Sublimation of matrix (DHB, CHCA,
and SA) produced an even layer of small crystals across the sample plate and the
deposition was readily controlled with time, temperature, pressure setting and was
highly reproducible from one sample to another.
3.5
IMS Data Acquisition
After a tissue section is coated with a MALDI matrix, an optical imaging is first
scanned and restored. Then, the MALDI target plate with the coated tissue section
is loaded into the MALDI source of the mass spectrometer. The tissue section is
moved in two dimensions (
X
,
Y
) to define the exact region of interest while the laser
beam remains fixed to the inlet of MALDI-MS and the mass spectra are acquired.
The mass spectrometer records the spatial distribution of molecular species such as
peptides, protein, and/or small molecules. The nanoparticle-assisted laser desorp-
tion/ionization (nano-PALDI) imaging MS [
137
] was used to visualize lipids and
peptides at a resolution of 15 mm in mammalian tissue. Bouschen et al. [
138
] intro-
duced the scanning microprobe matrix-assisted laser desorption/ionization mass
spectrometry (SMALDIMS) and reported that mass spectrometric images of lateral
distribution of sample components and impurities were obtained with a lateral reso-
lution of 1 mm. The image resolution is affected by many factors, including crystal
size and laser beam spot diameter. Once the crystal size is optimized, the laser spot
diameter remains the critical parameter. Generally, most commercially available
MALDI-TOF-MS spectrometers equipped with N
2
(337 nm) laser or a frequency
tripled Nd:YAG (355 nm) laser provide a laser spot size at 100 mm. The smaller the
diameter of a laser spot is, the higher the resolution of a MS image is.
3.6
IMS Data Processing and Evaluation
The size of data generated by an IMS experiment is dependent on the special
resolution and MS scan area. It can vary from 100 megabytes up to a few giga-
bytes per sample. After acquisition, mass spectral data are converted into image
data. A suitable image processing software package can be used to import data
from the mass spectrometer, to generate imaging from position correlated mass
spectra and to allow visualization and comparison with the optical imaging of the
biological sample. The BioMap software package [
139-
141
] provides visualiza-
tion and a storage plate form. It allows displaying the mass spectrum from selected
