Biomedical Engineering Reference
In-Depth Information
This chapter describes and discusses the mostly used IMS technology—MALDI-
TOF-IMS, including sample preparation, data interpretation/handling, and its
applications.
2
Principles of MALDI Mass Spectrometry
MALDI was introduced in the late 1980s and is one of the most successfully devel-
oped MS soft ionization techniques that uses the matrix assists laser ablation of
sample-coated target to vaporize gas-phase ions for injection into a mass spectrom-
eter. The advantage of MALDI is its gentleness compared with ESI and Atmospheric
Pressure Chemical Ionization (APCI) and its ability to analyze the polar, nonvola-
tile, and large molecules. It has been very successfully used for the analysis of both
biopolymers compounds and small molecular organic compounds (<1,500 Da).
2.1
The Principle of MALDI Ionization Method
The Laser Desorption Ionization (LDI) was investigated by Franz Hillenkamp and
Michael Karas [
2,
3
]. LDI involved sample bombardment with short and intense
pulses from a laser light to effect both desorption and ionization of the analyte mol-
ecules. It has become a soft desorption ionization method for mass spectrometric
analyses of biological macromolecules and small molecular compounds. The
MALDI technique was improved by Koichi Tanaka.
A laser beam serves as the source of desorption and ionization. Many different
types of laser light have been studied for MALDI-TOF-MS. The most used lasers
include pulsed N
2
laser with a wavelength of 337 nm and Nd-YAG solid-state laser
with a wavelength of 355 nm. The ideal laser should deliver an efficient and control-
lable quantity of energy to the samples, and in order to avoid thermal decomposition
this energy must be transferred quickly. The samples have absorption of the laser
energy radiation and ionization.
2.2
Choice of Matrix
MALDI-MS employs a matrix and the use of a matrix with the samples has several
purposes: extraction of analyte from the cocrystallization surface, formation of
analyte-dropped crystals, and absorption of the laser energy for soft-ionization of
analyte molecules into MS analyzer. The commonly used MALDI matrices include
a-cyano-4-hydroxycinnamic acid (CHCA), 2,5-Dihydroxybenzoic acid (DHB),
Sinapinic acid (SA), et al. [
61
]. A typical matrix solution would comprise the
matrix at a concentration of 10-20 mg/mL in a solvent that is compatible with the
