Matrix-assisted Laser Desorption/Ionisation (MALDI)
In the early 1960's, it was demonstrated that the irradiation of
low-mass organic molecules with a high-intensity laser pulse formed
ions that could be successfully mass analysed. This was the origins of
laser desorption (LD) ionisation. Other the next few decades, the
technique underwent substantial development, culminating in the
extension of the technique to the volatilisation of non-volatile
biopolymers and organic macromolecules. There was, however, a sharp
cut-off in mass at about 5-10 kDa, limiting the technique's
application. The other main limitation was that ions were created in
bursts which prevented the technique from being coupled to scanning
mass analysers. In fact LD was only really successful when coupled to
time-of-flight (TOF) mass analysers.
In 1987, Michael Karas and Franz Hillenkamp [1] successfully
demonstrated the use of a matrix (a small organic molecule) in LD to
circumvent the mass limitation. The matrix had a strong absorbance at
the laser wavelength and was highly sublimable [2]. A low
concentration of the analyte was mixed with this matrix onto a probe
or metal plate (see figure) and introduced into a pulsed laser beam. A
substantial burst of ions was produced with each laser pulse. An
unexpected side effect of the matrix was that it allowed for the laser
incidence spot to be refreshed between each pulse, thus greatly
enhancing shot-to-shot reproducibility. This was the foundation of
matrix-assisted laser desorption/ionisation (MALDI). Later
developments by Koichi Tanaka [3] demonstrated the application of
MALDI to a whole range of biological macromolecules. This gave him a
part share of the 2002 Noble prize for chemistry [4], making him the
5th mass spectrometry pioneer to receive such an honour.
The mechanism of MALDI is believed
to consist of three basic steps:
(i) Formation of a 'Solid
Solution': It is essential for the matrix to be in excess
thus leading to the analyte molecules being completely isolated from
each other. This eases the formation of the homogenous 'solid
solution' required to produce a stable desorption of the analyte.
(ii) Matrix Excitation:
The laser beam is focused onto the surface of the matrix-analyte
solid solution. The matrix chromophore absorbs the lase irradiation
causing rapid vibrational excitation, bringing about localised
disintegration of the solid solution. The clusters ejected from the
surface consist of analyte molecules surrounded by matrix and salt
ions. The matrix molecules evaporate away from the clusters to leave
the free analyte in the gas-phase.
(iii) Analyte Ionisation:
The photo-excited matrix molecules are stabilised through proton
transfer to the analyte. Cation attachment to the analyte is also
encouraged during this process. It is in this way that the
characteristic [M+X]+ (X= H, Na, K etc.) analyte ions are
formed. These ionisation reactions take place in the desorbed
matrix-analyte cloud just above the surface.
References:
[1]
M. Karas and F.
Hillenkamp,
International
Journal of Mass Spectrometry and Ion Processes, 78,
1987, p53.
[2] A series of papers covering matrix development were
published shortly after the initial MALDI papers - for example:
R.C. Beavis and B.T. Chait,
Rapid
Communications in Mass Spectrometry, 3,
1989,
p432.
R.C. Beavis and B.T. Chait,
Organic
Mass Spectrometry, 27,
1992,
p156.
M. Karas and F. Hillenkamp,
Organic
Mass Spectrometry, 28,
1993,
p1476.
[3] K. Tanaka,
et. al.,
Rapid Communications in Mass
Spectrometry, 2,
1988,
p151.
[4] K. Tanaka,
Angewandte
Chemie - International Edition, 42,
2003,
p3861.
[5] Several good reviews of MALDI have also been published - for
example:
M. Karas
et. al.,
Mass Spectrometry Reviews,
10,
1991, p335.
R.C. Beavis,
Organic
Mass Spectrometry, 27,
1992,
p653.
U. Bahr, M. Karas and F. Hillenkamp,
Fresenius'
Journal of Analytical Chemistry, 348,
1994,
p783.