Yq. Dai et al., CONFOCAL FLUORESCENCE MICROSCOPIC IMAGING FOR INVESTIGATING THE ANALYTE DISTRIBUTION IN MALDI MATRICES, Analytical chemistry, 68(15), 1996, pp. 2494-2500
The analytical performance of matrix-assisted laser desorption/ionizat
ion (MALDI) mass spectrometry is strongly influenced by the method of
analyte and matrix preparation. We report a nonintrusive method based
on laser confocal microscopic imaging technology to examine the MALDI
samples prepared by various protocols. In this method, the analyte is
tagged with a fluorescent group, The matric and analyte are prepared u
nder the same conditions as those used in conventional MALDI experimen
ts, It is demonstrated that: confocal microscopy can provide clear, th
ree-dimensional images of sample crystals as well as the analyte distr
ibution within the crystals. It is shown that the analyte is incorpora
ted into the matrix crystals for all the sample preparation protocols
examined. Moreover, the confocal microscopic images reveal that, with
the use of a dried-droplet method for sample/matrix preparation, the a
nalyte is not uniformly distributed within the matrix crystals, In sam
e crystals, no analyte is incorporated. In addition, it is found that
large crystals formed using a slow growth process display a more unifo
rm analyte distribution. Relatively more uniform analyte distribution
is observed for samples prepared with the formation of microcrystals.
The possible correlation between the ion signal variations observed in
MALDI and the uniformity of the analyte distribution obtained by the
confocal microscopic imaging method is discussed. Finally, a double-im
aging method involving the use of two analytes with different labeling
groups is demonstrated. It is found that different analytes are not c
oherently distributed in the matrix crystals.