Biomedical Engineering Reference
In-Depth Information
target compounds to be analyzed. The solvents used for MALDI-MS can be water,
ethanol, methanol, acetonitrile, acetone, chloroform, tetrahydrofuran, and a mix-
ture of solvents. Generally, the MALDI matrix solutions should be prepared freshly
as they are susceptible to light-induced decomposition. Often a matrix solution is
saturated and allowed to settle. Trifluoroacetic acid at 0.1-0.2 % could be added to
the matrix solution to enhance ionization.
2.3
MALDI Sample Preparation
To prepare a sample, an analyte of interest is dissolved in an appropriate solvent at
a concentration of about 20 mM. This solution and a matrix solution (50:50, vol/vol)
are mixed with an appropriate matrix-analyte ratio at ³10:1 in a small vial. If an
internal standard is used, the internal standard is dissolved in the sample solution
before combining with the matrix. A tiny drop (0.5-2.0 ml) of the final solution is
transferred and applied to the MADLI sample target plate which can be a stainless
steel or a Gold-coated MALDI sample plate. The sample-matrix solution is allowed
to dry at room temperature and under normal pressure for several minutes until all
of solvents are completely evaporated. The precipitate is the cocrystallization of
matrix and analyte, ready for MALDI analysis.
There are several ways to coat a MALDI plate. One is two-layer overlayer [
62,
63
] :
In this method, the first layer on the MALDI target plate comprises the densely
packed matrix microcrystals formed by the quick solvent evaporation of a matrix
solution in acetone or a mixture of acetonitrile and water at room temperature.
A mixture solution containing both matrix and sample is then deposited onto the
first layer of small crystals to form uniform analyte-matrix microcrystals. The
overlayer method is reported to provide improved results, particularly for analyses
of proteins and peptides [
64-
66
] .
The second method is sandwich method which was developed from the two-
layer method and used first for the analysis of the single mammalian cell lysates
[
67
]. The first thin layer is formed by the matrix-only solution. It is followed by
deposition of the analyte solution and then deposition of second layer of matrix on
the top of analyte layer. The sample is basically sandwiched between the two
matrix layers and preparation results in a matrix-sample-matrix sandwich. This
method is specifically used for detecting protein and peptides [
67-
69
] . The three-
layer matrix-sample preparation method was also used as matrix-matrix-sample
mode [
70
] .
Han and Schey [
71
] used a special matrix coating method in processing the
bovine lenses sections (30-40 mm) for MS imaging. The tissue sections were first
sprayed with an acetonitrile-water (50:50, vol/vol) solution resulting in a tightly
bound section. After drying, the tissue sections were coated with a thin layer matrix
of SA at 15 mg/mL in ethanol-water (50:50, vol/vol). After it was dried, the tissue
sections were finally sprayed with several cycles of SA matrix solution at 15 mg/mL
in ethanol-water-formic acid (44:44:12, vol/vol/vol).
