Agriculture Reference
In-Depth Information
sample
mixture on the target surface. Each has its advantages and disadvantages. For
MALDI-MS analysis, it is always bene
Several protocols have been developed for deposition of the matrix
-
cial to induce rapid formation of small
crystals, because slow growth of large crystals can cause improper incorporation of
analyte molecules. Slow growth can also compromise mass resolution because of
differences in distance between the top and bottom of the crystal [65]. The oldest
but still widely employed dried
droplet method is based on direct deposition of the
sample mixed with a saturated matrix solution and drying under ambient condi-
tions. The primary advantage of this method is its simplicity and tolerance to the
presence of salts and buffers. On the other hand, relatively large, inhomogeneous,
and irregularly distributed crystals are formed, which often results in the need for
searching for the
-
on the sampling surface [79]. To reduce the size of
the crystals, increase their homogeneity, and increase the speed of the procedure,
the drying process can be accelerated by vacuum, a stream of nitrogen, or heating.
Other methods that produce small and homogeneous crystals involve physical
crushing [65], rapid evaporation of matrix solution applied to the surface in a
volatile solvent [80], and electrospraying of the matrix
“
sweet spots
”
analyte mixture onto a
grounded metal sample plate [81]. The throughput of sample preparation in MALDI
can be signi
-
cantly increased by the use of matrix-precoated layers, on which
sample is directly deposited, either manually or using automated sample deposition
systems [82].
In general, it is dif
cult to characterize the overall time requirements for sample
preparation in MALDI-MS as this parameter is strongly application dependent and
can span from few seconds (dilute-and-shoot approach) to several hours (isolation,
cleanup, and enzymatic digestion).
Laser Parameters
In order to achieve good sensitivity and mass resolving power,
attention must also be paid to optimization of laser parameters. The most in
uential
parameters are the laser wavelength and laser strength, which can be adjusted by
changing the attenuation factor (the higher the attenuation, the lower the laser
strength). Although the most widely employed nitrogen laser (radiation at 337 nm)
usually provides good results in most applications, other alternatives such as
tunable Nd:YAG or excimer lasers are also available. Because the best analytical
results can be obtained only at laser wavelengths that correspond to a high
absorption of the matrix, optimization of this parameter can signi
cantly improve
the sensitivity of a particular method [83]. For each type of sample, there is a
minimum laser strength that is required for production of ions. At settings above
this threshold, a pronounced increase of ionization yield can be observed until a
plateau is reached. Contrarily, using a laser strength that is too high typically
induces fragmentation of analyte ions. The strength of the laser is also linked to the
mass resolving power. There is a relatively narrow range of laser strength values
that provide superior mass resolving power [65]. The mass resolving power can be
further improved by employing delayed extraction of ions, which can compensate
for variations in velocities of ions with the same
m
/
z
values caused by uneven
energy distribution during laser impact.
