Agriculture Reference
In-Depth Information
Preparation and deposition of matrix
-
sample mixture.
Laser parameters.
Matrix
ll a number of requirements. The matrix
compound has to absorb the laser wavelength, dissolve and/or cocrystallize with
the sample, be stable under the vacuum conditions, induce codesorption of the sample
components upon the laser impact while minimizing its thermal and chemical
degradation, and, last but not least, promote the ionization of the target analyte(s).
Given these requirements, it is apparent that the matrix choice is strongly linked to the
nature of the analyte; hence, a matrix that works well for one analyte/sample type
combination can be ineffective for another. A wide variety of matrices has been
proposed for different applications [57]. An overview of some matrices used in food-
related applications is provided in Table 2.3.
In addition, the choice of optimal matrix-to-analyte concentration ratio is
crucial for effective MALDI ionization. Considering the principle of MALDI,
the concentration of matrix has to be consistently maintained in excess with
respect
A MALDI matrix has to ful
50,000-fold molecular ratio range is typically
recommended. If the amount of analyte is too high or too low, no analyte-related
mass spectra are observed [65]. On the positive side, using an appropriate
concentration ratio can lead to diminution or complete elimination of matrix
ions, which can interfere with analyte signals [63]. Serious matrix-related prob-
lems may arise when applying MALDI to low molecular weight compounds,
because of abundant, low m / z matrix ion signals. To overcome this drawback, a
matrix-free approach has been proposed. However, such procedures are limited by
the stability of the target analytes as rapid thermal degradation occurs upon sample
exposure to the laser beam [66]. As an alternative, ionic liquids (combinations of
organic cations with a variety of anions) have been employed as matrices in order
to obtain matrix-free mass spectra. The use of ionic liquids has other bene
to the analyte. A 500
-
ts,
including the production of a much more homogeneous sample solution, greater
vacuum stability, and, in many cases, higher signal intensities compared with
conventional liquid and solid matrices [67]. Regardless of the type of matrix used,
it is a common practice to dope the sample with solutions of acids, alkali metals, or
silver salts to improve the ionization yield of analytes and enhance the formation
of particular ion types [57].
Sample Preparation
Because the time required for MALDI measurements is
typically under 1min, the time required to prepare the samples dictates the throughput
of the whole analytical procedure. The process of sample preparation for MALDI
consists of two steps: (i) pretreatment of sample, and (ii) deposition and drying of the
matrix
first phase can comprise a simple
procedure such as dilution with suitable solvent, in some cases, various extraction and/
or cleanup strategies such as SPE or dialysis [68,69] have to be employed to isolate
target analytes from the complex food material and to minimize undesired effects
caused by interfering compounds (see Table 2.3).
-
sample mixture on the surface. While the
Search WWH ::




Custom Search