Agriculture Reference
In-Depth Information
was applied for this study as it introduced the least errors. Evidently, it is not
appropriate just to report
r
2
or
r
without considering accuracy or error because a
reported concentration at the lower end may be erroneous and misleading, especially
for a wider calibration range. From the above example, it can be concluded that the
selection of weighting factors for calibration curve is necessary. Selecting appropriate
weighting factors for calibration curves has become a general practice for bioanalysis
in the pharmaceutical industry to get acceptable analytical results.
2.3 DIRECT TECHNIQUES
2.3.1 Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry
Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) enables
detection of analytes varying by molecular weight and rapid examination of small
quantities of complex samples, while exhibiting good tolerance toward contamina-
tion [57]. Although MALDI is largely used in biological studies to characterize large
molecules such as proteins, peptides, or carbohydrate polymers [58], it has also been
applied to the ionization of intermediate and low molecular weight compounds [59].
The unique features and versatility of MALDI have led to its use in diverse research
fields, including food quality, authenticity, and safety control. MALDI-based mass
spectrometry is undoubtedly an analytical technique that has signi
cant potential for
use in high-throughput food analysis.
2.3.1.1 Principles and Instrumentation
In MALDI, analytes are mixed with an excess of a UV-absorbing matrix compound
(usually a small organic acid) and dried to allow formation of crystals on a sample
plate. The sample plate is subsequently introduced into the MALDI ion source in
which a vacuum is maintained. The sample
matrix crystals are exposed to a short
pulse (few nanoseconds) from a laser and, as a consequence of heat energy absorption,
transferred into the gaseous phase, where analyte ionization takes place. The ions are
immediately extracted into the mass spectrometer. It should be noted that many laser
pulses (typically 10
-
ed
schemes of a MALDI source and ionization process are shown in Figure 2.3. Due to
the nature of the MALDI technique, it is widely used with re
-
500) are summed to form the mass spectrum. The simpli
ectron-type TOF mass
analyzers, which provide enhanced mass resolving power and enable analysis of ions
in a wide range of
m
/
z
values. It should be noted that in 2000, atmospheric pressure
(AP) MALDI was developed and enabled reduction in cost and enhanced ease of
operation [60].
The MALDI ionization process is complex and is not yet completely understood.
The most general mechanism described by Knochenmuss proposes a two-step process:
(i) primary ionization after laser impact resulting in the formation of matrix-derived
species, and (ii) secondary reactions taking part in the MALDI plume after evaporation
of matrix
-
analyte crystals [61]. According to the photoionization/pooling model
