Agriculture Reference
In-Depth Information
ionization techniques, these novel approaches enable straightforward examination of
various objects while requiring little or no sample pretreatment and signi
cantly
improving the overall throughput of these methodologies compared with methodol-
ogies that require multiple or complex sample preparation steps. Sample interrogation
can be performed in an open environment (i.e., at atmospheric pressure) by intro-
ducing the sample into the ionization region and exposing it to a stream of desorbing
and/or ionizing medium. Analyte ions arising from the ionization processes related to
ESI, atmospheric pressure chemical ionization (APCI), or atmospheric pressure
photoionization (APPI) are subsequently transferred through the open air to the inlet
of the mass spectrometer [89]. More than 30 ambient ionization techniques (including
some variants) have been developed and described to date. It should be noted,
however, that not all of them have been widely used or commercialized. The
pioneering techniques of desorption electrospray ionization (DESI) [92] and direct
analysis in real time (DART) [93] remain the most established. Considering the ease
of use and high throughput, ambient MS has been recognized as holding great
potential for rapid characterization of food components, detection of various contam-
inants and
ngerprinting/pro
ling [94
-
96], and in many other
fields of analytical
chemistry [97
-
99]. The ambient ionization techniques are anticipated to be applied in
the
field as powerful tools for early detection of various hazards related to food.
The following sections provide an overview of the most widely used ambient
desorption/ionization techniques that have been applied to various aspects of food
quality/safety and list example applications documenting both advantages and
limitations of these techniques. Additionally, experimental parameters that signifi-
-
cantly in
uence the outcome of DESI- and DART-based analyses are discussed.
2.3.3.1 Principles and Instrumentation
Desorption Electrospray Ionization
DESI, which was introduced in 2004 by
Takats et al., remains the most popular and widely used of all ambient desorption/
ionization techniques [92]. Ionization in DESI experiments occurs when electrospray-
generated charged droplets of solvent (typically amixture of water and organic solvent)
are directed toward the sample components deposited on a sampling surface. The
ionization mechanism is not yet completely understood, but it is believed to be
predominantly a multistage charged droplet pickup process. In the
rst phase, the
sampling surface is prewetted by the initial solvent droplets and dissolution of sample
components takes place. Subsequently, the surface solvent layer is impacted by
later arriving charged droplets to formmicrodroplets. The analytes in the multicharged
microdroplets are ionized through processes taking part in conventional ESI, that is, a
continuous decrease in droplets size and formation of analyte ions via
“
Coulombic
explosion.
In addition to droplet pickup, condensed-phase and gas-phase charge
transfer processes are also probably involved in ionization of some analytes [97]. The
ions thus formed are transferred into the mass spectrometer through an extended ion
transfer line that links the gap between the ionization region and the MS system
”
s
atmospheric pressure interface. Because of the similarity in ionization mechanisms,
DESI and ESI yield similar mass spectra that contain both singly and multiply charged
ions and are particularly effective in ionization of polar analytes.
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