Agriculture Reference
In-Depth Information
Several in-house and commercial autosamplers have been developed for repeat-
able and high-throughput sampling of liquid and solid samples. The most common
method of liquid sample introduction is its transfer into the metastable gas stream on a
surface of a glass melting point capillary either by robotic arm or by scanning
autosampler. Direct and automated desorption of sample deposits from various
surfaces (e.g., tablets or TLC plates) can also be performed provided the angle-
adjustable ion source is available. An alternative approach is represented by TM
sampling, within which a porous material (stainless steel wire mesh, fabric, or foam
swabs) is positioned between the DART source exit and the mass spectrometer inlet to
serve as the desorption/ionization surface [106
108]. In a study by Krechmer et al.,
TM sampling was combined with ohmic heating of the metal screen surface while
operating the DART ionization gas at ambient temperature. The control of surface
temperature through modulation of electrical current
-
cant
increase in sample vaporization rate, thus increasing the analysis throughput,
compared with a conventional DART setup [109].
flow enabled a signi
Desorption Atmospheric Pressure Chemical Ionization
Desorption atmospheric
pressure chemical ionization (DAPCI) is based on desorption of the sample surface by a
heated gas stream containing reagent species (electrons, protons, hydronium ions,
solvent ions, metastables, etc.) generated in an atmospheric pressure corona dis-
charge [110]. The dominant mechanism of ion formation is similar to that taking
part in a conventional APCI source in which gas-phase ion/molecule reactions seem to
play the crucial role in the ionization process [111]. The primary ions generated in
corona discharge collidewith the solvent molecules to formsecondary ions that transfer
charge to the analytes emitted from the sample surface. The setup employing a supply of
solvent (Figure 2.10) can be avoided if there is a suf
cient concentration of atmospheric
water present in the ionization region (H
3
O
+
ions are thenmainly involved in gas-phase
ion reactions) [91]. DAPCI provides superior ionization yield for nonpolar compounds
of rather lower molecular weight, thus offering an ionization method that is orthogonal
to DESI.
Atmospheric Pressure Solids Analysis Probe Ionization
Atmospheric pressure
solids analysis probe (ASAP) is another APCI-like technique similar to DAPCI. The
liquid or solid sample is loaded on a glass probe and inserted into a conventional
APCI source to be exposed to a stream of hot nitrogen gas without solvents [112].
As a result of vaporization, the sample components are transferred into the gas phase
and further carried to the discharge needle region where they are ionized through
corona discharge-based APCI processes (Figure 2.11). The type of ions formed in
ASAP ionization is strongly in
uenced by the environment in the ion source,
especially by the humidity. Depending on the conditions, either [M
H]
or radicals ions (M
+
and M
) are the predominant species observed [94]. Like
DESI and DART, the ASAP technique has been commercialized and can be
attached to most LC
H]
+
/[M
+
MS systems. However, no option for automated sample
introduction is currently available.
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