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Figure 2.9.
A scheme of DART ion source and ionization process.
Direct Analysis in Real-Time Ionization
DART ionization, introduced in 2005 by
Cody et al., is the second pioneering ambient ionization technique [93]. Like DESI,
DART is commercially available and its popularity is rising. In DART, excited-state
metastable atoms or molecules of gas (typically helium) are used as the medium
for ionization. Gas metastables are formed via a glow discharge taking part in a
compartment separated from the sample. In the next step, charged species are
removed from the gas stream by passing through a perforated electrode, leaving
only metastables. Gas can be optionally heated and directed to the sample (see
Figure 2.9). The grid electrode at the exit of the DART gun serves as an ion repeller,
which prevents ion
ion recombination resulting in signal loss. In the sampling region
between the ion source exit andmass spectrometer inlet, the metastable species interact
either directly with the (thermo)desorbed analyte molecules or with atmospheric
components to form reactive species that further ionize the analytes. The major
mechanism of DART ionization in positive ion mode involves Penning ionization
of atmosphericwater and nitrogen and subsequent proton transfer to analyte resulting in
[M
-
H]
+
ion formation. In negative ion mode, negatively charged oxygen clusters
formed by thermal electrons deprotonate molecules of analytes. In addition to proton
abstraction, electron capture, dissociative electron capture, and anion attachment
processes can take part under DART negative ion mode settings [96]. DART is
suitable for ionization of analytes with medium/low polarity and molecular weights
below 1000 Da. DART mass spectra show common features with those obtained by
APCI and APPI techniques. Contrary to DESI, multiply charged ions or metal
+
cation
adducts are not formed [93]. For some analytes, generation of adduct ions, such as
[M
-
Cl]
, can be induced by introducing vapors of suitable dopant
solvents into the ionization region. In DART-based experiments, the source optimiza-
tion is typically simpler compared with DESI and is mainly limited to tuning the
ionization gas temperature and adjusting the setup geometry.
NH
4
]
+
or [M
+
+
