Agriculture Reference
In-Depth Information
Figure 2.14.
Relationship between heater temperature of the DART ion source and actual temperature in
the ionization region at helium gas flow 2 L/min [123].
employed. Such a hardware setup can also improve the sensitivity of measurement by
sweeping the analyte-laden carrier gas to theMS inlet region, thus reducing the potential
for the gas to drift away into the surrounding atmosphere [124].
In most applications published to date, the impact of DART electrode voltages on
the sensitivity has not been studied in detail and settings similar to those shown in
Table 2.4 were used. The reports in which optimization of these parameters was
carried out provide rather contradictory results [115]. For example, the signals of
some organometallic compounds were intensi
ed by increasing the discharge needle
voltage (up to 4 kV) and were also strongly dependent on other electrode potential
settings [125]. Improved sensitivity was obtained for melamine when a relatively low
grid electrode potential of 50 V was used [120,126]. On the other hand, no signi
cant
impact of electrode voltages was observed in a study of the analysis of pharmaceut-
icals in biological
fluids. In any case, the optimal voltage settings may vary for various
compound classes and should be tuned when performing targeted analysis of a few
analytes.
As already mentioned, formation of adduct ions in DART can be facilitated by
allowing dopant vapors to access the ionization region. Depending on the particular
analyte, the use of a dopant can yield adducts that are of higher intensity than the
pseudomolecular ion or even facilitate ionization of compounds that would otherwise
not provide any ions under standard DART setup. For this purpose, aqueous
ammonia, dichloromethane, or tri
uoroacetic acid solution is typically used to induce
CF
3
COO]
ions, respectively. The
vapor introduction can be achieved either by placing an autosampler vial containing
dopant in the proximity of the ionization region or by adding it to the sample [96]. The
use of dopants has been demonstrated by Vaclavik et al., who employed ammonia to
enhance formation of [M
NH
4
]
+
,[M
Cl]
, and [M
formation of [M
+
+
+
NH
4
]
+
from triacylglycerols in olive oil [127]. The adduct
+
