Agriculture Reference
In-Depth Information
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0
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Gas temperature (°C)
Figure 2.13.
The impact of ionization gas temperature on signal intensity of melamine (
□
) and cyanuric
acid (
) in powder extract. Error bars are standard deviations (
n
=
5) Ref. [96], Figure 2, p. 207.
Reproduced with permission of Elsevier Science Ltd.
■
formation of charged water clusters, thus further ionizing the analytes. Nitrogen- and
argon-derived metastables are of lower energy and can ionize only some compounds,
thus limiting the application scope [115]. Nevertheless, this phenomenon can be used
for selective ionization of target analytes while avoiding ion formation of other
sample components [120]. Additional obstacles to wider application of nitrogen are
the need for higher electric
fields for metastable formation and possible oxidation of
analytes during ionization [121].
The temperature of the ionization gas is often the key factor affecting the results in
DART-based experiments. The optimal gas temperature for a particular analyte depends
on its physicochemical properties, such as boiling point, polarity, andmolecular weight.
A gas temperature that is too low will not facilitate thermodesorption of nonvolatile
analytes, while a gas temperature that is too high can lead to rapid volatilization and
signal drop due to insuf
cient acquisition rate of the mass spectrometer. In addition,
analyte thermal degradation, extensive ion fragmentation, or even sample pyrolysis can
occur under high ionization gas temperatures [96,122]. Typically, a bell-shaped curve is
obtained when plotting the ion intensity against the gas temperature used, as shown in
Figure 2.13 for melamine and cyanuric acid spiked into milk powder extract. From the
practical point of view, it is important to note that the actual temperature in the ionization
region is different from that of the gas heater as a consequence of mixing with the cooler
ambient atmosphere (see Figure 2.14). This temperature difference is even more
pronounced when higher gas
flow rate settings are applied [123].
Regarding the ionization gas
flow, an increase in signal intensity due to promoted
thermodesorption process was reported at higher helium gas
flows. When operating
DART at high
flow rates, one should be aware of the risk of MS system contamination,
as the sample can be easily blown off the sampling surface. The upper gas
flow limit is
also determined by the stability of the vacuum system of the mass spectrometer because
high gas
flow rates can increase the pressure in the atmospheric pressure interface and
cause automatic shutdown of the instrument. To overcome this drawback, a gas ion
separator
flange enclosing the MS inlet and providing additional pumping has to be
