GAS BreathSpec, Руководство пользователя

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BreathSpec®
– User Manual
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Figure 2: Ion Mobility Spectrometer -Working Principle
Atmospheric Ionization of molecules can be obtained by several techniques. G.A.S.
uses photoionization with a 10.6eV UV-lamp or soft chemical-ionization initiated by a
low-radiation tritium (H3) source (below exemption limits of EURATOM). While the
first directly produces positive ions, the latter generates
reactant ions with the gas
atmosphere by a cascade of reactions following the collision of a fast electron
emitted from the
β
-radiator H3. The so-called Reaction Ion Peak (RIP) representing
the total of all ions available is formed as a first step. In nitrogen and air, resp., the
reactant ions can be described as H+(H2O)
n
and O2-(H2O)
n
. Chemical ionization of
analytes by reactant ions then result in the formation of specific analyte ions, when
the affinity of the analyte towards the reactant ion is higher in case compared to
water (using the positive ionization mode). The proton affinity of water is 691kJ/mol,
so all molecules with a higher proton affinity will be ionized by a proton transfer,
which is typically given for all heteroatom-organic compounds. The ionization takes
place at ambient pressure, so that the analyte concentration is not diluted as
compared to other analytical methods where a vacuum must be applied. Therefore,
IMS is extremely sensitive. The detection limits typically are in the low ppb-range for
volatile organic compounds (VOC).