Electrochemical reduction of NOx - DTU Orbit

3.5.1 Mass spectrometer
3 Experimental
The mass spectrometer used for determination of the O2, N2 and N2O concentrations in this work
was a type Omnistar TM GSD 301 from Pfeiffer Vacuum. The basic principle of a mass spectrometer
is the gas components are first ionized, then sorted according to mass and finally detected. A very
simplified sketch of the components in a mass spectrometer is shown in Figure 6. In the
Omnistar TM GSD 301 mass spectrometer used in this work the sample gas was continuously sucked
into the mass spectrometer through a 1 m long capillary, while the vacuum in the system was
established by the combination of a turbo pump and a diaphragm pump. The ion source used was
a tungsten filament and the mass analyser a quadropole mass analyser. The detector used was a
channeltron detector, where channeltron is an abbreviation of channel electron multiplier. For
control of the mass spectrometer and data acquisition the Quadstar 32-bit software from Pfeiffer
Vacuum was used. This software can be used for logging of the ionic current signal detected during
the measurements, when the MID (Multiple Ion Detection) operating mode is chosen.
Furthermore the software can be used converting the ionic current signal into concentrations after
a suitable calibration when the MCD (Multiple Concentration Detection) operating mode is
chosen. The MCD operating mode was used for the conversion measurements in this work.
3.5.2 Chemiluminiscence detector
Figure 6. General schematic of mass spectrometer.
The concentration of NO, NO2 and NOx was detected by a chemiluminiscence detector Model 42i
High Level from Thermo Scientific. The operating principle of the chemiluminiscence detector is
based on the detection of light emission from excited chemical species, also known as
chemiluminiscence. In the chemiluminiscent detector the NO reacts with ozone and forms excited
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