GE Analytical Instruments ©2006
DLM 14291 Rev. A
1-7
Nitric oxide and ozone are mixed in a small volume (~20 mL) reaction cell. This
small volume permits measurement of low concentrations of NO at low flow rates
and produces sharp peaks for analysis of liquid samples. For maximum sensitivity,
the reaction cell is operated at low pressure (typically 4-7 torr). A few other
chemicals such as sulfur-containing compounds, undergo a chemiluminescent
reaction with ozone but emit light at shorter wavelengths. To minimize
interference from these species, an optical filter that transmits only red
wavelengths (>600 nm) is installed between the reaction cell and the
photomultiplier tube.
Photomultiplier Tube and Cooled Housing
The light from the chemiluminescent reaction of NO with O
3
is measured using a
red-sensitive photomultiplier tube. For maximum sensitivity, the PMT is cooled to -
12 °C using a thermoelectric cooled housing. The cooler is operated continuously
whenever the main power switch is on. The temperature of the cooled housing is
measured using a K-type thermocouple and monitored on the front panel display.
Vacuum Pump and Ozone Destruction Trap.
A vacuum pump is used to draw the sample into the NOA and maintain the reaction
cell at low pressure. The exhaust from the reaction cell exits the analyzer at the
rear of the instrument using a metal tube connected to Tygon tubing. Ozone in the
exhaust is removed using a chemical trap containing Hopcalite™. This material
reacts with ozone, removing it from the exhaust before the gas reaches the
vacuum pump. Since the Hopcalite is consumed, the chemical trap must be
periodically replaced. The oil used in the vacuum pump is a synthetic motor oil
(Mobil 1™ weight 10W-30) which provides better protection than conventional
pump oil. For long pump lifetime, the oil must be changed at regular intervals.
The exhaust from the pump contains some oil mist, which can be removed using a
charcoal trap. The microprocessor keeps track of the trap and oil lifetimes, and
notifies the user when it is time to replace the traps or change the oil.
Electronics
There are 7 circuit boards in the NOA:
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