Introduction
Principle of Operation
1-2
Model 42
i
Trace Level Instruction Manual
Thermo Fisher Scientific
Thermo Fisher Scientific is pleased to supply this NO-NO
2
-NO
x
analyzer.
We are committed to the manufacture of instruments exhibiting high
standards of quality, performance, and workmanship. Service personnel are
available for assistance with any questions or problems that may arise in the
use of this analyzer. For more information on servicing, see Chapter 7,
“
Servicing
”.
The Model 42
i
Trace Level operates on the principle that nitric oxide
(NO) and ozone (O
3
) react to produce a characteristic luminescence with
an intensity linearly proportional to the NO concentration. Infrared light
emission results when electronically excited NO2 molecules decay to lower
energy states. Specifically:
Nitrogen dioxide (NO
2
) must first be transformed into NO before it can
be measured using the chemiluminescent reaction. NO
2
is converted to
NO by a molybdenum NO
2
-to NO converter heated to about 325 °C.
The ambient air sample is drawn into the Model 42
i
Trace Level through
the
sample
bulkhead, as shown in
Figure 1–1
. The sample flows through a
capillary, and then to the mode solenoid valve.
The mode solenoid valve determines whether the sample flows through the
NO
2
-to-NO converter (NO
x
mode) or bypasses the NO
2
-toNO converter
(NO mode). The sample then flows through the converter output valve
and a flow sensor to the prereactor solenoid valve.
The prereactor solenoid valve directs the sample either to the reaction
chamber, where it mixes with ozone to give an NO reading, or to the
prereactor, where it reacts with ozone prior to the reaction chamber giving
a dynamic zero reading for the analyzer. The prereactor is sized so that
greater the 99% of a 200 ppb NO sample will react prior to entering the
reaction chamber, yet is small enough to allow other potential interferents
to pass through to the reaction chamber.
Dry air enters the Model 42
i
Trace Level through the dry air bulkhead,
passes through a flow switch, and then through a silent discharge ozonator.
The ozonator generates the ozone needed for the chemiluminescent
reaction. At the reaction chamber, the ozone reacts with the NO in the
sample to produce excited NO
2
molecules. A photomultiplier tube (PMT)
housed in a thermoelectric cooler detects the luminescence generated
during this reaction.
Principle of
Operation
ν
h
+
O
+
NO
O
+
NO
2
2
3
→
