Thermo 42i - Principle of Operation

Introduction
Principle of Operation
1-2 42i Low Source Instruction Manual Thermo Electron Corporation
• “Specifications” on page 1-3 is a list of the analyzer’s performance
specifications.
Thermo Electron is pleased to supply this chemiluminescence
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”.
Principle of Operation The Model 42i Low Source 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 NO
2
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 stainless steel NO
2
-to-NO converter heated to about 625 °C (the optional
molybdenum converter is heated to 325 °C).
The ambient air sample is drawn into the Model 42i Low Source through
the sample bulkhead, as shown in Figure 1-1. The sample flows through a
capillary, and then to the mode solenoid valve. The solenoid valve routes the
sample either straight to the reaction chamber (NO mode) or through the
NO
2
-to-NO converter and then to the reaction chamber (NO
x
mode). A
flow sensor prior to the reaction chamber measures the sample flow.
Dry air enters the Model 42i Low Source 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. From the reaction chamber, the exhaust travels through the
ozone (O
3
) converter to the pump, and is released through the vent.
The NO and NO
x
concentrations calculated in the NO and NO
x
modes
are stored in memory. The difference between the concentrations is used to
calculate the NO
2
concentration. The Model 42i Low Source outputs NO,
υ hONOO NO
223
++→+