Teledyne 9110E - Nitrogen Oxides Analyzer - Manual (Page 177)

Model 9110E Instruction Manual Theory of Operation
M9110E Rev B 163
Figure 10-4: Reaction Cell During the AutoZero Cycle.
10.1.5. Measurement Interferences
It should be noted that the chemiluminescence method is subject to interferences from a
number of sources. The M9110E has been successfully tested for its ability to reject
interference from most of these sources. Table 10-1 lists the most important gases, which
may interfere with the detection of NO in the M9110E.
10.1.5.1. Direct Interference
Some gases can directly alter the amount of light detected by the PMT due to chemilumi-
nescence in the reaction cell. This can either be a gas that undergoes chemiluminescence
by reacting with O
3
in the reaction cell or a gas that reacts with other compounds and
produces excess NO upstream of the reaction cell.
10.1.5.2. Third Body Quenching
As shown in Equation 10-3, other molecules in the reaction cell can collide with the excited
NO
2
* , preventing the chemiluminescence of Equation 10-2, a process known as quenching.
CO
2
and H
2
O are the most common quenching interferences, but N
2
and O
2
also contribute
to this interference type.
Quenching is an unwanted phenomenon and the extent to which it occurs depends on the
properties of the collision partner. larger, more polarized molecules such as H
2
O and CO
2
quench NO chemiluminescence more effectively than smaller, less polar and electronically
“harder” molecules such as N
2
and O
2
.
The influence of water vapor on the M9110E measurement can be eliminated with an
optional, internal sample gas dryer. The concentrations of N
2
and O
2
are virtually constant