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Principles of Operation Teledyne API - Model T200H/T200M Operation Manual
270
8.2.4. MEASUREMENT INTERFERENCES
It should be noted that the chemiluminescence method is subject to interferences from a
number of sources. The T200H/M has been successfully tested for its ability to reject
interference from most of these sources. Table 8-1 lists the most important gases, which
may interfere with the detection of NO in the T200H/M.
8.2.4.1. 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.
8.2.4.2. Third Body Quenching
As shown in Equation 9-3, other molecules in the reaction cell can collide with the
excited NO
2
*
, preventing the chemiluminescence of Equation 9-2, 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 T200H/M measurement can be eliminated with an
optional, internal sample gas dryer. The concentrations of N
2
and O
2
are virtually
constant in ambient air measurements, hence provide a constant amount of quenching and
the interference of varying CO
2
amounts is negligible at low concentrations.
The T200H and T200M analyzers are typically used in high CO
2
concentration
environments. The pneumatic setup of these two analyzer models minimizes the
interference from CO
2
such that the analyzers conform to the standards set forth by the
US-EPA in Method 20 - NOx from Stationary Gas Turbines , available at
http://www.epa.gov/ttn/emc/promgate.html
07270D DCN7141