Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 324)

Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 324
13.1.5.2. Third Body Quenching
As described by Equation 13-3, other molecules in the reaction cell can collide with the excited NO
2
* ,
causing the excited NO
2
* to return to its ground state without releasing a photon of light. This is known
as third party quenching.
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
are the most significant quenching
interferents of NO chemiluminescence.
 The influence of water vapor on the 9110T measurement can be eliminated with an optional,
internal sample gas dryer (see Section 3.3.2.6).
 The interference of varying CO
2
amounts at low concentrations (less that 0.5%) is
negligible.
 In cases with excessively high CO
2
concentrations (larger than 0.5%), the effect can be
calibrated out by using calibration gases with a CO
2
content equal to the measured air.
 Only very high and highly variable CO
2
concentrations will then cause a measurable
interference. For those applications, it is recommended to use other analyzer models.
Please consult TAI Sales Department or our website (see Section 12.10).
 Smaller less polar and electronically “harder” molecules such as N
2
and O
2
can cause interference
of this type as well, however, the concentrations of N
2
and O
2
are virtually constant in ambient air
measurements, hence provide a constant amount of quenching that is accounted for in the
calibration of the instrument .
13.1.5.3. Light Leaks
The 9110T sensitivity curve includes a small portion of the visible light spectrum (see Figure 13-2),
therefore it is important to ensure that the reaction cell is completely sealed with respect to light. To
ensure this:
 All pneumatic tubing leading into the reaction cell is opaque in order to prevent light from entering
the cell.
 Light penetration is prevented by stainless steel filters and orifices.