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Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 325
Table 13-1: List of Interferents
Gas Interference Type Rejection Method
CO
2
Dilution: Viscosity of CO
2
molecules causes them to
collect in aperture of Critical Flow Orifice altering flow
rate of NO.
If high concentrations of CO
2
are suspected,
special calibration methods must be performed to
account for the affects of the CO
2
.
Contact TAI’s Customer Service Department (see
Section 12.10) for details.
3
rd
Body Quenching: CO
2
molecules collide with
NO
2
* molecules absorbing excess energy kinetically
and preventing emission of photons.
SO
X
Some SO
X
variants can also initiate a
chemiluminescence reaction upon exposure to O
3
producing excess light.
Wavelengths of light produced by
chemiluminescence of SO
X
are screened out by
the Optical Filter.
Chemically reacts with NH
3
, O
2
and H
2
O in O
3
generator to create (NH
3
)
2
SO
4
(ammonium sulfate)
and NH
3
NO
2
(ammonium nitrate) which form opaque
white deposits on optical filter window. Also forms
highly corrosive HNO
3
(Nitric Acid)
Most of the ammonium sulfate and ammonium
nitrate produced is removed from the sample gas
by an air purifier located between the O
3
Generator and the reaction cell.
3
rd
Body quenching: SO
X
molecules collide with NO
2
*
molecules absorbing excess energy kinetically and
preventing emission of photons.
If high concentrations of SO
X
are suspected,
special calibration methods must be performed to
account for the affects of the SO
2
.
Contact Teledyne Customer Service Department
(see Section 12.10) for details.
H
2
O
3
rd
Body quenching: H
2
O molecules collide with NO
2
*
molecules absorbing excess energy kinetically and
preventing emission of light.
Analyzer’s operating in high humidity areas must
have some drying applied to the sample gas (see
Section 3.3.2.6 for more details).
Water also reacts with NH
3
and SO
X
in the O
3
generator to create (NH
3
)
2
SO
4
(ammonium sulfate)
and NH
3
NO
2
(ammonium nitrate) which form opaque
white deposits on the optical filter window. This also
forms highly corrosive HNO
3
(nitric acid)
Water is effectively removed from the O
3
gas
stream by the Perma Pure
®
Dryer (Section
13.2.3.2 for more details). We offer several
Perma Pure
®
dryers for the sample stream (see
Section 3.3.2.6 for more details).
NH
3
Direct Interference: NH
3
is converted to H
2
O and NO
by the NO
2
converter. Excess NO reacts with O
3
in
the reaction cell creating a chemiluminescence
artifact.
If a high concentration of NH
3
is suspected, steps
must be taken to remove the NH
3
from the sample
gas prior to its entry into the NO
2
converter (see
Section 3.3.2.6 for more details).
NH
3
also reacts with H
2
O, O
2
and SO
X
in the O
3
generator to create (NH
3
)
2
SO
4
(ammonium sulfate)
and NH
3
NO
2
(ammonium nitrate) which form opaque
white deposits on optical filter window. Also forms
highly corrosive HNO
3
(nitric acid).
The Perma Pure
®
dryer built into the 9110T is
sufficient for removing typical ambient
concentration levels of NH
3
.
13.1.5.4. Reaction Cell Temperature Control
The stability of the chemiluminescence reaction between NO and O
3
can be affected by changes in the
temperature and pressure of the O
3
and sample gases in the reaction cell. In order to reduce temperature
effects, the reaction cell is maintained at a constant 50 C, just above the high end of the instrument’s
operation temperature range.
Two AC heaters, one embedded into the bottom of the reaction cell, the other embedded directly above
the chamber’s exhaust fitting, provide the heat source. These heaters operate off of the instrument’s
main AC power and are controlled by the CPU through a power relay on the relay board (see Section
13.3.4.4).
A thermistor, also embedded in the bottom of the reaction cell, reports the cell’s temperature to the CPU
through the thermistor interface circuitry of the motherboard (see Section 13.3.3.3).