Teledyne T200, Operation Manual, Page: 286 / 425

Principles of Operation Teledyne API
– T200 NO/NO2/NOx Analyzer Operation Manual
284
Equation 13-1
2
*
2 3
O NO O NO
  

The second step occurs because the laws of thermodynamics require that systems
seek the lowest stable energy state available, therefore the excited NO
2
molecule
quickly returns to its ground state, releasing the excess energy. This release takes
the form of a quantum of light (
h

). The distribution of wavelengths for these quanta
range between 600 and 3000 nm, with a peak at about 1200 nm.
Equation 13-2
nm
h NO NO
1200 2
*
2

 
All things being constant (temperature, pressure, amount of ozone present, etc.), the
relationship between the amount of NO present in the reaction cell and the amount of
light emitted from the reaction is very linear. If more NO is present, more IR light is
produced. By measuring the amount of IR light produced with a sensor sensitive in the
near-infrared spectrum (see Figure 13-2) the amount of NO present can be determined.
In addition, sometimes the excited NO
2
collides with other gaseous molecules in the
reaction cell chamber or even the molecules of the reaction cell walls and transfers its
excess energy to this collision partner (represented by
M in the equation 12-3 below)
without emitting any light at all. In fact, by far the largest portion of the excited NO
2
returns to the ground state this way, leaving only a few percent yield of usable
chemiluminescence.
Equation 13-3
M NO M NO
  
2
*
2
The probability of a collision between the NO
2
* molecule and a collision partner M
increases proportionally with the reaction cell pressure. This non-radiating collision
with the NO
2
* molecules is usually referred to as third body quenching , an unwanted
process further described in Section 13.1.5.2.
Even under the best conditions only about 20% of the NO
2
that is formed by the reaction
described in equation 12-1 is in the excited state. In order to maximize
chemiluminescence, the reaction cell is maintained at reduced pressure (thereby
reducing the amount of available collision partners) and is supplied with a large,
constant excess of ozone (about 3000-5000 ppm) from the internal ozone generator.
06858E DCN7057