Teledyne T200 User Manual page 179

The second step occurs because the laws of thermodynamics require that systems
•
seek the lowest stable energy state available, therefore the excited NO
quickly returns to its ground state, releasing the excess energy. This release takes
the form of a quantum of light (hν). The distribution of wavelengths for these quanta
range between 600 and 3000 nm, with a peak at about 1200 nm.
• 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 6-2) the amount of NO present can be
determined.
In addition, sometimes the excited NO
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 Equation 6-3 below) without
emitting any light at all. In fact, by far the largest portion of the excited NO
ground state this way, leaving only a few percent yield of usable chemiluminescence.
The probability of a collision between the NO
increases proportionally with the reaction cell pressure. This non-radiating collision with
the NO * molecules is usually referred to as third body quenching, an unwanted process
2
further described in Section 6.1.5.2.
Even under the best conditions only about 20% of the NO
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.
083730200A DCN7962
*
NO → NO
2
*
NO + M →
2
Teledyne API T200 NO/NO
ν
+ h
2 1200 nm
collides with other gaseous molecules in the
2
NO + M
2
* molecule and a collision partner M
2
that is formed by the reaction
2
/NO Analyzer with NumaView™ Software
2 x
molecule
2
Equation 6-2
returns to the
2
Equation 6-3
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