Teledyne 200E, Инструкция по эксплуатации, Страница: 171 / 302

Theory of Operation Model 200E Instruction Manual
supplies the reaction cell with a large, constant excess of ozone (about 3000-5000 ppm)
from the internal ozone generator.
Figure 10-1: M200E Sensitivity Spectrum
However, only about 20% of the NO
2
that is formed through reaction 10-1 is in the excited
state. In addition, the excited NO
2
can collide with another collision partner M in the
reaction cell (mostly other molecules but also cell walls) and transfer its excess energy to
its collision partner without emitting any light at all (Equation 10-3). In fact, by far the
largest portion of the NO
2
* returns to the ground state this way, leaving only a few percent
yield of usable chemiluminescence.
Eq 10-3 )
M NO M NO
+ → +
2
*
2
In order to enhance the light yield of the reaction, the reaction cell is maintained at reduced
pressure. 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 quenching , an unwanted process further described
in Section 10.1.5.2.
10.1.2. NO
X
and NO
2
Determination
The only gas that is truly measured in the M200E is NO. Any NO
2
contained in the gas is not
detected in the above process since NO
2
does not react with O
3
to undergo chemilumi-
nescence.
In order to measure the concentration of NO or NO
X
(which is defined here as the sum of
NO and NO
2
in the sample gas), the M200E periodically switches the sample gas stream
through a converter cartridge filled with molybdenum (Mo, “moly”) chips (Figure 10-6)
156 044100102 Rev A