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Theory of Operation Teledyne API - Model 200EH/EM Operation Manual
190
Model 200E Instrument Response
0 a.u.
20 a.u.
40 a.u.
60 a.u.
80 a.u.
100 a.u.
120 a.u.
140 a.u.
0.5µm 0.7µm 0.9µm 1.1µm 1.3µm 1.5µm 1.7µm 1.9µm
Wavelength
Intensit
y
Optical Hi-Pass Filter Performance
NO + O
3
Emission Spectrum
PMT
Response
M200EH/EM
Sensitivity Window
Figure 10-10-1: M200EH/EM 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.
MNOMNO ++
2
*
2
→
(Equation 10-3)
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 10.2.4.2.
10.1.2. NO
X
AND NO
2
DETERMINATION
The only gas that is truly measured in the M200EH/EM 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 chemiluminescence.
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 M200EH/EM periodically switches the sample gas stream through a converter cartridge filled
with molybdenum (Mo, “moly”) chips heated to a temperature of 315° C. The heated molybdenum reacts with
NO
2
in the sample gas and produces a variety of molybdenum oxides and NO according to Equation 10-4.
)°315(++
2
CatOMxNOyMoxNO
zy
→
(Equation 10-4)
Once the NO
2
in the sample gas has been converted to NO, it is routed to the reaction cell where it undergoes
the chemiluminescence reaction described in Equations 10-1 and 10-2.
04521C (DCN5731)