Thermo Scientific 42iQ, Инструкция по эксплуатации

Страница: 290 / 320

Optional Equipment
Lag Volume
9-18 42iQ Instruction Manual Thermo Scientific
The 42iQ is available with the following Lag Volume option.
The 42iQ is based on the principle that nitric oxide (NO) and ozone react
to produce a characteristic luminescence with an intensity linearly
proportional to the NO concentration. Infrared light emission results when
electronically excited NO
2
molecules decay to lower energy states.
Specifically,

h O NO O NO
2 2 3
   
Nitrogen dioxide (NO
2
) must first be transformed into NO before it can
be measured using the chemiluminescent reaction. NO
2
is converted to
NO by a molybdenum NO
2
-to-NO converter heated to about 325 °C.
3 2
MoO NO 3 Mo NO 3
  
The ambient air sample enters the 42iQ through an inline PTFE
particulate filter, through a flow control capillary, and then splits between
the NO and the NO
x
channel. In the NO channel, the split sample is
directed to the common port of the three-way solenoid valve. The sample is
then routed either to the inlet tee of the reaction chamber or joins the
exhaust of the reaction chamber. In the NO
x
channel, the split sample is
directed to the common port of a second three-way solenoid valve after
having passed through the NO
2
converter and a lag volume, as shown in
Figure 9–10 . The “delayed” sample is then routed identically to the NO
channel.
The two channels operate 180° out of phase, that is, when the instrument is
monitoring NO, the NO
x
sample is being bypassed, and when the
instrument is monitoring NO
x
, the NO sample is being bypassed. The
solenoids switch every 5 seconds and the size of the lag volume has been
chosen so the same original sample is being monitored by both the NO and
the NO
x
channels. In this way, any positive or negative errors in the NO
2
signal (determined by the difference between the NO
x
and NO readings) is
minimized - especially in a situation where the sample is changing rapidly,
e.g., an urban traffic environment.
Lag Volume
Principle of
Operation