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The first chemiluminscence analyser to measure NO was built in 1970 utilizing these
principles of chemical reaction. A vacuum of reduced pressure (approximately negative l-15
mmHg) is used to draw the gaseous sample into a reaction chamber through a valve where
ozone is generated by electrical discharge. The vacuum is also necessary to evacuate gases
that could potentially absorb energy from NO2., and to stabilise NO by removing 02, to
prevent any 02 reacting with the NO to produce NOz which does not emit light (Hampl,
Walters et al. 1996). The NO and 03 are then mixed in front of a photomultiplier tube
sensitive to low levels of light at the red sensitive end of the spectrum (660 - 900nm). The
photons from the reaction strike a photosensitive surface and the impact releases electrons
which are accelerated toward an electron sensitive surface (the first dynode) by an electric
field. Each electron impact on this first dynode then causes emission of several electrons and
these are accelerated to a second dynode. This step is repeated but the electrons are attracted
to the terminal electrically charged element - the anode - and the resulting current is
measured. This amplification achieved by the photomultiplier is necessary to measure the
signal as the NO2'rosction emits a relatively weak red light (Hampl, Walters et al. 1996) and
means that each electron emitted from the original photosensitive surface becomes a signal
from millions of electrons at the anode (Turner 1985).
Figure 4.2: Diagram of the chemiluminscence analyser
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