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The original group demonstrated that the light emitted was indeed directly proportional to the
linear content of the specimen and I have included the graph of their results (see Figure 4.3).
They used increasing volumes from one to one hundred micro-litres of a commercially
available gas in gas tight, Nz flushed syringes. The chemiluminscence signal was recorded as
the peak height at an integration time of two seconds. This showed a linear relationship of
chemiluminscence measured in millivolts to NO concentration measured from zero to fifty
picomoles.
Figure 4.3: Relationship between chemiluminscsnce in millivolts to nitric oxide concentration in
picomoles.
m
e*
Eto
plo
tr*
!-
E-
610
dgnrl.l7.9'l{Onurr-tt
0
0tr0t6m?lgl$'|oa5
t{Oernln[(pfllol)
Taken from Archer S. Measurement of Nitric oxide in biological models. FASEB Journal 1993; 7(2):
349-360 (Archer 1993).
The background output of the photomultiplier is relatively stable, unlike the other light source
meters which can have surges. The dark current (as it is known) can be affected by changes in
temperature, light and alterations in voltage, and by very high levels of NO. Large
disturbances in the dark current may take days to stabilise. For this reason most are equipped
with a cooler as cooling the photomultiplier tube improved the signaUnoise ratio. Preferably
the analyser should also be kept in a temperature controlled room and away from any other
equipment that potentially generates heat.
A red cut off filter separates the reaction chamber and the photomultiplier to prevent the
detection of light with wavelengths below that of the NO/O3 reaction. These include blue or
ultraviolet emission of alkenes and sulphur containing species such as the reaction of
hydrogen sulphide (HzS). These reactions are unlikely to be a major problem in practice as the
compounds are present on a greatly reduced scale compared to NO with far less or no
biological activity, and they are not volatile. As the NO/OI reaction takes place in the gas
phase, this is ideal for measurement of gaseous samples such as exhaled air. The resulting
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