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amplified and detected as a current. As mentioned in the previous chapter, the dark current of
the photomultiplier can be affected by changes in temperature and this model had an inbuilt
cooler that kept the photomultiplier temperature at -200C to improve the signaUnoise ratio.
The machine itself should ideally also be kept in a temperature controlled room (as was
suggested in the machine manual). In our case it was placed in a small air conditioned room
within the hospital away from any other equipment that potentially generated heat - though
there was one window into the room which resulted in some temperature change. The NO
analyser was re-serviced just prior to our experiments taking place.
The NO analyser was calibrated with azero and two known NO gas concentrations before and
after every subject during the methodological studies and between every two subjects when
later studies in children were being undertaken. Two of the gases used were commercial
preparations of NO made for calibration purposes by BOC Gases (BOC Gases, 10 Priestley
Rd, Surrey Research Park, Guildford, Surrey, UK) and as the studies progressed included
concentrations of 37ppb, 55ppb lO3ppb, l lOppb and I lSppb with the balance being nitrogen
(Nz). Calibration gases at these low levels were extremely difficult to get at the
commencement of these studies as these gas mixtures had no known use other than
calibration, and this was not widely used at this time. The third gas was a zero calibration and
this was prepared by passing room air through the ozone generator of the chemiluminescence
analyser, which converted all NOx to NOz. This was then passed through a soda lime and
activated charcoal column to remove the NOz and Ot respectively and then passed direct into
the analyser. The resulting NOz and 03 from the experiment were removed in the same
manner by passing through the soda lime column and the charcoal column. To minimise NO
loss by absorption or interaction with other surfaces (see Section 5.4), teflon tubing (4mm
extemal diameter) was used as the most inert substance to connect the machinery and to
sample from the subject.
5.2.2 Capnograph, pressure transducer, Jlow meter and chart recorder
In addition to measuring NO itself, we wanted to be able to measure other parameters. At the
time, the early publications available on exhaled NO from the lung, which will be reviewed in
the next chapter (see Section 6.3), seemed to show a consistency in the trends found in NO
results in different subject groups but the absolute levels of NO reported between the
investigators were very different. This suggested that some of the measurement techniques
were altering the NO results recorded. There was also discussion in the literature about where
the measured NO from exhalation was anatomically being generated. In view of this, we
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