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NO can be detected by standard gas chromatograph techniques @ai, Payne et al. 1987; Tsikas,
Boger et al. 1994), although initially this was found to be much less sensitive than most of
other methods discussed here, partly because of the necessity for periodic manipulation and
thus disturbance of the reaction mixture during the sample processing. It is now increasingly
combined with scanning mass spectrophotometers where NO in air can be detected (Kelm,
Feelisch et al. 1988). Sensitivity has been limited by the resolving power (13500) required to
differentiate between ttN2 1m/z 30.00022) and NO (m1229.99799). euantification was further
complicated by the vast difference in the concentration of these two gases, namely parts per
thousand for l5N2 and parts per billion for NO. Specific experiments can be designed using the
isotope of lsN as a specific label for NO. In the mass spectrometer, gases are admitted under
very low pressures into an ion source where they are ionized in a high energy electron flux.
The positive ions formed are accelerated in an electric field and then deflected in a magnetic
field. The radius of the deflection is inversely proportional to the number of the particles. A
multi-collector device for a number of particles can be used. It can also be used to measure
the evolution of gas from a liquid sample. The reaction mixture is 'sparged' with an inert
compound such as argon (sparging is to agitate by introducing a compressed gas), a plunger is
lowered to the liquid surface and closed to the atmosphere. The reaction is then commenced
with injections of reagents, inhibitors, enzymes or cells - and in the case of NO stripping,
obtaining NO back from the compounds to which has bonded by acidification. The gas then
diffuses into a vacuum line through a cold trap which removes water vapour, and the
remaining gases canying NO and other compounds of interest are scanned and analysed at
brief intervals (Payne, Le Gall et al. 1996). This has been used to study nitrogen metabolism,
not only in biological fluids but also, for example, to assess bacterial activity in river water
etc. However using labelled arginine, it has been used to measure NOS activity (Tsikas 2OO4)
and very recently to assess whole body NO synthesis in healthy children after subjects had
oral doses of the labelled substrate with the measurement of the subsequently collected urine
(Forte, Ogborn et al. 2006). For all these measurements, isotope labelling is required, and
therefore they are not so useful for repeated exhalation measurement under different
conditions.
4.6
Chemiluminescence
The method that held the most promise for measuring NO in direct gas samples such as
exhaled air was chemiluminescence.
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